Genetics, Reproduction, and Evolution
This domain tests inheritance and variation, the difference between asexual and sexual reproduction, and the evidence and mechanism of evolution.
Core rules
- Genes are sections of DNA that carry hereditary information. Alleles are alternative forms of a gene. A dominant allele expresses its trait even when only one copy is present; a recessive allele expresses only when two copies are present.
- Genotype is the allele combination; phenotype is the observable trait. A monohybrid cross of two heterozygotes, Tt × Tt, gives genotype ratio 1 TT : 2 Tt : 1 tt and phenotype ratio 3 dominant : 1 recessive.
- Asexual reproduction uses one parent and produces genetically similar offspring; sexual reproduction uses two parents and gametes, producing variation.
- Evolution is change in heritable characteristics over generations. Natural selection favours traits that improve survival and reproduction in a particular environment.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Each parent can give T or t. The Punnett square gives TT, Tt, Tt, and tt. TT and Tt are tall because T is dominant; tt is short. The phenotype ratio is 3 tall : 1 short. The genotype ratio is 1 : 2 : 1. The short plant reappears because both parents carried the recessive allele.
Traps that cost marks
Do not say a dominant allele is more common; dominance is about expression, not frequency. Do not confuse genotype with phenotype. Acquired traits such as a weightlifter's muscles are not inherited. In evolution, individuals do not evolve; populations do.
Key points
- Genes carry hereditary information; alleles are alternative forms.
- Dominant alleles express in heterozygotes; recessive alleles need two copies.
- Tt × Tt gives a 3:1 phenotype ratio.
- Natural selection acts on heritable variation in populations.