Key inheritance terms

Laura Armstrong

Teacher

Topic Explainer Video

Key Inheritance Terms – Definitions & Examples

Additional Key Inheritance Terms

Laura Armstrong

Head of Biology

This topic requires prior knowledge of inheritance terms from GCSE Biology. You can test your knowledge on these below.

What is a genotype?

The genetic makeup of an organism – the combination of alleles it possesses for a gene.

What does homozygous mean?

An organism that carries two identical alleles for a gene.

What does heterozygous mean?

An organism that carries two different alleles for a gene.

Term	Definition	Example
Gene	A sequence of DNA that codes for a specific polypeptide (protein). It is found at a specific locus	Gene for eye colour
Allele	A version of a gene. Different alleles may produce different phenotypes.	Blue vs brown eye colour alleles
Locus	The specific, fixed position of a gene on a chromosome.	Chromosome 7 may contain CFTR gene at a locus
Genotype	The alleles an organism has for a particular gene or genes.	BB, Bb, or bb for eye colour
Phenotype	The observable characteristics of an organism, determined by genotype and environment.	Blue eyes, tall stature
Homozygous	Both alleles for a gene are the same.	BB or bb
Heterozygous	Two different alleles are present for a gene.	Bb
Dominant allele	Expressed in the phenotype even if only one copy is present.	B is dominant over b
Recessive allele	Only expressed when there is no dominant allele present.	b is recessive – only shows in bb
Codominant	Both alleles are expressed in the phenotype if present.	AB blood group
Diploid	A cell with two sets of chromosomes (one from each parent).	Human body cells
Haploid	A cell with one set of chromosomes (single chromosomes)	Gametes (sperm or egg cells)

Term	Definition	Example / Context
Epistasis	When one gene affects or masks the expression of another gene at a different locus.	A gene that suppresses pigment production masks the effect of a colour gene.
Sex Linkage	When a gene is located on a sex chromosome (usually the X chromosome in humans).	Haemophilia and colour blindness are X-linked recessive conditions.
Autosomal Linkage	When two or more genes are located on the same autosome (none sex-chromosomes) and are inherited together.	Genes for hair colour and eye colour being inherited together on chromosome 15.

Examiners often penalise vague or incorrect terminology. Be exact – learn the definitions provided!

No answer provided.

Define the term genotype and explain the difference between homozygous and heterozygous genotypes. (4 marks)

Genotype is the genetic makeup of an organism – the alleles it possesses.
A homozygous genotype has two identical alleles for a gene (e.g. AA or aa).
A heterozygous genotype has two different alleles for a gene (e.g. Aa).
In heterozygous individuals, the dominant allele is expressed in the phenotype.