Genes and alleles
Characteristics are inherited through genes — sections of DNA on chromosomes that code for a feature. Most genes come in different versions called alleles; you inherit one of each pair from each parent. An allele can be dominant (shown even if only one copy is present, written with a capital letter) or recessive (shown only when both copies are recessive, written with a small letter).
Genotype and phenotype
The genotype is the alleles an organism carries; the phenotype is the characteristic you actually see. An organism with two identical alleles is homozygous (BB or bb); with two different alleles it is heterozygous (Bb). A heterozygote shows the dominant phenotype but can still pass on the recessive allele — it is a carrier.
| Term | Meaning |
|---|---|
| Dominant allele | shown with one copy (capital letter) |
| Recessive allele | shown only with two copies (small letter) |
| Homozygous | two identical alleles (BB or bb) |
| Heterozygous | two different alleles (Bb) |
Punnett squares
A Punnett square predicts the offspring of a cross. You write each parent's two alleles, combine them in all four ways, and read off the ratio of genotypes and phenotypes. Crossing two heterozygotes (Bb × Bb), for example, gives offspring in the ratio 1 BB : 2 Bb : 1 bb — a 3 : 1 ratio of dominant to recessive phenotypes.
Sex determination
In humans, sex is decided by one pair of chromosomes: females are XX and males XY. Every egg carries an X; a sperm carries either an X or a Y. So the sperm decides the sex of the child, and a Punnett square of XX × XY gives a 50 : 50 chance of a boy or a girl.
Dominant hides, recessive needs two
A dominant allele shows even when paired with a recessive one. A recessive feature appears only when both alleles are recessive — which is why recessive traits can skip generations through carriers.
A worked example
In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants (Tt) are crossed. Work out the ratio of tall to short offspring.
- Each parent is Tt, so each can pass on T or t.Gametes carry one allele each.
- Combine in a Punnett square: TT, Tt, Tt, tt.All four combinations of the parents' alleles.
- Genotypes: 1 TT : 2 Tt : 1 tt.Reading the four boxes.
- TT and Tt are tall (T dominant), tt is short, so phenotypes are 3 tall : 1 short.The dominant allele shows in any plant that has it.
The offspring are in a 3 : 1 ratio of tall to short plants.
Codominance and multiple alleles
Not every gene is a simple dominant-versus-recessive pair. In codominance, both alleles show in the heterozygote at once — as in the AB blood group, where both the A and B alleles are expressed. Some genes also have multiple alleles: human ABO blood group is controlled by three alleles (Iᴬ, Iᴮ and i), where Iᴬ and Iᴮ are codominant and both are dominant to i. This gives four possible blood groups (A, B, AB and O).
Codominant vs dominant
With a dominant allele, only one of the pair shows. With codominance, BOTH alleles show together in the heterozygote — like the A and B antigens in blood group AB.
Genetic disorders and pedigrees
Some diseases are inherited. A recessive disorder (such as cystic fibrosis) appears only when a person inherits two recessive alleles; a carrier has one and is healthy but can pass it on. A dominant disorder shows with just one faulty allele. A pedigree (family tree) diagram tracks a condition through the generations, and Punnett squares can then work out the risk of a child inheriting it — the basis of genetic counselling.