Genetics and inheritance: study guide

Key ideas

Genetics predicts how alleles pass from parents to offspring. Punnett squares and probability rules handle crosses, the Hardy–Weinberg equation handles populations, and recombination frequency measures linkage.

• A monohybrid cross of two heterozygotes (Aa × Aa) gives 3 dominant : 1 recessive, and a dihybrid cross (AaBb × AaBb) gives 9 : 3 : 3 : 1 when the genes assort independently.
• Independent events multiply: the chance of two outcomes together is the product of their chances. A heterozygote with n heterozygous loci makes 2ⁿ gamete types.
• Hardy–Weinberg: allele frequencies p + q = 1 and genotype frequencies p² + 2pq + q² = 1 in a non-evolving population. The recessive phenotype frequency is q².
• Recombination frequency = recombinant offspring ÷ total offspring × 100. One percent is one map unit (centimorgan).

Key equations

• p + q = 1
• p² + 2pq + q² = 1
• dihybrid 9 : 3 : 3 : 1
• recombination % = recombinants ÷ total × 100
• gamete types = 2ⁿ

Worked example

If 4 % of a population shows a recessive trait, then q² = 0.04, so q = 0.2 and p = 0.8. The carriers are 2pq = 2 × 0.8 × 0.2 = 0.32, which is 32 %. In a test cross, 18 recombinants among 200 offspring give 18 ÷ 200 = 9 %, so the two genes are 9 map units apart.

Common mistakes

• Using q² as the carrier frequency. Carriers are 2pq; q² is the recessive homozygotes.
• Applying the 9 : 3 : 3 : 1 ratio to linked genes, which do not assort independently.
• Adding probabilities that should be multiplied, such as two independent births.

Practice

Test yourself with practice questions on this topic: Genetics and inheritance