Key inheritance terms
Laura Armstrong
Teacher

Contents
Recall Questions
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.
Topic Explainer Video
Key Inheritance Terms – Definitions & Examples
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) |
Additional Key Inheritance Terms
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. |
Exam Tip
Examiners often penalise vague or incorrect terminology. Be exact – learn the definitions provided!
Define the term genotype and explain the difference between homozygous and heterozygous genotypes. (4 marks)
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Genotype is the genetic makeup of an organism – the alleles it possesses.
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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.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!