<h5 id="61-introduction-to-inheritance">6.1 Introduction To Inheritance</h5> <ul> <li>The transmission of characters from one generation to the next, or the phenomenon of offsprings inheriting parental traits, is known as ‘Heredity’.</li> <li>The inherited characters are present on the chromosomes in the form of genes.</li> <li>Differences between offsprings and their parents are known as variations.</li> <li>The study of scientific facts of heredity and variation is referred to as Genetics.</li> <li>Understanding of genetics and heredity of traits is essential to use biotechnological tools for manipulating genes.</li> <li>The study of the principles of inheritance is important to identify genetic constituents (genes and their allelic forms in the population) regulating a trait, for its manipulation.</li> </ul> <p>======</p> <h5 id="611-mendels-work-the-foundation">6.1.1 Mendel’s work: The foundation</h5> <ul> <li>The stem height of the plant can be classified as tall or dwarf.</li> <li>The flower color can be either violet or white.</li> <li>The flower position can be axial or terminal.</li> <li>The pod shape can be inflated or constricted.</li> <li>The pod color can be green or yellow, and the seed shape can be round or wrinkled.</li> <li>The seed color can be yellow or green.</li> </ul> <p>======</p> <h5 id="incomplete-dominance">Incomplete dominance</h5> <ul> <li>Incomplete dominance is a phenomenon where the heterozygous condition of two alleles of a gene results in a reduced expression of the dominant allele, allowing both alleles to be partially expressed.</li> <li>This is different from the traditional dominant and recessive relationship, where the dominant allele completely masks the recessive allele.</li> <li>An example of incomplete dominance is seen in the four-o’clock plant (Mirabilis jalapa), where a cross between homozygous red-flowered plants (RR) and homozygous white-flowered plants (rr) produces $\mathrm{F} _{1}$ plants with pink flowers (Rr).</li> <li>When these $\mathrm{F} _{1}$ plants with pink flowers undergo selfing, they yield a 1:2:1 ratio of red, pink, and white flowers.</li> <li>Incomplete dominance is also known as intermediate inheritance, as the phenotype of the heterozygote is intermediate between the two homozygotes.</li> </ul> <p>======</p> <h5 id="codominance">Codominance</h5> <ul> <li>Codominance is a relationship between alleles in heterozygous condition where both alleles get equally expressed.</li> <li>It is different from the dominant-recessive relationship which expresses only the dominant trait.</li> <li>Codominance is observed in the inheritance of coat color in cattle, such as horses, cows, and dogs.</li> <li>An example of codominance is the Roan coat color (RW) in cattle, which is a mixture of white and pigmented coat colors.</li> <li>In $F_{1}$ generation, both red (RR) and white (WW) traits are equally expressed, resulting in the Roan coat color.</li> </ul> <p>======</p> <h5 id="law-of-independent-assortment">Law of independent assortment</h5> <ul> <li> <p>In a dihybrid cross between round-yellow and wrinkled-green seeded pea plants, all F1 progeny were round and yellow, indicating that round and yellow are dominant over wrinkled and green.</p> </li> <li> <p>The F2 generation upon selfing resulted in a ratio of 9:3:3:1 of offspring with 9 round-yellow, 3 wrinkled-yellow, 3 round-green, and 1 wrinkled-green seeds, which is observed in dihybrid crosses.</p> </li> <li> <p>This observation led to the proposal of the Law of Independent Assortment, which states that factors or genes controlling specific traits are inherited independent of each other.</p> </li> <li> <p>In F2 progeny, new combinations of traits, such as round shaped seed with green colour and wrinkled seed with yellow colour, are possible due to the independent assortment of alleles.</p> </li> <li> <p>The genotypic ratio of F2 progeny can be worked out using the Punnett square, which would be 1:2:2:4:2:1 for RRYY, RRYy, Rryy, rrYy, rryY, and rryy respectively.</p> </li> </ul> <p>======</p> <h5 id="62-linkage-and-crossing-over">6.2 Linkage and Crossing Over</h5> <ul> <li>Genes are located at specific gene loci of homologous chromosomes (autosomes or sex-chromosomes) and organisms may have numerous genes for its various phenotypic traits.</li> <li>The phenomenon of the inheritance of genes together and to retain their parental combination even in the offspring is known as linkage. Genes located on the same chromosomes and being inherited together are known as linked genes and the characters controlled by these genes are known as linked characters.</li> <li>Bateson and Punnett provided evidence in favour of linkage but Morgan and his colleagues later explained that genes for flower colour and pollen length are located on the same chromosome, that is they are linked.</li> <li>Morgan suggested that genes are present in a linear fashion in chromosome and all the genes present in the same chromosome are inherited together generation after generation retaining the parental combination.</li> <li>The phenomenon of interchange of chromosome segments during gametogenesis is known as crossing over. This results in the appearance of non-parental combinations and the linked genes do not always stay together but get separated many a times.</li> </ul> <p>======</p> <h5 id="63-sex-linked-inherttance">6.3 Sex-Linked Inherttance</h5> <ul> <li>The concept of sex-linked inheritance was introduced by Thomas H. Morgan in 1910.</li> <li>Morgan noted the sudden appearance of a white-eyed male in a culture of wild red-eyed Drosophila. This white-eyed mutation (w) was found to be recessive to red eye color (W).</li> <li>When F1 flies (both male and female) were crossed, the flies of F2 generation appeared in the ratio of 3:1 (red-eyed and white-eyed), with all the white-eyed flies being male.</li> <li>This led Morgan to conclude that the gene for eye color is located on the X chromosome, making it a sex-linked gene.</li> <li>Examples of sex-linked inheritance in human include color blindness and haemophilia.</li> </ul> <p>======</p> <h5 id="64-extrachromosomal-inheritance">6.4 Extrachromosomal Inheritance</h5> <ul> <li>DNA is present not only in the nucleus but also in mitochondria and plastids.</li> <li>During fertilization, the sperm cell loses most of its cytoplasm and cytoplasmic organelles, resulting in the zygote receiving plastids and mitochondria genome only from the maternal parent. This is known as extrachromosomal or cytoplasmic inheritance.</li> <li>Traits controlled by genes present in plastids or mitochondrial genomes do not follow Mendelian principles of inheritance and are mostly inherited through the maternal line, also known as maternal or uniparental inheritance.</li> <li>Examples of such traits include enzymes required for cellular respiration, coded by DNA present in mitochondria, and DNA present in plastids coding for chlorophyll or other pigments.</li> <li>The phenomenon of extrachromosomal inheritance was conclusively demonstrated in the Four O’clock plant (Mirabilis jalapa), where the cross of female green, white, or variegated plant with either of those males resulted in offspring with female phenotype only.</li> </ul> <p>======</p> <h5 id="65-polyplotidy">6.5 Polyplotidy</h5> <ul> <li>The text presents the chromosome numbers and gamete sizes of various plant species.</li> <li>Diploid Rice has 24 chromosomes and produces 12 gametes.</li> <li>Diploid Sorghum has 20 chromosomes and produces 10 gametes.</li> <li>Diploid or Triploid Banana has either 22 or 33 chromosomes and produces 11 gametes.</li> <li>Diploid or Triploid Apple has either 34 or 51 chromosomes and produces 17 gametes.</li> <li>Diploid or Triploid Peanuts have 40 chromosomes and produce 10 gametes.</li> <li>Tetraploid Cotton has 52 chromosomes and produces 13 gametes.</li> <li>Tetraploid Potato has 48 chromosomes and produces 12 gametes.</li> <li>Tetraploid Wheat has 42 chromosomes and produces 7 gametes.</li> <li>Hexaploid Strawberry has 56 chromosomes and produces 7 gametes.</li> <li>Octoploid Sugarcane has 80 chromosomes and produces 10 gametes.</li> </ul> <p>======</p> <h5 id="66-reverse-genetics">6.6 ReVerse Genetics</h5> <ul> <li>In genetic studies, the analysis of phenotypic variations in the population to determine genetic constituents (DNA sequences or genes) regulating these variations is called forward genetics.</li> <li>Reverse genetics, on the other hand, starts with the analysis of a DNA or protein sequence rather than phenotypic variations. It can be used to investigate the function of any gene/protein in an organism.</li> <li>The reverse genetics approach starts with a candidate gene or a protein sequence with an unknown function. The gene is disrupted using various experimental procedures, and the effect on the development of the organism is analyzed.</li> <li>Reverse genetics can be used to characterize genes with unknown functions, as not all genes in an organism result in visible variations.</li> <li>Techniques used for causing variation (disruption or altering) in target genes in reverse genetics include specific methods like gene silencing through RNA interference (RNAi) or targeted gene disruption by homologous recombination. RNAi is a regulatory biological process that uses small double stranded RNA molecules to inhibit gene expression or translation.</li> </ul> <p>======</p> <h5 id="sumimary">SUMIMARY</h5> <ul> <li>Heredity is the transmission of characters from one generation to the next, based on genetic information inherited from parents.</li> <li>Genetics is the study of scientific facts of heredity and variation.</li> <li>Mendel’s studies on inheritance of traits led to the concepts of alleles, monohybrid cross, phenotypic and genotypic ratios.</li> <li>Key principles in genetics include law of dominance, segregation, incomplete dominance, codominance, and independent assortment of alleles.</li> <li>Inheritance patterns can be sex-linked, extra-chromosomal, maternal, or polyploid/aneuploid.</li> <li>Forward and reverse genetics are approaches used to analyze phenotypic variations and genes.</li> <li>Characters are controlled by genes, which are part of the genome in a cell.</li> </ul> <p>======</p>