Principles of Inheritance and Variation

Key Scientists and Historical Milestones

James Dewey Watson and Francis Harry Compton Crick proposed the double-helical configuration for DNA in March $1953$ and were awarded the Nobel Prize in $1962$ .
Gregor Mendel conducted hybridization experiments on garden peas for seven years ( $1856-1863$ ) to establish fundamental rules of inheritance.
Hugo de Vries, Carl Correns, and Erich von Tschermak independently rediscovered Mendel's work in $1900$ .
Walter Sutton and Theodore Boveri proposed the Chromosomal Theory of Inheritance in $1902$ .
Thomas Hunt Morgan experimentally verified the Chromosomal Theory of Inheritance using Drosophila melanogaster, establishing concepts of linkage and recombination.
Alfred Sturtevant mapped gene positions on chromosomes using recombination frequencies.

Principles of Mendelian Inheritance

Law of Dominance: Characters are controlled by discrete units called factors occurring in pairs; in a dissimilar pair, one factor dominates the other.
Law of Segregation: Alleles segregate during gamete formation so that each gamete receives only one allele, exhibiting no blending in heterozygous conditions.
Monohybrid Cross: A cross involving a single character produces an $F_1$ generation displaying the dominant phenotype and an $F_2$ generation with a $3:1$ phenotypic ratio ( $3/4$ dominant : $1/4$ recessive) and a $1:2:1$ genotypic ratio ( $1/4\,TT : 1/2\,Tt : 1/4\,tt$ ).
Test Cross: Cross between an organism displaying a dominant phenotype and a recessive parent to determine the unknown genotype.
Law of Independent Assortment: Segregation of one pair of traits is independent of another pair during gamete formation, yielding an $F_2$ dihybrid phenotypic ratio of 9:3:3:1$.\n\n# Deviations from Mendelian Genetics\n\n* **Incomplete Dominance**: The F_1 $phenotype is intermediate between both parents, as seen in snapdragon (*Antirrhinum sp.*) flower color where crossing red ($ RR $) and white ($ rr $) yields pink ($ Rr $) offspring, giving an$ F_2 $phenotypic ratio of$ 1\,\text{Red} : 2\,\text{Pink} : 1\,\text{White}.\n* **Co-dominance**: Both alleles express simultaneously in heterozygous condition, such as human ABO blood groups governed by gene I $with alleles$ I^A $,$ I^B $, and$ i.\n* **Multiple Alleles**: Presence of more than two alleles governing a single trait within a population, as observed in ABO blood grouping.\n* **Polygenic Inheritance**: Traits controlled by three or more genes that exhibit continuous variation with additive effects, such as human skin color controlled by genes A $,$ B $, and$ C$.
Pleiotropy: A single gene mutation influences multiple phenotypic traits, such as Phenylketonuria causing mental retardation and reduced pigmentation.

Chromosomal Theory, Linkage, and Recombination

Homologous chromosomes segregate during meiosis (anaphase I) in a manner parallel to Mendelian factors.
Linkage describes the physical association of genes located on the same chromosome, whereas recombination generates non-parental gene combinations.
Tightly linked genes exhibit low recombination frequency (e.g., yellow body and white eye in Drosophila show $1.3\%$ recombination), while loosely linked genes show higher recombination frequency (e.g., white eye and miniature wing show $37.2\%$ recombination).

Sex Determination Mechanisms

$XO$ Type: Females carry $XX$ and males carry $XO$ (e.g., grasshoppers), demonstrating male heterogamety.
$XY$ Type: Females carry $XX$ and males carry $XY$ (e.g., humans and Drosophila), demonstrating male heterogamety.
$ZW$ Type: Females carry $ZW$ and males carry $ZZ$ (e.g., birds), demonstrating female heterogamety.
Haplodiploid System: In honeybees, females are diploid ( $32$ chromosomes) developing from fertilised eggs, while males (drones) are haploid ( $16$ chromosomes) developing from unfertilised eggs via parthenogenesis; males produce sperm by mitosis.

Mendelian Genetic Disorders

Colour Blindness: Sex-linked recessive defect in red/green cone cells of the eye due to mutation on the $X$ chromosome, affecting $8\%$ of males and $0.4\%$ of females.
Haemophilia: Sex-linked recessive disorder impairing blood clotting protein cascades, causing non-stop bleeding from simple cuts.
Sickle-Cell Anaemia: Autosomal recessive trait caused by a single base substitution from $GAG$ to $GUG$ at the sixth codon of the $\beta$ -globin gene, converting glutamic acid to valine ( $\text{Glu} \rightarrow \text{Val}$ ) and distorting red blood cells into a sickle shape under low oxygen.
Phenylketonuria: Autosomal recessive inborn error of metabolism caused by lack of phenylalanine hydroxylase, accumulating phenylalanine into phenylpyruvic acid and resulting in mental retardation.
Thalassemia: Autosomal recessive quantitative blood disorder reducing globin chain synthesis; $\alpha$ -Thalassemia is controlled by $HBA1$ and $HBA2$ on chromosome $16$ , while $\beta$ -Thalassemia is controlled by $HBB$ on chromosome 11$.\n\n# Chromosomal Genetic Disorders\n\n* **Aneuploidy**: Gain or loss of individual chromosomes due to failure of chromatid segregation during cell division.\n* **Polyploidy**: Gain of entire sets of chromosomes due to failure of cytokinesis after telophase, common in plants.\n* **Down's Syndrome**: Trisomy of chromosome 21 $($ 47 $chromosomes), described by Langdon Down ($ 1866), characterized by short stature, small round head, furrowed tongue, broad palm crease, and mental retardation.\n* **Klinefelter's Syndrome**: Presence of an additional X $chromosome in males ($ 47, XXY), leading to overall masculine development with feminine features (Gynaecomastia) and sterility.\n* **Turner's Syndrome**: Absence of one X $chromosome in females ($ 45, X0$$), resulting in sterility, rudimentary ovaries, and lack of secondary sexual characteristics.