3.7.1Genetics, Evolution and EcosystemsCore

Inheritance: Monohybrid and Sex-Linked Crosses

A genetic cross is a probability model, not a guarantee of exact offspring counts. Define alleles and parental genotypes before predicting gametes and phenotypes.

35 min Inheritance and Gene Pools

What you'll be able to do

Use genotype, phenotype and dominance precisely
Construct labelled monohybrid crosses
Interpret X-linked inheritance and pedigrees

State what the symbols represent

Genotype is an organism’s genetic constitution at the loci considered; phenotype reflects expression and interaction with the environment. A homozygote carries matching alleles at a locus and a heterozygote different alleles. Dominance concerns phenotype in a heterozygote, not an allele’s frequency, strength or desirability.

Define symbols before writing a cross and distinguish a genotype from an observed phenotype. For a dominant phenotype, either a homozygous or heterozygous genotype may be possible. A recessive phenotype in the simple model identifies a homozygous recessive genotype, but only under the assumptions stated for that trait.

Build a monohybrid probability model

Write parental genotypes, list the alleles in possible gametes, combine one gamete from each parent and translate offspring genotypes to phenotypes using the dominance rule. A heterozygote-by-heterozygote cross under complete dominance gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio.

Each fertilisation is a probability event. Four offspring do not have to consist of exactly three dominant and one recessive phenotype. Larger samples tend to give more stable proportions, but observed and expected frequencies can differ; an appropriate chi-squared analysis can test whether a departure is unusually large under the model.

Follow X-linked alleles through each parent

For an X-linked locus, write allele symbols on the X chromosome and distinguish the Y where it lacks the corresponding allele. A typical XY individual is hemizygous at such a locus, so a recessive X-linked allele can be expressed with only one copy.

A father passes his X chromosome to daughters and his Y to sons in the simplified model, so direct father-to-son transmission is not the expected pattern for an X-linked allele. Pedigree conclusions should consider family size, missing genotypes and alternative explanations instead of relying on one affected individual.

Apply the biology

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.

Example 1Cross two heterozygotes Aa and Aa with complete dominance. Give the probability of aa and of a dominant phenotype.

1Gametes from each parent are A and a.

2Offspring probabilities are AA=1/4, Aa=1/2 and aa=1/4.

AnswerP(aa)=1/4; P(dominant phenotype)=3/4.

Example 2A carrier female XᴺXⁿ and unaffected male XᴺY have children. In the simple recessive X-linked model, what fraction of sons is expected to be affected?

1Sons receive Y from the father and either maternal X with equal probability.

2Half receive Xⁿ and have genotype XⁿY.

AnswerHalf of sons, or one quarter of all children if the two sexes are equally likely in the model.

Common mistakes to avoid

Calling dominant alleles automatically common or beneficial.

Define dominance by heterozygote expression.

Reporting affected sons as a fraction of all children without stating the denominator.

Distinguish conditional and overall probabilities.

Key takeaways

Label alleles, genotypes and gametes before combining them.
Expected ratios describe probabilities.
Sex-linked crosses require explicit chromosome tracking.

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.

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