Monohybrid inheritance and sex linkage

Key terms

• Gene: a length of DNA coding for a polypeptide. Allele: a different version of the same gene.
• Genotype: the alleles an organism has (for example Ff). Phenotype: the characteristics that result.
• Dominant: shown whenever present. Recessive: shown only when two copies are present.
• Homozygote: two identical alleles (FF or ff). Heterozygote: two different alleles (Ff).
• Codominance: both alleles are expressed in the heterozygote (for example AB blood group).

Monohybrid crosses and pedigrees

A monohybrid cross follows one gene. Two heterozygous parents (Ff x Ff) give offspring genotypes FF : Ff : ff in a 1 : 2 : 1 ratio, so a 3 : 1 phenotype ratio and a 1 in 4 chance of a homozygous recessive child.

In a pedigree diagram, squares are males, circles are females, and shaded shapes are affected. Two unaffected parents having an affected child shows the condition is recessive and both parents are carriers.

Sex linkage

Genes on the X chromosome are sex-linked. Females are XX, males XY; the Y carries few genes. So males have only one copy of X-linked genes and a recessive allele is always expressed in them.

Red-green colour blindness is X-linked recessive. A carrier mother (X^B X^b) and unaffected father (X^B Y) have:

• daughters: 1/2 X^B X^B, 1/2 carriers X^B X^b (none affected)
• sons: 1/2 X^B Y, 1/2 colour blind X^b Y.

Worked example

Two parents are both carriers (Ff) of cystic fibrosis. What is the probability their child has CF?

1. Gametes from each parent: F or f.
2. Offspring: FF, Ff, fF, ff.
3. Only ff is affected: 1 in 4.

Answer: 0.25 (25%)

Check yourself

1. Two carriers (Ff) of a recessive disorder have a child. What is the percentage chance the child is a carrier (heterozygous)?
   Show answer

   50 %. Ff appears in 2 of 4 outcomes: 50%.

2. A heterozygous parent (Ff) and a homozygous recessive parent (ff) have a child. What is the percentage chance the child is ff?
   Show answer

   50 %. Ff x ff gives Ff : ff in 1 : 1, so 50%.

3. A carrier mother (X^B X^b) and an unaffected father (X^B Y) have a son. What is the percentage chance he is colour blind?
   Show answer

   50 %. Sons get Y from father and either X^B or X^b from mother: 50%.

4. What is a heterozygote?
   • An organism with two identical alleles
   • An organism with two different alleles of a gene
   • An organism showing the recessive phenotype
   • A version of a gene
   Show answer

   An organism with two different alleles of a gene. Hetero = different.

5. In codominance, a heterozygote
   • shows only the dominant allele
   • shows only the recessive allele
   • has no phenotype
   • shows the effects of each of its two alleles
   Show answer

   shows the effects of each of its two alleles. Both alleles are expressed, for example blood group AB.

6. Why are X-linked recessive conditions more common in males?
   • Males have two X chromosomes
   • Males have only one X chromosome, so a single recessive allele is expressed
   • The Y chromosome carries the dominant allele
   • Females cannot inherit X-linked alleles
   Show answer

   Males have only one X chromosome, so a single recessive allele is expressed. There is no second X to carry a dominant allele.

7. Two unaffected parents have an affected child. What does this show about the condition?
   • It is caused by a dominant allele
   • It is not genetic
   • Only the father passed it on
   • It is caused by a recessive allele and each parent is a carrier
   Show answer

   It is caused by a recessive allele and each parent is a carrier. Each parent must carry one hidden recessive allele.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.