5.2 Genotype, Phenotype and Punnett Squares
Notes
A Punnett square predicts the possible offspring of a cross.
- Genotype = the alleles (TT, Tt, tt). Phenotype = the trait you see (tall or short).
- Homozygous = two of the same allele (TT or tt). Heterozygous = two different alleles (Tt).
- Monohybrid cross $\text{Tt} \times \text{Tt}$: genotypes 1 TT : 2 Tt : 1 tt; phenotypes $3:1$ dominant to recessive.
- $\text{Tt} \times \text{tt}$ gives $1:1$ (half dominant, half recessive).
- A test cross crosses an unknown dominant individual with a homozygous recessive one to find its genotype.
- A dihybrid cross $\text{AaBb} \times \text{AaBb}$ gives a $9:3:3:1$ phenotype ratio.
Practice
In guinea pigs, black fur (B) is dominant to white (b). Two heterozygous black guinea pigs mate. What fraction of offspring are expected to be white?
Bb x Bb gives 1 BB : 2 Bb : 1 bb, so 1/4 are bb.
A black guinea pig is crossed with a white one (bb). Half the babies are white. What is the black parent's genotype?
A BB parent could only pass B, so all babies would be black.
Which genotype is homozygous recessive?
Two copies of the recessive allele.
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