AP® Biology · Unit 5 · Topic 5.3
Mendel's Law of Segregation
Each parent carries two alleles for a gene but passes only one to each gamete, because meiosis splits homologous chromosomes apart. This single rule explains why traits can skip a generation and reappear.
Every diploid organism carries two alleles for each gene, one from each parent, sitting on homologous chromosomes. When a cell undergoes meiosis, those homologous chromosomes separate during anaphase I, so each gamete ends up with only one allele from the pair. A heterozygous parent, genotype Tt, therefore makes two kinds of gametes in equal numbers: half carry T, half carry t. Segregation is this physical fact, that the alleles separate cleanly rather than blending, and it is why a recessive trait can vanish for a generation and then reappear once two carriers happen to mate.
A Punnett square just tracks the two possible gametes from each parent. Cross two Tt pea plants: each parent contributes T or t with equal probability, giving offspring genotypes TT, Tt, Tt, and tt in a 1:2:1 ratio. Since T is dominant, TT and Tt plants all look tall, so the phenotype ratio is 3 tall to 1 short. Change the cross to Tt x tt, a test cross, and the short parent contributes only t. Offspring are Tt or tt in a 1:1 ratio, so about half the offspring are tall and half are short.
Segregation is the rule that makes Mendelian ratios predictable, and those ratios are exactly what chi-square tests later check against observed data, and what pedigree charts try to reconstruct backward. It also sets up independent assortment, the next layer, which asks how two different genes behave relative to each other during that same meiotic split.
Key terms
- Allele
- One version of a gene, located at the same spot on a chromosome as its paired version.
- Homozygous
- Carrying two copies of the same allele for a gene, such as TT or tt.
- Heterozygous
- Carrying two different alleles for a gene, such as Tt.
- Test cross
- A cross between an individual of unknown genotype and a homozygous recessive individual, used to reveal hidden alleles.
- Phenotype ratio
- The proportion of observable traits among offspring, such as 3 tall to 1 short.
Watch it explained
- How it worksHow does meiosis physically separate the two alleles a parent carries?
- How it worksWalk me through a Punnett square for a cross between two heterozygous pea plants.
- Worked exampleFind the phenotype ratio when a heterozygous tall plant is crossed with a short one.
- Common mistakeWhy doesn't a dominant allele have to be the most common allele in a population?
Check yourself
End of topic.