Genetics and Heredity
Genetics and Heredity
This domain tests Mendelian inheritance, monohybrid and dihybrid crosses, codominance, sex linkage, pedigrees and variation. You should be able to set out gametes, use a Punnett square, and distinguish genotype from phenotype.
Core rules
- Each diploid organism carries two alleles per gene. A dominant allele is expressed in the heterozygote; a recessive phenotype requires two recessive alleles.
- A monohybrid cross Tt × Tt gives genotype ratio 1 TT : 2 Tt : 1 tt and phenotype ratio 3 dominant : 1 recessive when dominance is complete.
- A test cross uses a homozygous recessive partner to reveal an unknown dominant phenotype's genotype.
- In X-linked inheritance, males have one X and are affected more often by recessive X-linked alleles. Fathers pass X chromosomes to daughters and Y chromosomes to sons.
Worked example
In peas, tall T is dominant to dwarf t. Cross Tt × tt. Parent gametes are T or t, and t or t. Offspring are 1 Tt : 1 tt, so 50% tall and 50% dwarf. The reasoning is that the recessive parent contributes only t; any tall offspring must receive T from the heterozygous parent.
Traps
The 3:1 ratio is a phenotype ratio; the genotype ratio is 1:2:1. A carrier has one recessive allele but usually no recessive phenotype. Codominance is not blending: both alleles are expressed, as in AB blood group. Do not assume an affected son inherited the allele from his father in an X-linked recessive condition.