Genetics and inheritance

Characteristics are passed from parents to offspring in genes. Most genes come in different forms called alleles, and each of us has two alleles of every gene, one from each parent. A genetic diagram such as a Punnett square shows the possible offspring of a cross and how likely each one is.

Key facts

  • A gene is a short section of DNA on a chromosome. Different forms of a gene are alleles. The alleles you have are your genotype; the characteristic you show is your phenotype.
  • A dominant allele (capital letter) shows when only one copy is present; a recessive allele (small letter) shows only when there are two copies. Two identical alleles: homozygous; two different alleles: heterozygous.
  • A Punnett square shows each parent's alleles along the sides and the possible offspring in the boxes. Each box is a 1 in 4 (25%) chance.
  • Crossing two heterozygous parents (Bb × Bb) gives a 3 : 1 ratio of dominant to recessive phenotypes; crossing a heterozygote with a homozygous recessive (Bb × bb) gives 1 : 1.
  • Sex is decided by one pair of chromosomes: females are XX and males are XY. Every child has a 50% chance of being a boy.
  • Some disorders are inherited: polydactyly (extra fingers or toes) is caused by a dominant allele; cystic fibrosis is caused by a recessive allele, so two carriers can have an affected child.
  • Family trees show how a characteristic is passed down the generations. In codominance both alleles show (blood group AB); sex-linked alleles, such as for haemophilia, are on the X chromosome.

Worked examples

Example 1

In mice, brown fur (B) is dominant to white fur (b). Two heterozygous brown mice are crossed. What fraction of the offspring is expected to be white?

  1. Each parent is Bb, so each can pass on B or b.
  2. Fill in the boxes: BB, Bb, Bb, bb.
  3. Only bb is white: 1 box out of 4.
  4. So 1 in 4, or 25%, are expected to be white; the ratio of brown to white is 3 : 1.

Answer: 1 in 4 (25%)

Example 2

In pea plants, tall (T) is dominant to short (t). A heterozygous tall plant is crossed with a short plant. What is the expected ratio of tall to short offspring?

  1. The tall plant is Tt; the short plant must be tt.
  2. Gametes: T or t from the first plant; only t from the second.
  3. Offspring: Tt, Tt, tt, tt.
  4. Tall : short = 2 : 2 = 1 : 1.

Answer: 1 : 1

Example 3

The family tree shows a recessive disorder. Parents 1 and 2 are unaffected but child 3 is affected. What are the genotypes of 1 and 2?

  1. The disorder is recessive, so the affected child 3 is ff.
  2. Child 3 got one f from each parent.
  3. Parents 1 and 2 do not show the disorder, so each also has an F.
  4. So both parents are Ff: carriers.

Answer: both heterozygous carriers (Ff)

Example 4

Two heterozygous pea plants have 200 offspring. How many would you expect to show the recessive characteristic?

  1. A cross of two heterozygotes gives 3 dominant : 1 recessive.
  2. So 1 in 4 show the recessive characteristic. 200 ÷ 4 = 50
  3. The real number will be close to 50, because fertilisation is random.

Answer: about 50