5.4 Non-Mendelian Genetics

Beyond simple dominance

Not all traits follow simple dominant/recessive rules. Incomplete dominance: the heterozygote shows a blend (red × white flowers → pink). Codominance: both alleles are fully expressed at once (AB blood type). Multiple alleles: a gene has more than two versions in the population (the ABO blood system).

Many genes, many effects

Polygenic traits are controlled by many genes, giving a continuous range (height, skin color). Pleiotropy is the opposite: one gene affects many traits. Some traits are also sex-linked or shaped by the environment.

Key terms

Incomplete dominance

Heterozygotes show a blended, intermediate phenotype (red × white → pink).

Codominance

Both alleles are fully and separately expressed (AB blood type).

Multiple alleles

A gene with more than two versions in the population (ABO blood).

Polygenic inheritance

A trait controlled by many genes, giving a continuous range.

Pleiotropy

One gene affecting many different traits.

Try an AP-style question

A color-blind man (XᶜY) and a woman with normal vision whose father was color-blind have children. What is the probability that a son is color-blind?

1. 1
2. 0
3. 1/2
4. 1/4