Predicting Inheritance: Monohybrid Crosses (AQA A Level Biology): Revision Note
Exam code: 7402
Monohybrid crosses
Monohybrid inheritance involves tracking the inheritance of one gene with two alleles
It uses known genotypes, phenotypes, and the process of meiosis to predict offspring outcomes
During gamete formation, each allele from a homologous pair has an equal chance of being passed on
This means the zygote has an equal probability of inheriting either parental allele
Genetic diagrams (especially Punnett squares) are used to present this information clearly
Predicted genotypes are based on probability, not certainty
Observed ratios in offspring may differ from predictions due to random fertilisation of gametes
Worked Example
One of the genes for the coat colour of horses has the following two alleles:
B, a dominant allele → produces a black coat when present
b, a recessive allele → produces a chestnut coat when present in a homozygous individual
In this example, a heterozygous male is crossed with a heterozygous female
Parental phenotype: black coat x black coat
Parental genotype: Bb Bb
Parental gametes: B or b B or b
Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb
Codominance
When working with codominant alleles, the genetic diagrams can be constructed in a similar way, however, the genotypes are represented using a capital letter for the gene and superscript letters for the alleles (eg. IAIA)
There will be more possible phenotypes and so the predicted ratios will be different
Worked Example
The gene for blood type has three alleles:
A, a dominant allele → produces blood type A
B, a dominant allele → produces blood type B
O, two recessive alleles → produces blood type O
In this example, a blood type A person is crossed with a blood type B person
Parental phenotype: Blood type A x Blood type B
Parental genotype: IAIO IBIO
Parental gametes: IA or IO IB or IO
Predicted ratio of phenotypes in offspring - 1 Blood type AB : 1 Blood type A : 1 Blood type B : 1 Blood type O
Predicted ratio of genotypes in offspring: 1 IAIB : 1 IAIO : 1 IBIO : 1 IOIO
Sex-linkage
Sex-linked genes are located on the X chromosome (rarely on Y)
Males (XY) have one copy of X-linked genes, so are more likely to express recessive traits
For example,
XᴬY = unaffected
XᵃY = affected
Females (XX) have two copies, so can be unaffected, carriers, or affected
For example,
XᴬXᴬ = unaffected
XᴬXᵃ = carrier
XᵃXᵃ = affected
Males cannot be carriers and do not pass X-linked traits to sons, as they only pass on the Y chromosome
Worked Example
Haemophilia is a sex-linked disorder caused by a gene on the X chromosome
The gene codes for Factor VIII, a protein essential for blood clotting
F = dominant allele → produces normal Factor VIII
f = recessive allele → results in no Factor VIII, leading to haemophilia
Males only need one copy of f (XᶠY) to be affected, as they have only one X chromosome
Females must inherit two copies (XᶠXᶠ) to be affected; heterozygous females (XᶠXF) are carriers
Two parents with normal clotting (e.g. XFXᶠ mother and XFY father) can produce a child with haemophilia:
Parental phenotypes: carrier female x normal male
Parental genotypes: XFXf XFY
Parental gametes: XF or Xf XF or Y
Predicted ratio of phenotypes in offspring - 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting
Predicted ratio of genotypes in offspring: 1 XFXF : 1 XFXf : 1 XFY : 1 XfY
Examiner Tips and Tricks
Tips for drawing Punnet squares
Always show your working:
Include the possible gametes produced by each parent
Do not skip straight to the Punnett square
State both the genotype and phenotype of offspring
For sex-linked inheritance questions:
Read carefully to see if the question refers to all children or a specific sex (e.g. only boys or girls)
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