Genetics and inheritance · Year 12
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.