Key terms
- Gene: a unit of heredity. Allele: a version of a gene.
- Genotype: the allele combination (for example Aa). Phenotype: the observable trait.
- Homozygous: two identical alleles (AA or aa). Heterozygous: two different alleles (Aa).
- Dominant alleles (capital letter) show in the phenotype when present. Recessive alleles (lowercase) show only when two copies are present.
Mendel's laws
Gregor Mendel experimented with pea plants and found:
- Law of segregation: the two alleles for a gene separate when gametes form, so each gamete carries one.
- Law of independent assortment: alleles of different genes sort independently (true for genes on different chromosomes).
Punnett squares
A Punnett square shows possible offspring from the gametes of each parent.
Worked example
In peas, tall (T) is dominant to short (t). Cross two heterozygous plants (Tt × Tt).
- Each parent produces gametes T and t in equal numbers.
- The square gives: TT, Tt, Tt, tt.
- Genotype ratio: 1 TT : 2 Tt : 1 tt.
- Phenotype ratio: 3 tall : 1 short, so 25 percent are short.
Now cross Tt × tt (a test cross): the offspring are 1/2 Tt and 1/2 tt, so half are tall and half short. A test cross uses a homozygous recessive partner to reveal the genotype of an organism with the dominant phenotype. If any short offspring appear, the tall parent must be Tt.
Using probability
Independent events multiply. If Tt × Tt, the chance of two children (or plants) both being short is 1/4 × 1/4 = 1/16. The chance that a given offspring is short is always 1/4, whatever happened to earlier ones; chance has no memory.
Beyond simple dominance
- Incomplete dominance: the heterozygote is intermediate. Crossing red and white snapdragons can give pink.
- Codominance: both alleles show fully, as in the AB blood type.
- Polygenic traits: many genes contribute, as with human height and skin colour.
Sex linkage
The X chromosome carries many genes the Y does not. Males (XY) have only one X, so a single recessive allele on it shows. This is why X-linked recessive conditions such as red-green colour blindness are more common in males. A son inherits his X from his mother, never from his father.
Common mistakes
- Treating "dominant" as "most common". Dominance describes expression, not frequency.
- Mixing up genotype and phenotype.
- Forgetting that each child's probability is independent.