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7.1Genetics, Populations, Evolution and Ecosystems (A Level only)

Inheritance (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

15 min read 22 parts Lesson 2 of 25 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

Key Terms in Genetics, Codominance and Sex Linkage — AQA A LEVEL BIOLOGY 7402 notes: Define the key genetic terms and explain codominance, multiple alleles and X-linked inheritance with haemophilia as the example.A gene is a DNA sequence that codes for a polypeptide or protein.

  • homozygous
  • heterozygous
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Sex-linked

Part 1 of 5Key terms in genetics

A gene is a DNA sequence that codes for a polypeptide or protein.

Locus
Each gene occupies a specific locus on a chromosome, and a chromosome contains many genes in one long DNA molecule. Alleles are different forms of the same gene.
Homozygous
Individuals with two identical alleles are homozygous; those with different alleles are heterozygous.
The genotype is the allele combination; the phenotype is the observable characteristics, determined by the genotype and the environment.
Horses, for example, have two copies of the coat-colour gene in each cell (diploid).
A dominant allele is expressed in the heterozygote; a recessive allele is expressed only when homozygous.

In a cross between two homozygous parents (AA × aa), all F1 offspring are heterozygous (Aa) and show the dominant phenotype.

Part 2 of 5Codominance and multiple alleles

In codominance both alleles are expressed in the phenotype of the heterozygote.

1
The gene for human ABO blood type has three alleles: IA and IB (codominant) and IO (recessive).
2
Genotypes and phenotypes: IAIA or IAIO give group A; IBIB or IBIO give group B; IAIB gives group AB; IOIO gives group O.
3
When working with codominant alleles, genetic diagrams are built in the same way,
  • but genotypes are written with a capital letter for the gene and superscript letters for the alleles (e.g. IAIA).
4
There are more possible phenotypes, so the predicted ratios differ.

Part 3 of 5Monohybrid inheritance

Monohybrid inheritance tracks one gene with two alleles. It uses known genotypes, phenotypes and the process of meiosis to predict offspring: during gamete formation, each allele from a homologous pair has an equal chance of being passed on.

Part 4 of 5Sex linkage and haemophilia

Haemophilia is a recessive X-linked disorderthe gene is on the X chromosome and codes for Factor VIII, a protein essential for blood clotting.

Sex linkage and haemophilia
Autosomal genes lie on autosomes (any chromosome that is not a sex chromosome).
Females (XX) have two copies,
  • so can be unaffected, carriers or affected (e.g. a female heterozygote XHXh is a carrier);
    • males (XY) have one X,
  • so a single recessive allele (XhY) causes the disease.
Males inherit their X from their mother, so the condition is more common in males.

Part 5 of 5Worked examples

  1. Define the terms locus and heterozygous. [2]2 marks

  2. Give all the genotypes that give blood group A, and explain why a person of genotype IAIB has group AB. [3]3 marks

  3. Explain why haemophilia is more common in males. [3]3 marks

Key terms

TermMeaning
LocusFixed position of a gene on a chromosome.
HomozygousHaving two identical alleles of a gene.
HeterozygousHaving two different alleles of a gene.
CodominanceBoth alleles are expressed in the phenotype of the heterozygote.
Sex-linkedA gene located on a sex chromosome (usually X).
CarrierHeterozygous individual with a recessive allele that does not show the condition.

Avoid these mistakes

1
Confusing gene and allele.
2
Writing the carrier female as XhXh.
3
Saying the phenotype is determined only by the genotype.
4
Treating codominance as blending.

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