Key Terms in Genetics (AQA A Level Biology): Revision Note

Key terms in genetics

Genes & alleles

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

• Each gene is located at a specific locus on a chromosome

• A chromosome contains many genes made of a long DNA molecule

• Genes can exist in different versions called alleles

• Alleles have slightly different base sequences but occupy the same locus

Example

• The Agouti gene in horses controls coat colour

• There are two alleles:

  • A → black coat

  • a → chestnut coat

Genotype & phenotype

• Eukaryotic cells have homologous chromosomes, so each gene is present in two copies

• Individuals can have the same alleles (homozygous) or different alleles (heterozygous)

• An organism’s genotype is the alleles it possesses

• The phenotype is the observable characteristics, determined by the genotype and the environment

Example

• Horses have two copies of the coat colour gene in each cell

• Genotype AA (homozygous) → black coat phenotype

• Genotype Aa (heterozygous) → also black coat phenotype, assuming A is dominant over a

Dominance

• Not all alleles affect the phenotype in the same way

• Some alleles are dominant: they are always expressed in the phenotype

  • This means they are expressed in both heterozygous and homozygous individuals

• Others are recessive: they are only expressed in the phenotype if no dominant allele is present

  • This means that it is only expressed when present in a homozygous individual

Example of dominance

• If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur:

  • Genotype AA → black coat

  • Genotype Aa → black coat

  • Genotype aa → chestnut coat

Codominance

• Codominance occurs when both alleles in a heterozygous genotype are fully expressed

  • This means that features of both alleles are observed in the phenotype

• Genotype notation uses a capital letter to represent the gene (e.g. I) and superscript letters for the alleles (e.g. Iᴬ, Iᴮ, Iᴼ)

Example of codominance

• A good example of codominance can be seen in human blood types

• The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O

• Gene controlling blood type = I

• Three alleles: Iᴬ, Iᴮ (codominant), Iᴼ (recessive)

• Genotypes and resulting phenotypes:

  • IᴬIᴬ or IᴬIᴼ → Blood group A

  • IᴮIᴮ or IᴮIᴼ → Blood group B

  • IᴬIᴮ → Blood group AB (codominance)

  • IᴼIᴼ → Blood group O

F₁, F₂ & test crosses

• A homozygous dominant (e.g. AA) crossed with a homozygous recessive (e.g. aa) produces the F₁ generation

• All F₁ offspring are heterozygous (Aa) and show the dominant phenotype

• Crossing two F₁ individuals (Aa × Aa) gives rise to the F₂ generation

  • Expected phenotypic ratio: 3 dominant : 1 recessive

  • Expected genotypic ratio: 1 AA : 2 Aa : 1 aa

Test cross (black cross)

A test cross is used to determine the genotype of an individual showing the dominant phenotype

• The unknown individual is crossed with a homozygous recessive (aa)

• Offspring outcomes:

  • If all offspring show the dominant phenotype → The unknown genotype is homozygous dominant (AA)

  • If the offspring are 50% dominant and 50% recessive → The unknown is heterozygous (Aa)

Linkage

• Linkage refers to genes that are inherited together because they are on the same chromosome

• Two main types: Sex linkage and Autosomal linkage

Sex Linkage

• Women have two copies of the X chromosome (XX), whereas men have one X chromosome and one shorter Y chromosome (XY), which together are known as the sex chromosomes

• Sex-linked genes are located on the X chromosome (rarely on Y)

• Males are more likely to express recessive sex-linked traits (only one X allele)

• Genotype notation is used as follows:

  • Haemophilia is an example of a recessive X-linked disorder

    • Female heterozygote: X^HX^h

    • Male with dominant allele:X^HY

Autosomal Linkage

• This occurs on the autosomes (any chromosome that isn’t a sex chromosome)

• Two or more genes on the same chromosome do not assort independently during meiosis

• These genes are linked and they stay together in the original parental combination