Genetics deals with the inheritance (passing of characters from parents to progeny) and variation (degree of difference between progeny and parents).

Mendel’s Laws of Inheritance: Gregor Mendel conducted hybridisation experiments on garden peas (Pisum sativum) for seven years (1856-1863). He selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits (e.g., Tall vs Dwarf, Violet vs White flowers).

Inheritance of One Gene (Monohybrid Cross): Mendel crossed tall and dwarf plants. The F1 generation was always tall (resembled one parent). Self-pollination of F1 yielded F2 generation where 1/4th were dwarf and 3/4th were tall (3:1 Phenotypic ratio). The Genotypic ratio was 1:2:1 (TT:Tt:tt). Mendel proposed that “factors” (now called Genes) are passed down unchanged. Genes coding for a pair of contrasting traits are Alleles.

Based on this, Mendel proposed two laws:

1. Law of Dominance: Characters are controlled by discrete units called factors. Factors occur in pairs. In a dissimilar pair of factors, one dominates (dominant) and the other is recessive.

2. Law of Segregation: Alleles do not show any blending and both characters are recovered as such in the F2 generation. During gamete formation, the alleles separate (segregate) so that each gamete receives only one of the two alleles.

Incomplete Dominance: Seen in Antirrhinum (Snapdragon) or Mirabilis jalapa (4 O’clock plant). Crossing red (RR) and white (rr) flowers produces pink (Rr) F1 hybrids. The phenotypic ratio in F2 is 1:2:1 (Red:Pink:White), identical to the genotypic ratio.

Co-dominance: Both alleles express themselves fully. Example: ABO blood grouping in humans controlled by gene I. The gene has three alleles: $I^A$, $I^B$, and $i$. $I^A$ and $I^B$ produce slightly different sugars, while $i$ produces none. When $I^A$ and $I^B$ are present together, they both express their own types of sugars (Blood group AB).

Inheritance of Two Genes (Dihybrid Cross): Mendel crossed plants differing in two characters (e.g., Round Yellow seeds vs Wrinkled Green seeds). The F1 was Round and Yellow. The F2 generation showed a phenotypic ratio of 9:3:3:1 (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green).

This led to the Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

Chromosomal Theory of Inheritance: Proposed by Sutton and Boveri. They noted that the behavior of chromosomes was parallel to the behavior of genes and used chromosome movement to explain Mendel’s laws.

Linkage and Recombination: T.H. Morgan worked with Drosophila melanogaster (fruit flies) and found that genes located on the same chromosome are physically linked and do not assort independently (Linkage). The generation of non-parental gene combinations is Recombination. He observed that tightly linked genes show very low recombination, while loosely linked genes show higher recombination. Alfred Sturtevant used recombination frequencies to map gene positions on chromosomes.

Sex Determination:

• XO type: Male has one X chromosome (XO), female has two (XX). Example: Grasshopper.

• XY type: Male has X and Y (heterogametic), female has XX (homogametic). Example: Humans, Drosophila.

• ZW type: Female has Z and W (heterogametic), male has ZZ (homogametic). Example: Birds.

  Sex Determination in Humans: Genetic makeup of the sperm determines the sex of the child (50% probability of boy or girl).

Mutation: Alternation of DNA sequences resulting in changes in genotype and phenotype. Point mutation involves a change in a single base pair (e.g., Sickle cell anemia). Frameshift mutation involves deletion or insertion of base pairs.

Genetic Disorders:

1. Mendelian Disorders: Determined by alteration in a single gene.

   • Haemophilia: Sex-linked recessive. A single protein involved in blood clotting is affected. Carrier females transmit to sons.

   • Sickle-cell anaemia: Autosomal recessive. Substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain. RBCs become sickle-shaped under low oxygen tension.

   • Phenylketonuria: Autosomal recessive. Lack of enzyme to convert phenylalanine to tyrosine. Accumulation causes mental retardation.

2. Chromosomal Disorders: Caused by absence or excess or abnormal arrangement of chromosomes.

   • Down’s Syndrome: Trisomy of chromosome 21 (47 chromosomes). Short stature, furrowed tongue, mental retardation.

   • Klinefelter’s Syndrome: Additional copy of X chromosome in males (47, XXY). Overall masculine development but with feminine features (gynaecomastia). Sterile.

   • Turner’s Syndrome: Absence of one X chromosome in females (45, XO). Sterile, lack secondary sexual characters.