Summary

Mendel’s laws: Law of Dominance (one allele masks the other in a heterozygote), Law of Segregation (alleles separate during gamete formation), and Law of Independent Assortment (genes on different chromosomes assort independently). The monohybrid cross gives a 3:1 phenotypic ratio; the dihybrid cross gives 9:3:3:1.

Incomplete dominance: In snapdragon, F1 hybrids show an intermediate phenotype (pink flowers from red × white). The F2 ratio is 1:2:1 (red : pink : white), matching the genotypic ratio.

Co-dominance: Both alleles express equally in the heterozygote, e.g., human ABO blood group — IA and IB are co-dominant, while i is recessive. The possible genotypes and phenotypes: IA IA or IA i (type A), IB IB or IB i (type B), IA IB (type AB), ii (type O).

Multiple alleles: A gene can have more than two allelic forms in a population. ABO blood groups are controlled by three alleles (IA, IB, i). An individual carries only two, but the population pool has all three.

Test cross: Crossing an individual with a homozygous recessive to determine its genotype. A 1:1 phenotypic ratio in the progeny indicates the parent was heterozygous.

Chromosomal theory of inheritance: Sutton and Boveri proposed that genes are located on chromosomes, and the behaviour of chromosomes during meiosis (segregation and independent assortment) parallels Mendel’s laws.

Linkage and recombination: Genes on the same chromosome are linked and tend to be inherited together. Crossing over during prophase I produces recombinant gametes; genes that recombine less often are closer together, which is how Sturtevant used recombination frequency to work out the order of genes on a chromosome.

Sex determination: In humans, females are XX, males are XY. The male determines the sex of the offspring (50% chance of XX or XY). In birds, females are ZW, males are ZZ. In honeybees, females are diploid (from fertilised eggs), males (drones) are haploid (from unfertilised eggs) — haplodiploidy.

Sex-linked inheritance: A gene located on the X chromosome (colour blindness, haemophilia) is inherited differently from a gene on an autosome. Because males have only one X, a single recessive allele on it is enough to show the disorder; a carrier mother has a 50% chance of passing it to each son.

Mendelian disorders: Caused by single gene mutations, follow Mendelian patterns. Examples: Haemophilia (X-linked recessive), Sickle-cell anaemia (autosomal recessive — beta-globin gene mutation, GAG to GUG, valine replaces glutamic acid), Phenylketonuria (autosomal recessive — deficiency of phenylalanine hydroxylase), Thalassemia (autosomal recessive — reduced alpha or beta globin chain synthesis). …