Principles of Inheritance and Variation
Genetics is the branch of biology dealing with inheritance (passing of characters from parent to progeny) and variation (the degree by which progeny differ from parents).
Mendel's work
Gregor Mendel conducted hybridisation experiments on garden pea (Pisum sativum) for seven years (1856-1863), selecting 14 true-breeding varieties (7 contrasting pairs, e.g. tall/dwarf, round/wrinkled, yellow/green seeds). He applied statistical analysis to biology for the first time.Mendel's Laws
- Law of Dominance (First Law): characters are controlled by discrete factors that occur in pairs; in a dissimilar pair one dominates the other. Explains the F1 showing one parental trait and the 3:1 ratio at F2.
- Law of Segregation (Second Law): alleles of a pair segregate during gamete formation; a gamete receives only one. No blending occurs.
- Law of Independent Assortment (from dihybrid crosses): when two pairs of traits combine in a hybrid, segregation of one pair is independent of the other. Gives the 9:3:3:1 F2 phenotypic ratio.
Monohybrid F2: phenotypic 3:1, genotypic 1:2:1. A test cross (dominant phenotype x homozygous recessive) reveals an unknown genotype; a heterozygote gives a 1:1 test-cross ratio.
Deviations from Mendelism
- Incomplete dominance: F1 intermediate (Antirrhinum RR red x rr white gives pink Rr; F2 = 1 Red : 2 Pink : 1 White).
- Co-dominance: both alleles expressed (ABO blood groups - IA and IB together give AB).
- Multiple alleles: gene I has three alleles (IA, IB, i) giving 6 genotypes and 4 phenotypes.
- Pleiotropy: one gene, many phenotypes (phenylketonuria).
- Polygenic inheritance: trait controlled by 3+ genes with additive, environment-influenced effects (human skin colour, height).
Chromosomal theory
Mendel's work (1865) was rediscovered in 1900 by de Vries, Correns and von Tschermak. Sutton and Boveri noted chromosome behaviour parallels gene behaviour - the Chromosomal Theory of Inheritance. T.H. Morgan verified it in Drosophila, discovering linkage (physical association of genes on a chromosome) and recombination (non-parental combinations). Sturtevant used recombination frequency to map genes.Sex determination
- XO type (grasshopper): male XO, female XX - male heterogamety.
- XY type (humans, Drosophila): male XY, female XX - male heterogamety. Sex of human child is determined by the sperm (X or Y); each pregnancy is 50:50.
- ZW type (birds): female ZW, male ZZ - female heterogamety.
- Haplodiploidy (honey bee): female diploid (32), male haploid (16) from unfertilised eggs.
Genetic disorders
- Mendelian (single gene): colour blindness and haemophilia (X-linked recessive); sickle-cell anaemia, phenylketonuria and thalassaemia (autosomal recessive). Studied via pedigree analysis.
- Chromosomal (aneuploidy): Down's syndrome (trisomy 21, 47), Klinefelter's (47, XXY), Turner's (45, X0).