Genetic inheritance · GCSE Biology

Key words

Alleles are different forms of the same gene. You inherit one allele of each gene from each parent.
A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present.
Homozygous means both alleles are the same (AA or aa); heterozygous means the alleles are different (Aa).
The genotype is the alleles present (for example Bb); the phenotype is the characteristic shown (for example brown eyes).
Some characteristics are controlled by a single gene, but most result from multiple genes interacting.

A Punnett square shows the possible combinations of alleles in the offspring of two parents.
Example: two heterozygous parents (Aa × Aa) give offspring AA, Aa, Aa and aa, a 3 : 1 ratio of dominant to recessive phenotype.
Ratios are probabilities: each child has the same chance, whatever happened to its brothers and sisters.

Human body cells have 23 pairs of chromosomes. 22 pairs control characteristics only; one pair determines sex.
Females have two X chromosomes (XX); males have one X and one Y (XY).
A Punnett square shows a 50% chance that a baby will be a boy or a girl.

Polydactyly (having extra fingers or toes) is caused by a dominant allele, so only one parent needs to pass it on.
Cystic fibrosis is a disorder of cell membranes caused by a recessive allele, so a child must inherit it from both parents. Parents who are heterozygous are carriers.
Embryo screening can identify embryos with a disorder, and gene therapy may one day correct faulty alleles.
These raise economic, social and ethical issues: screening is expensive, embryos may be destroyed, and some worry it could lead to choosing embryos for other characteristics.

Allele: A different form of the same gene.
Dominant: An allele that is expressed even if only one copy is present.
Recessive: An allele that is expressed only when two copies are present.
Homozygous: Having two identical alleles of a gene.
Heterozygous: Having two different alleles of a gene.
Genotype: The combination of alleles an organism has.
Phenotype: The characteristic that is shown, resulting from the genotype.
Punnett square: A diagram used to predict the outcomes of a genetic cross.
Carrier: A person with one recessive allele for a disorder who does not have the disorder.
Polydactyly: An inherited disorder causing extra fingers or toes, caused by a dominant allele.
Cystic fibrosis: An inherited disorder of cell membranes caused by a recessive allele.