Answer

(b) All the genes, present on a particular chromosome form a linkage group. The number of linkage group of a species correspond to the total number of different chromosomes of that species. It is not simply the number of chromosomes in haploid set.

e.g., in human male $=22$ pairs of autosomes $+1 \mathrm{X}$-chromosome $+1 \mathrm{Y}$-choromosme i.e., 24 linkage groups and in female $=22$ pairs autosomes $+2 X$-chromosomes i.e., 23 linkage groups.

While options (a), (c) and (d) are incorrect.

• Option (a) is incorrect: Genes located on the same chromosome do not form different groups based on their relative distance. Instead, they are part of the same linkage group regardless of their distance from each other.

• Option (c) is incorrect: Genes located on the same chromosome will form a linkage group. The concept of linkage groups is based on the physical connection of genes on the same chromosome.

• Option (d) is incorrect: While genes on the same chromosome can interact and affect the phenotype, this does not define the concept of a linkage group. A linkage group is specifically defined by the physical connection of genes on the same chromosome, not by their interactive effects on the phenotype.

Thinking Process

Numerical changes in chromosome number are referred to as changes in ploidy.

Answer

(a) Aneuploidy involves changes in chromosome number by additions or deletions of less than a whole set. In this case organism gains or loses one or more chromosomes but not a complete set. Polyploidy is defined as the addition of entire set of chromosome. The polyploidy can be triploidy (3n), tetraploidy (4n), pentaploidy (5n), etc.

Allopolyploidy is the polyploidy in which chromosome sets are non-homologous. In other words we can say that the allopolyploids are derived from a stock which is heterozygous. Monosomy is the process in which one chromosome is removed from diploid set of chromosome (2n-1).

• Polyploidy: Polyploidy is defined as the addition of entire sets of chromosomes, such as triploidy (3n), tetraploidy (4n), pentaploidy (5n), etc. The conditions $2n \pm 1$ and $2n \pm 2$ involve changes in chromosome number by additions or deletions of less than a whole set, which is not the case in polyploidy.

• Allopolyploidy: Allopolyploidy is a type of polyploidy where the chromosome sets are non-homologous, meaning they come from different species. The conditions $2n \pm 1$ and $2n \pm 2$ do not involve the combination of chromosome sets from different species but rather the gain or loss of individual chromosomes within the same species.

• Monosomy: Monosomy specifically refers to the loss of a single chromosome from a diploid set (2n-1). The conditions $2n \pm 1$ and $2n \pm 2$ include both the gain and loss of chromosomes, not just the loss of one chromosome as in monosomy.

Thinking Process

Crossing over (recombination) is the mutual exchange of the corresponding segments of the adjacent paternal and maternal chromatids of the synapsed homologous chromosomes producing new combinations of genes.

Answer

(a) Crossing over separates genes away from each other. So, the distance between the genes and percentage of recombination shows an direct relationship, i.e., when genes are close together they have high linkage and exhibit low recombination frequencies.

Thus, the other option are wrong as it does not show parallel or inverse relationship.

• Option (b) is incorrect because an inverse relationship would imply that as the distance between genes increases, the percentage of recombination decreases, which is not the case. In reality, as the distance between genes increases, the percentage of recombination also increases.

• Option (c) is incorrect because a parallel relationship would imply that the distance between genes and the percentage of recombination change in the same manner without any direct correlation. However, there is a direct correlation where an increase in distance leads to an increase in recombination percentage.

• Option (d) is incorrect because stating there is no relationship between the distance between genes and the percentage of recombination is false. There is a well-established direct relationship where the percentage of recombination increases with the distance between genes.

Thinking Process

Sex-linked disorders are those genetic diseases where the defective genes are carried on either the $X$ or $Y$-chromosomes.

Answer

(d) Most sex-linked (X-linked) conditions are recessive. This means that in a person with two $X$-chromosomes (females), both copies of a gene (i.e., one on each X-chromosome), must have a change or mutation whereas in a person with one $X$-chromosomes (males), only one copy of a gene must have a mutation.

A female with a mutation in one copy of a gene on the $\mathrm{X}$-chromosome is said to be a ‘carrier’ for an X-linked condition.

For X-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the $\mathrm{X}$-chromosome can transmit either the $\mathrm{X}$-chromosome with this mutation or a normal X-chromosome to her children.

Autosomal dominant inheritance refers to the pattern of inheritance of a condition directly or indirectly due to a dominant faulty gene located on autosome.

Autosomal recessive inheritance is the condition caused directly or indirectly due to a recessive faulty gene copy on autosome.

Sex-linked dominant is a rare trait that is caused by a single abnormal gene on the X-chromosome.

• (a) Autosomal dominant: This option is incorrect because autosomal dominant diseases are inherited when a single copy of a faulty gene located on one of the autosomes (non-sex chromosomes) is sufficient to cause the disease. In this case, the disease would not be limited to only male progeny and would not be transferred specifically from a carrier female to male offspring.

• (b) Autosomal recessive: This option is incorrect because autosomal recessive diseases require two copies of a faulty gene (one from each parent) for the disease to manifest. The disease would not be limited to only male progeny and would not be transferred specifically from a carrier female to male offspring.

• (c) Sex-linked dominant: This option is incorrect because sex-linked dominant diseases are caused by a single abnormal gene on the X-chromosome. If a female carries a dominant mutation on one of her X-chromosomes, she would typically show symptoms of the disease herself, and the disease would not be limited to only male progeny. Both male and female offspring could inherit the disease.

Answer

(d) Sickle-cell anaemia is an autosome linked recessive trait. This disease is controlled by a single pair of allele $\mathrm{Hb}^{A}$ and $\mathrm{Hb}^{s}$ only the homozygous individuals for $\mathrm{Hb}^{s}$, i.e., $\mathrm{Hb}^{s} \mathrm{Hb}^{s}$ shows the diseased phenotype. The heterozygous individuals are carriers $\left(\mathrm{Hb}^{A} \mathrm{Hb}^{S}\right)$.

Due to point mutation, glutamic acid (Glu) is replaced by valine (Val) at sixth position of $\beta$-chain of haemoglobin molecule. This substitution occurs due to the single base substitution of the beta globin gene from GAG (Glu) to GUG (Val).

Whereas, the other codes GGG, AAG, GAA do not codes for valine.

• GGG: This triplet code corresponds to glycine, not valine.
• AAG: This triplet code corresponds to lysine, not valine.
• GAA: This triplet code corresponds to glutamic acid, not valine.

Thinking Process

The alleles which are able to express themselves independently when present together are called codominant alleles and this biological phenomenon of expressing together is called codominance.

Answer

(b) A B O blood grouping in humans is an example of codominance. A B O blood groups

$\text { are controlled by gene } \mathrm{I} . \text { Gene } \mathrm{I} \text { has three alleles } \mathrm{I}^{\mathrm{A}}, \mathrm{I}^{\mathrm{B}} \text { and } \mathrm{I}^{\mathrm{oi} . \mathrm{A}} \text { and } \mathrm{I}^{\mathrm{B}} \text { are the }$ dominant alleles. When $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ are present together, both express equally and produce the surface antigens $A$ and $B$, whereas $I$ is the recessive allele and does not produce any antigen.

Pleiotropy referred the genetic effect of a single gene on multiple phenotypic traits.

Incomplete dominance is a genetic term in which does not completely dominate another allele.

Segregation is the separation of allele during the process of gametogenesis. This is the basis of reappearance of recessive character in $\mathrm{F}_{2}$-generation.

• Pleiotropy is incorrect because it refers to the genetic effect of a single gene on multiple phenotypic traits, not the expression of two alleles equally.
• Incomplete dominance is incorrect because it describes a situation where one allele does not completely dominate another allele, resulting in a blended phenotype, not the equal expression of both alleles.
• Segregation is incorrect because it refers to the separation of alleles during the process of gametogenesis, which is the basis for the reappearance of recessive characters in the F2 generation, not the simultaneous expression of two dominant alleles.

Thinking Process

In birds the ZW case i.e., ZZ/ZW type of sex determination is seen.

Answer

(d) In ZZ/ZW case, the female has heteromorphic (ZW) sex chromosomes and the male has homomorphic (ZZ) sex chromosomes. Thus, peacock shows ZZ/ZW sex determination type.

In platypus the sex determination is of $X X-X Y$ type. Both male and females has ten sex chromosome each. The male has $X Y, X Y, X Y, X Y, X Y$ and female has $X X X X X X X X X X$. In snails the sex determination is environmentally induced, while in cockroaches it is of XX-XO types.

In this type $\mathrm{Y}$-chromosome is completely lacking. In this the presence pf unpaired $X$-chromosomes determines the masculine sex.

• In platypus, the sex determination is of $XX-XY$ type. Both males and females have ten sex chromosomes each. The male has $XY, XY, XY, XY, XY$ and the female has $XXXXXXXXXX$.
• In snails, the sex determination is environmentally induced.
• In cockroaches, the sex determination is of $XX-XO$ type. In this type, the Y-chromosome is completely lacking. The presence of an unpaired X-chromosome determines the masculine sex.

Thinking Process

On the basis of monohybrid cross (a cross involving only one trait) Mendel formulated the law of segregation.

Answer

(b) Tt and Tt let’s use Mendel’s cross of tall and dwarf pea plants as an example. The $\mathrm{F}_{1}$ plants of genotype $\mathrm{Tt}$ are self-pollinated. (both tall $(\mathrm{T})$ but with dwarf ( $\mathrm{t}$ ) alleles).

Phenotypic ratio : Tall : Dwarf

$3: 1$

Genotypic ratio : Pure tall : Hybrid : Pure dwarf

$1: 2$ : 1

The letters $T$ and $t$ are used to represent the alleles of the gene that determine plant height by conventions. The upper case letter $(T)$ represents the dominant allele and the recessive allele (t) is represented by the same letter in lower case.

Thus, the tall parents plants having heterozygous alleles, results in offsprings which comprises of both tall and dwarf plants.

For the parental cross, both the parents are true breeding plants, the tall plant is homozygous for the tall allele ’ $T$ ‘, while the dwarf plant is homozygous for the dwarf allele ’ $t$ ‘. Mendel tracked each trait through two generations.

When true breeding plants were crossed to each other, this is called a parental cross and offspring comprise the first filial or $ F_{1}$-generation. When the members of the $F_{1}$-generation were crossed, this produced the $F_{2}$-generation or second filial generation.

A cross between true breeding tall and dwarf plants of the parent generation yield phenotypically tall plants.

The cross between TT and Tt is called back cross, which results into two homozygous and two heterozygous dominant gametes. The cross between $\mathrm{Tt}$ and $\mathrm{tt}$ is called test cross which results into $1: 1$ ratio of gametes.

• (a) TT and Tt: This option is incorrect because a cross between a homozygous tall plant (TT) and a heterozygous tall plant (Tt) would not produce any dwarf plants. The offspring would either be homozygous tall (TT) or heterozygous tall (Tt), resulting in all tall plants.

• (c) TT and TT: This option is incorrect because a cross between two homozygous tall plants (TT) would produce only homozygous tall offspring (TT). There would be no dwarf plants in the offspring.

• (d) Tt and tt: This option is incorrect because a cross between a heterozygous tall plant (Tt) and a homozygous dwarf plant (tt) would produce a 1:1 ratio of tall (Tt) and dwarf (tt) plants. This does not match the scenario where only a few dwarf plants are observed among predominantly tall plants.

Thinking Process

A cross that involves the analysis of two independent traits is termed a dihybrid cross.

The law of independent assortment was deduced from Mendel’s experiment with dihybrid cross.

Answer

(d) Alleles of two genes are segregating independently. It can be explained as. Suppose crosses are made between a pea plant with round and yellow seeds and one with wrinkled and green ones.

All $F_{1}$ hybrids give yellow and round seeds. Since yellow colour is dominant over the green and the round shape is dominant over the wrinkled.

When the $F_{1}$-hybrid plants are crossed to each other or allowed to self fertilise, an $\mathrm{F}_{2}$-generation form as represented in the following figure

Phenotypic ratio - 9:3:3:1

The outcome of the dihybrid cross make it very clear that segregation of the seed colour is independent of the seed shape and both the parental and new combinations of the characters appear in the $F_{2}$ offspring, i.e., assortment of genes of one pair is independent of the other pair.

When the alleles of two genes are interacting with each other one may dominate over other or become recessive. During a multigenic inheritance we describe a characteristics that is specified by a combination of multiple genes. Multiple allelism is a type of non-mendelian inheritance pattern that involves more than just the typical two alleles.

• (a) the alleles of two genes are interacting with each other: This option is incorrect because the 9:3:3:1 ratio indicates independent assortment, not interaction between alleles. Interaction between alleles would result in different phenotypic ratios due to epistasis or other genetic interactions.

• (b) it is a multigenic inheritance: This option is incorrect because multigenic inheritance involves multiple genes contributing to a single trait, often resulting in a continuous range of phenotypes rather than discrete categories. The 9:3:3:1 ratio is specific to the independent assortment of two genes, each with two alleles.

• (c) it is a case of multiple allelism: This option is incorrect because multiple allelism refers to the presence of more than two alleles for a single gene within a population. The 9:3:3:1 ratio pertains to the interaction of two different genes, each with two alleles, not multiple alleles of a single gene.

Thinking Process

Linkage refers to the physical association of genes on chromosome.

Answer

(c) Linkage will not result in variations among siblings. Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.

Morgan came to know that the genes were located on the X-chromosome and also observed that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.

It indicates that due to the physical association of the two genes. There will be no variations among siblings. Independent assortment of genes means that allele pair separate during the formation of gametes independently. It means that traits are transmitted to offspring independently of one another.

Crossing over is the exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination. Mutation is the sudden inheritable change in genetic material of an organism which transfers to next generation.

• Independent assortment of genes: Independent assortment of genes means that allele pairs separate during the formation of gametes independently. This results in different combinations of alleles being passed on to offspring, leading to genetic variation among siblings.

• Crossing over: Crossing over is the exchange of genetic material between homologous chromosomes during meiosis. This process creates new combinations of genes that are different from either parent, contributing to genetic variation among siblings.

• Mutation: Mutation is a sudden inheritable change in the genetic material of an organism. These changes can be passed on to the next generation, introducing new genetic variations among siblings.

Answer

(a) Non-homologous Chromosome The law of independent assortment holds true as long as two different genes are on separate chromosomes. When the genes are on separate chromosom, the two alleles of one gene ( $A$ and a) will segregate into gametes independently of the two alleles of the other gene ( $B$ and $b$ ).

Equal numbers of four different gametes will form $A B, a B, A b, a b$. But if the two genes are on the same chromosome, then they will be linked and will segregate together during meiosis, producing only two kinds of gametes.

Genes A and B on different chromosomes Genes A and B on same chromosome

Homologous chromosomes are similar but not identical. Each carries the same gene in same order but the alleles for each trait may not be the same. Extranuclear genetic elements are also called as plasmids and shows the pattern of maternal inheritance.

• (b) Homologous chromosomes: Homologous chromosomes are pairs of chromosomes that have the same structure and carry the same genes in the same order, but they may have different alleles. Mendel’s law of independent assortment does not apply to genes located on homologous chromosomes because these genes are not independently assorted; they are linked and tend to be inherited together.

• (c) Extra nuclear genetic element: Extra nuclear genetic elements, such as plasmids, are not located on chromosomes within the nucleus. They are found in organelles like mitochondria and chloroplasts and follow a different pattern of inheritance, often maternal. Therefore, Mendel’s law of independent assortment, which applies to nuclear genes, does not apply to these extranuclear elements.

• (d) Same chromosome: Genes located on the same chromosome are said to be linked and do not assort independently. They tend to be inherited together unless crossing over occurs during meiosis. This linkage violates Mendel’s law of independent assortment, which states that genes on different chromosomes assort independently of one another.

Answer

(c) Occasionally, a single gene may express more than one trait. This phenomenon is called pleiotropy. Sometimes, one trait will be very evident and others will be less evident, e.g., a gene for white eye in Drosophila also affect the shape of organs in male responsible for sperm storage as well as other structures.

Similarly, sickle-cell anaemic individuals suffer from a number of problems, all of which are pleiotropic effects of the sickle-cell alleles.

Multiple allelism is a seies of three or more alternative or allelic forms of a gene, only two of which can exist in any normal diploid individual, e.g., genes of blood groups in humans.

Mosaicism describes the occurrence of cells that differ in thier genetic component from other cells of the body.

Polygeny refers to a single characteristic that is controlled by more than two genes. (it is also known as multifactorial inheritance).

• Multiple allelism is incorrect because it refers to a series of three or more alternative or allelic forms of a gene, only two of which can exist in any normal diploid individual, e.g., genes of blood groups in humans.

• Mosaicism is incorrect because it describes the occurrence of cells that differ in their genetic component from other cells of the body.

• Polygeny is incorrect because it refers to a single characteristic that is controlled by more than two genes. (It is also known as multifactorial inheritance).

Answer

(a) In certain insects, such as cockroach, and some roundworms, the Y-chromosome is missing so that the male has only one sex chromosome, i.e., ’ $X$ ‘. The condition in the male is $\mathrm{XO}$ ( $\mathrm{O}$ means absence of one sex chromosome) and in the female it is $\mathrm{XX}$., thus males showing 17 chromosomes while females show 18 chromosome. All the other option given are wrong.

• (b) females and males, respectively: This option is incorrect because it suggests that females have 17 chromosomes and males have 18 chromosomes. However, in the given taxon of insects, males have 17 chromosomes (XO) and females have 18 chromosomes (XX).

• (c) all males: This option is incorrect because it implies that all organisms with 17 and 18 chromosomes are males. In reality, only the males have 17 chromosomes (XO), while the females have 18 chromosomes (XX).

• (d) all females: This option is incorrect because it implies that all organisms with 17 and 18 chromosomes are females. In reality, only the females have 18 chromosomes (XX), while the males have 17 chromosomes (XO).

Thinking Process

A pedigree is a family tree that diagrams the relationships among parents and children across several generations which shows the inheritance pattern of a particular phenotyphic character.

Answer

(b) Mendelian inheritance in humans is difficult to study. Current understanding of mendelian inheritance in humans is gained by analysis of family pedigrees or the results of matings that have already occured. By analysing a pedigree, we may be able to predict how the trait is inherited.

It is a visual tool for documenting the biological relationship in families and to determine the mode of inheritance (dominant, recessive etc.) of genetic diseases.

Whereas quantitative trait, polygenic trait and maternal traits are not studied by pedigree analysis.

Continuous traits are often measured and given a quantitative value, they are often referred as quantitative traits, e.g., crop yield, weight, gain in animals, $\mid Q$, etc.

Polygenic traits are another exception to mendels rule, which occurs when a trait is controlled by more than one gene. This means that each dominant allele adds to the expression of the next dominant allele.

Maternal traits are the traits inherited and expressed from the maternal parent to the subsequent offsprings.

• Quantitative trait: Quantitative traits are often measured and given a quantitative value, such as crop yield or weight gain in animals. These traits are typically influenced by multiple genes and environmental factors, making them unsuitable for pedigree analysis, which focuses on the inheritance patterns of single-gene traits.

• Polygenic trait: Polygenic traits are controlled by more than one gene, with each dominant allele contributing to the expression of the trait. Because these traits involve multiple genes, they do not follow simple Mendelian inheritance patterns and are not typically studied using pedigree analysis.

• Maternal trait: Maternal traits are inherited and expressed from the maternal parent to the offspring. These traits are often influenced by the maternal environment and mitochondrial DNA, which is inherited exclusively from the mother. Pedigree analysis is not designed to study traits that are solely inherited from one parent, such as maternal traits.

Thinking Process

Law of segregation states that the factors or alleles of a pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show any blending.

Answer

(b)

Results of a dihybrid cross where the two parents differed in two pairs of contrasting traits seed colour and seed shape

Rest of the options does not support the Mendel’s low of segregation.

• Option (a): The results of the $F_{3}$-generation of a cross do not directly demonstrate the discrete and independent nature of the factors controlling characters. Mendel’s conclusions were primarily based on the $F_{1}$ and $F_{2}$ generations.

• Option (c): Self-pollination of $\mathrm{F}{1}$ offsprings leads to the $F{2}$ generation, which shows the segregation of traits. However, it does not directly address the observation that offspring of a cross between plants with contrasting characters show only one character without blending.

• Option (d): Cross-pollination of the $\mathrm{F}{1}$-generation with the recessive parent (test cross) is used to determine the genotype of the $\mathrm{F}{1}$ generation, but it does not directly support the proposition that the factors controlling characters are discrete and independent.

Thinking Process

When a progeny of $F_{1}$ is crossed with the homozygous recessive parent, it is called test cross.

Answer

(a) 1:1 It can be explained by the following test cross.

Genotypic ratio $-1: 1: 1$

The other options are incorrect.

• Option (b) $9: 3: 3: 1$: This ratio is typical of a dihybrid cross involving two independently assorting genes, not linked genes. Since genes $A$ and $B$ are linked, they do not assort independently, and this ratio does not apply.

• Option (c) $3: 1$: This ratio is typical of a monohybrid cross involving a single gene with complete dominance, not a dihybrid cross involving two linked genes. Therefore, it does not apply to the given scenario.

• Option (d) $1: 1$: This ratio is typical of a test cross involving a single gene, not a dihybrid cross involving two linked genes. The correct ratio for the given scenario is $1: 1: 1: 1$, not $1: 1$.

Answer

(d) Mendel’s dihybrid cross

Other combinations do not show dihybrid cross ratio of mendelien inheritance.

• Option (a) phenotypes-4, genotypes-16: This option is incorrect because, in a Mendelian dihybrid cross, the number of genotypes is 9, not 16. The 16 genotypes would imply a different type of genetic cross or a misunderstanding of the dihybrid cross results.

• Option (b) phenotypes-9, genotypes-4: This option is incorrect because, in a Mendelian dihybrid cross, the number of phenotypes is 4, not 9. The 9 phenotypes would suggest a more complex genetic interaction than a simple dihybrid cross.

• Option (c) phenotypes-4, genotypes-8: This option is incorrect because, in a Mendelian dihybrid cross, the number of genotypes is 9, not 8. The 8 genotypes would indicate an incomplete account of the possible genetic combinations in a dihybrid cross.

Thinking Process

The child with blood group $\mathrm{O}$ will have homozygous recessive alleles. Therefore, both the parents should be heterozygous, i.e., the genotype of father will be $I^{A} _{i}$ and of mother will be ${I}^{B} _i$.

Answer

(c) When a cross is carried out between heterozygous father (for blood group B) and heterozygous mother (of blood group A) to get four children with different blood groups.

All the four blood groups are controlled by three allelic genes $\mathrm{I}^{\mathrm{A}}, \mathrm{B}, \mathrm{i}$ and thus it shows phenomenon of multiple allelism. Both $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ is dominant over i. However, when together, both are dominant and show the phenomenon of codominance forming the blood group AB. Six genotypes are possible with combination of these three alleles. Thus, other option are wrong or incorrect.

• Option (a) is incorrect: If the mother were homozygous for ‘A’ blood group (genotype $\mathrm{I}^{\mathrm{A}}\mathrm{I}^{\mathrm{A}}$) and the father were heterozygous for ‘B’ blood group (genotype $\mathrm{I}^{\mathrm{B}}\mathrm{i}$), their child could not have an ‘O’ blood group (genotype $\mathrm{i}\mathrm{i}$) because the mother would always pass on an $\mathrm{I}^{\mathrm{A}}$ allele.

• Option (b) is incorrect: If the mother were heterozygous for ‘A’ blood group (genotype $\mathrm{I}^{\mathrm{A}}\mathrm{i}$) and the father were homozygous for ‘B’ blood group (genotype $\mathrm{I}^{\mathrm{B}}\mathrm{I}^{\mathrm{B}}$), their child could not have an ‘O’ blood group (genotype $\mathrm{i}\mathrm{i}$) because the father would always pass on an $\mathrm{I}^{\mathrm{B}}$ allele.

• Option (d) is incorrect: If both the mother and father were homozygous for ‘A’ and ‘B’ blood groups respectively (genotypes $\mathrm{I}^{\mathrm{A}}\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}\mathrm{I}^{\mathrm{B}}$), their child could not have an ‘O’ blood group (genotype $\mathrm{i}\mathrm{i}$) because the mother would always pass on an $\mathrm{I}^{\mathrm{A}}$ allele and the father would always pass on an $\mathrm{I}^{\mathrm{B}}$ allele.

Answer

When a progeny of $F_{1}$ is crossed with the homozygous recessive parent, it is called test cross.

Test cross between pure dominant (A) and hybrid dominant (B) individuals with recessive parent is shown below

Such a cross is useful to determine the genotype of an unknown trait, i.e., whether it is heterozygous or homozygous dominant for the trait.

Answer

If the characters are present on the same chromosome they would not assort independently as they are linked on the same chromosome. Percentage of linkage depends on the distance between the genes. With linkage no conclusive laws can be drown.

Thinking Process

Controlled cross-pollination is one of the major approaches of crop improvement programme. In such experiments it is important to make sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination (from unwanted pollen).

Answer

Steps of controlled cross-pollination are

(i) Selection of parents with desired characters.

(ii) Emasculation, i.e., if the female parent bears bisexual flowers, before dehiscence anther should be removed by forceps.

(iii) Bagging, i.e., emasculated flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen.

(iv) When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma

(v) The flowers are rebagged and the fruits are allowed to develop.

Emasculation is not always needed in a cucurbit plant. Emasculation is essential only in case of bisexual flowers to prevent self-pollination. In case of cucurbit plant, female parent produces usually unisexual flowers but may sometimes have bisexual flowers.

Note If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower rebagged.

Answer

The criteria for selecting the organism to study inheritance are

(i) Easily visible and different traits

(ii) Short life span

(iii) Simple pollination procedure

(iv) Organisms must be true breeds

(v) Mating of gametes has to be random

(vi) Can be easily manipulated

Answer

The pedigree chart shows that the trait is autosome linked and recessive in nature. But, the parents are carriers (i.e., heterozygous) hence, among the offsprings only few show the trait irrespective of sex. The other offsprings are either normal or carrier.

Answer

Characters segregate during gamete formation. Pure-breeding parents give rise to $F_{1}$ with heterozygous conditions. Only self-pollination of heterozygotes can result in all possible recombinations of characters in progeny as mating is random.

Answer

Genes contain the information required to express a particular trait can be explained by the following experiment.

G Beadle and E Tatum set an experiment to prove that one gene possess a particular trait and is responsible for the production of one enzyme or protein. They performed their experiment on Neurospora crassa which were nutritionally mutant.

It was proved that a single protein contains several polypeptide and each polypeptide is controlled by separate gene. Thus, each gene expresses a particular trait. This theory was called one-gane-one enzyme or one gene-one polypeptide hypothesis.

But after the discovery of cistron (the functional unit of gene), the theory was named as one-cistron-one polypeptide hypothesis.

Answer

Alleles are polymorphs that differ in their nucleotide sequence resulting in contrasting phenotype expression. Alleles are the alternative forms of a same gene for, e.g., genes for height have two allele, one for dwarfness (t) and one for tallness (T).

Significance

(i) A character may have two or more contrasting phenotypic expression, thus resulting variation in the population.

(ii) These are used in the studies of inheritance and in understanding their behaviour.

Answer

Phenotypic ratio Red flower : White flower $3: 1$

Genotype ratio $\quad \mathrm{RR} \quad: \mathrm{Rr}: \mathrm{rr} \quad 1: 2: 1$

Generally, upper case letters are used as symbols for dominant and lower case for recessive traits of the same gene (alleles). Experiment shows that it is a monohybrid cross with $3: 1$ ratio in $\mathrm{F}_{2}$-generation.

This shows parents must be true-breeds. As parents are diploid and homologous chromosomes carry alleles with similar type they are represented with RR and $\mathrm{rr}$.

Thinking Process

Phenotype $=$ Genotype + Environment

(Trait) (potentiality) (opportunity)

Answer

Obviously, genes are not the only factors that determine phenotype. Environment also plays an important role in the expression of traits. Genes are actually quite active throughout our lives, switching their expression on and off in response to the environment.

Besides the effect of internal factors like hormones and metabolism on gene expression, external factors like temperature, light, nutrition, etc., also affect the gene expression and ultimately exhibiting phenotypic changes.

So, we can say that genes provide only the potentiality and the environment provides the opportunity for the expression of traits.

Answer

The given cross $A a b b D D X$ aa bb dd, is a trihybrid cross, Accordingly the type of offspring produced would be,

Thinking Process

The sex determining chromosome in case of humans is of $X Y$ type.

Answer

It is unfortunate that in our society women are blamed for giving birth to female children and have been ostracised and ill-treated because of this false notion. Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females, these are the autosomes.

A pair of $X$-chromosomes are present in the female, whereas the presence of an $X$ and Y-chromosome are determinant of the male characteristic. During spermatogenesis among males, two types of gametes are produced.

50 per cent of the total sperm produced carry the X-chromosome and the rest $50 \%$ has Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an $\mathrm{X}$-chromosome. There is an equal probability of fertilisation of the ovum with the sperm carrying either $\mathrm{X}$ or $\mathrm{Y}$-chromosome.

In case when the ovum fertilises with a sperm carrying $X$-chromosome the zygote develops into a female $(X X)$ and the fertilisation of ovum with $Y$-chromosome carrying sperm results into a male offspring. Thus, it is evident that it is the genetic makeup of the sperm (male) that determines the sex of the child.

It is also evident that in each pregnancy there is always $50 \%$ probability of having either a male or a female child.

Answer

Seed shape is determined by a single gene, with the allele $(R)$ for round peas dominant over the allele ( $r$ ) for wrinkled peas (recessive trait).

It the alleles for the gene controlling the seed shape are homozygous in a plant, it will show the character or phenotype of same alleles, i.e.,-RR- round seed, rr-wrinkled seed.

On the other hand, if the alleles of gene are heterozygous. They will express the phenotype of dominant allele.

$\mathrm{Rr}$ - Round seed ( $r$ - wrinkled is recessive)

This is the genetic basis of wrinkled phenotype of pea seed.

Answer

Multiple alleles are the multiple forms of a gene which occur on the same gene locus, but distributed in different organisms in the gene pool with an organism, which carry only two alleles and the gamete have only one allele.

Despite multiple allelism, an individual will have only two alleles because an individual develops from a zygote which is the result of fusion of sperm (carrying father set of (n)haploid chromosomes) and an egg (carrying mother set of haploid chromosomes)

Sperm and an egg have only one gene (allele) for each trait. A zygote when becomes diploid, have two alleles for each trait. It is the maximum number of alleles an individual can have. e.g., genes of blood groups.

Answer

Mutagens may be physical, i.e., ionising radiations X-ray,UV rays, gamma rays, DNA reactive chemicals, i.e., hydroxyl radicals, $H_{2} O_{2}$, etc., or biological such as virus.

A mutagen can induce mutation by inducing, a change in the base sequence by insertion deletion or substitution.

e.g., a single base sequence substitution at the sixth codon of the $\beta$-globin gene changes the codon from GAG to GUG. This results in the substitution of glutamic acid (Glu) by valine (Val) at the sixth position of the $\beta$-globin chain of the haemoglobin molecule.

The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to the elongated sickle, i.e., like structure which is not functional.

Thinking Process

In a monohybrid cross starting with parents which are homozygous dominant and homozygous recessive, $F_{1}$ would be heterozygous for the trait and would express the dominant allele. But in case of incomplete dominance the result will be different.

Answer

In case of incomplete dominance, a monohybrid cross shows the result as follows

Here, the phenotypic and genotypic both ratios are the same. So, we can conclude that when genotypic and phenotypic ratios are the same, alleles show incomplete dominance. i.e., none of the two alleles shows dominance thus producing hybrid intermediate from the expression of two homozygous alleles.

Thinking Process

The child with blood group $O$ will have homozygous recessive alleles. Therefore, both the parents should be heterozygous, i.e., genotype of father will be $I^{A} i$, or $I^{B} i$ and of mother will be $I^{A}$ i or $I^{B}$ i.

Answer

A child have blood group $\mathrm{O}$ in the following two cases

Case I When father is $\mathrm{I}^{\mathrm{A}} \mathrm{i}$ and mother is $\mathrm{I}^{\mathrm{B}} \mathrm{i}$.

The offsprings will have the above possible blood groups. i.e., $A B, A, B$ and $O$

Case II When father is $\mathrm{I}^{\mathrm{B}} \mathrm{i}$ and mother is $\mathrm{I}^{\mathrm{A}} \mathrm{i}$.

The offsprings will have the above possible blood groups, i.e., $A B, A, B$ and $O$. Thus, a child can have blood group ’ $O$ ’ if parents have heterozygous alleles for group ’ $A$ ’ and ’ $B$ ‘.

Answer

Down’s syndrome is a human genetic disorder caused due to trisomy of chromosome number 21. Such individuals are aneuploid and have 41 chromosomes, i.e., $(2 n+1)$

Symptoms of down’s syndrome are

(i) Mental retardation

(ii) Growth abnormalities

(iii) Constantly open mouth

(iv) Dwarfness, etc., gonads and genitalia under developed

The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome (a pair 21 during meiotic division. The chances of having a child with Down’s syndrome increases with the age of the mother $(+40)$ because age adversely affects meiotic chromosome behaviour.

Meiosis in the egg cells is not completed, until after fertilisation. During this long gap (till meiosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater the time they remain upaired greater the chance for unpairing and chromosome non-disjunction.

Answer

Chromosome theory of inheritance was proposed by Sutton and Boveri independently in 1902. The theory believes that chromosomes are vehicles of heriditary information, possess Mendelian factors or genes and it is the chromosomes which segregate and assort independently during transmission from one generation to the next.

Thinking Process

If any of the alleles does not fully dominate in a heterozygous condition, it is called incomplete dominance.

Answer

Incomplete dominance is the phenomenon where neither of the two alleles shows dominance thus producing intermediate hybrid between the expression of two alleles in homozygous state. In this case, a new phenotype in between the two original phenotype appears.

Answer

True breeding is a stable trait inheritance and expression for several generations as a result of continuous self-pollination.

Characteristic features of a true-breeding line

(i) They are used as parents in artificial hybridisation as they provide gametes with all similar traits.

(ii) Homozygous recessive plants are used in test cross to determine the genotype.

Answer

The result shows that the four types of offspring are in a ratio of $1: 1: 1: 1$. Such a result is observed in a test cross progeny of a dihybrid cross.

The cross can be represented as

Parents Tall and red $(\mathrm{TtRr}) \times$ Dwarf and white $(\mathrm{ttrr})$

Offsprings

Answer

Colourblindness is a X-inked sex inheritance. For becoming colourblind, the female must have the allele for it in both her $\mathrm{X}$-chromosomes and if only one $\mathrm{X}$-chromosome of female possess allele for colour blind character she becomes the carrier for this characteristics But males develop colourblindness when their sole $\mathrm{X}$ - chromosome has the allele for it. Thus males are more prone to colour blindness while females are carriers.

Answer

Gene for colourblindness is X-chromosome linked, and sons receive their sole $X$-chromosome from their mother, not from their father. Male to male inheritances is not possible for X-linked traits in humans.

In the given case the mother of the son must be a carrier (heterozygous) for colour blindness gene, thus transmitting the gene to her son.

Answer

Morgan worked with the tiny fruit flies, Drosophila melanogaster, which were found to be suitable for genetical studies due to the following characteristics

(i) They could be grown on simple synthetic medium in the laboratory.

(ii) They complete their life-cycle in about two weeks.

(iii) A single mating could produce a large number of progeny flies.

(iv) A clear differentiation of the sexes- the male and female flies are easily distinguishable.

(v) It has many types of variations (hereditary) that can be seen with low power microscopes.

Answer

By 1902, the chromosome movement during meiosis had been worked out.

Walter Sutton and Theodore Boveri, (1902) noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel’s Laws.

They studied the behaviour of chromosomes during mitosis (equational division) and during meiosis (reduction division). The chromosomes as well as genes occur in pairs and the two alleles of a gene pair are located of homologous sites of homologous chromosomes.

Chromosome movement in meiosis and germ cell formation in a cell with four chromosomes. Chromosomes segregate when germ cells are formed

Answer

Recombination refers to the generation of new combination of genes which is different from the parental types. It is produced due to crossing over that occurs during meiosis prior to gamete formation.

Applications of Recombination

(i) It is a means of introducing new combinations of genes and hence new traits.

(ii) It increases variability which is useful for natural selection and under changed environment.

(iii) Since, the frequency of crossing over depends upon the distance between the two genes, the phenomenon is used for preparing linkage chromosome maps.

(iv) It has proved that genes lie in a linear fashion in the chromosome.

(v) Breeders have to select small or large population for obtaining the required cross-overs. For obtaining cross-overs between closely linked genes, a very large population is required.

(vi) Useful recombinations produced by crossing over are picked up by breeders to produce useful new varieties of crop plants and animals. Green revolution has been achieved in India due to this selective picking up of useful recombinations. Operation flood or white revolution is also being carried out on the similar lines.

Answer

Artificial selection (or selective breeding) describes intentional breeding for certain traits or combination of traits by humans, for exploiting the variations existing among species. It is of three types-mass selection, pure-line selection and clonal selection.

Yes, it affects the process of natural selection. Natural selection selects for/or against traits based on their effect on the fitness of the organism. In artificial selection, traits are selected based on human preference for improving traits.

The process of natural selection leads to evolutionary change in the expression of the trait in the population, whereas the artificial selection, though being the same process, involves the traits preferred by humans for its own benefit. It is a much faster process than the natural selection but it may impose threat on diversity in long run making it unfit to the environment.

Answer

Incomplete dominance is a phenomenon where two contrasting alleles are present together but neither of the alleles is dominant over other and the phenotype formed is intermediate of the two alleles.

e.g., the kind of inheritance in the dog flower (Snapdragon or Antirrhinum species) in which the intermediate trait is expressed in $\mathrm{F}_{1}$-generation.

Codominance is a phenomenon in which when two contrasting alleles are present together and both of the alleles express themselves.

e.g., $A B$ blood group in humans where both the alleles are expressed to produce RBC surface antigens $A$ and $B$.

(i) Coss showing incomplete dominance

(ii) Blood group showing co-dominance

Answer

Sickle-cell anaemia is an autosomal recessive disease caused by haemoglobins an oxygen carrying protein in blood cells.

Despite the disease’s lethal symptoms, it protects the carrier from malaria. Its allele are most common in the people of African descent (about 7 % people of African descent carry an allele) and some other are as where malaria in prevalent.

It provides the vital protection from malaria. Individuals with HbAS heterozygotes tend to survive better than individuals with HbSS (homozygotes) as they are not exposed to the same severity of risk. e total number of genes is estimated

Answer

The standard dihybrid ratio is 9:3:3:1. Yes, the values will show deviation if the two genes in the above case are interacting with each other. When the genes are linked, they do not assort independently but remain together in the gametes and the offsprings, give a dihybrid ratio of $3: 1$ and show a test cross ratio of 1:1 instead of 1:1:1:1.

Answer

(a) The term homogametic and heterogametic refers to the organism depending upon whether all the gametes contain one type of sex chromosome (homo same) or two different types of sex chromosomes (hetero different).

Humans show $X X / X Y$ type of sex determination, i.e., females contain 2 copies of $\mathrm{X}$-chromosome and males contain $1 \mathrm{X}$ and $1 \mathrm{Y}$-chromosome. Therefore, ova produced by females contain the same sex chromosome, i.e., $X$.

On the other hand the sperms contain 2 different types of chromosomes, i.e., $50 \%$ sperms have $X$ and $50 \%$ have $Y$-chromosomes (meiosis). Therefore, the sperms are different with respect to the composition of sex chromosome.

In case of humans, females are considered to be homogametic while males are heterogametic. Yes, there are examples where males are homogametic and females are heterogametic. In some birds the mode of sex determination is denoted by ZZ (males) and ZW (females). Certain moths and butterflies also show homogametic males and heterogametic females.

(b) As a rule the heterogametic organism determines the sex of the unborn child. In case of humans, since males are heterogametic it is the father and not the mother who decides the sex of the child. In some animals like crocodiles, lower temperature favour hatching of female offsprings and higher temperatures lead to hatching of male offsprings.

Answer

The genotype of parents are

$50 \%$ daughters are normal visioned but $50 \%$ will be carries and $50 \%$ of sons are likely to be colour blind and $50 \%$ are normal visioned.

Answer

T H Morgan (1866-1945) was given the Nobel Prize in 1933.

His contributions are

(i) Morgan worked on fruit fly Drosophila melanogaster and proposed the chromosomal theory of linkage. N

(ii) He stated and established that genes are located on the chromosome.

(iii) He established the principle of linkage, crossing over, sex-linked inheritance and discovered the relation between gene and chromosome.

(iv) He established the technique of chromosome mapping.

(v) He observed and worked on mutation.

Alfred Henry Sturtevant (1891-1970) student of morgan was given the National Medal of Science in 1967. His contributions are

(i) He constructed the first genetic map of a chromosome while working on the Drosophila genome.

(ii) His main contributions to science include his analysis of genetic ’linkage groups,’ which became classical method of chromosome mapping that is still used today. In 1913, he determined that genes were arranged on chromosomes in a linear fashion, like beads on a necklace. He also showed that the gene for any specific trait was in a fixed location (locus).

(iii) His work on Drosophila proved that two closely related species showed newly recurring mutations that were allelic and thus probably identical. His work also helped to determine genetic role in sexual selection and development and displayed the importance of chromosomal crossing over in mutations.

(iv) One of Sturtevant’s principal contributions was his introduction to the concept that the frequency of crossing over between two genes could help to determine their proximity on a linear genetic map. His experiments determined that the frequency of double crossing over can be used to deduce gene order.

Answer

Aneuploidy is a phenomenon which occurs due to non-disjunction, resulting in gain or loss of one or more chromosomes during meiosis.

Aneuploidy is different from polyploidy. Polyploidy is a phenomenon in which the organisms contain more than two monoploid value or basic sets of chromosomes. i.e., $-3 n$, 4netc. Example of such organisms are certain fish and salamanders and is commonly found in plants like grapes, banana.

Chromosomal Abnormalities

(a) Down’s syndrome is an autosomal disorder that is caused by the trisomy of chromosome 21.

The individual is short statured with round head, open mouth, protruding tongue, short neck, slanting eyes, and broad short hands. The individual also shows retarded mental and physical growth,under developed gonads and genitats, etc.

(b) Klinefelter’s syndrome is the chromosomal disorder that is caused by the presence of an additional copy of $\mathrm{X}$-chromosome resulting in the karyotype $45+\mathrm{XXY}$.

In this disorder sex of the individual is masculine but possess feminine characters also. The individual shows gynaecomastia, i.e., development of breasts. The individual will be often sterile having poor beard growth and feminine pitched voice.

(c) Turner’s syndrome is the chromosomal disorder that is caused by the absence of one of the X-chromosomes, resulting in the karyotype 45+XO.

In this disorder the individual (female) will be sterile with rudimentary ovaries. Other symptoms include shield-shaped thorax, webbed neck, poor development of breasts, short stature, small uterus and puffy fingers.