Patterns of inheritance: study guide

Key ideas

Inheritance patterns go beyond simple dominance. ABO blood groups have three alleles, sex-linked traits depend on the sex of the child, and some alleles blend. Each is solved with a Punnett square and the multiplication rule.

• Complete dominance: Aa shows the dominant trait, and a cross of two carriers gives ¾ dominant, ¼ recessive, and ⅔ of the dominant offspring are carriers.
• ABO: Iᴬ and Iᴮ are codominant and both dominant over i. Iᴬi gives group A, Iᴮi group B, IᴬIᴮ group AB and ii group O.
• X-linked recessive: affected males are XʰY and females need XʰXʰ. A carrier mother and an unaffected father give half of the sons affected and none of the daughters affected, but half carriers.
• Incomplete dominance gives an intermediate heterozygote, so the cross of two heterozygotes gives 1 : 2 : 1 in both genotype and phenotype.

Key equations

• Aa × Aa → ¼ AA : ½ Aa : ¼ aa
• Iᴬi × Iᴮi → A, B, AB, O each ¼
• XᴴXʰ × XᴴY → ½ of sons affected
• P(A and B) = P(A) × P(B)

Worked example

A mother of blood group A (Iᴬi) and a father of group B (Iᴮi) can have children of any group: A, B, AB or O, each with probability ¼. A carrier mother and normal father (XᴴXʰ × XᴴY) have sons that are 50 % affected, daughters that are 0 % affected and 50 % carriers.

Common mistakes

• Forgetting that a heterozygote Iᴬi has blood group A, not a mixture.
• Giving daughters the same chance of being affected as sons in an X-linked recessive cross.
• Taking ¼ rather than ⅔ as the carrier fraction among unaffected children of two carriers.

Practice

Test yourself with practice questions on this topic: Patterns of inheritance