A female carrier for haemophilia has a child with a father who does not have the condition. Calculate the probability of their next child being a son with haemophilia.
Step 1: Identify the parent genotypes.
Step 2: Construct the Punnett square.
| (from Father) | (from Father) | |
|---|---|---|
| (from Mother) | (Normal Female) | (Normal Male) |
| (from Mother) | (Carrier Female) | (Affected Male) |
Step 3: Analyze the specific outcomes.
Step 4: Calculate the final probability.
Students frequently lose marks by labeling males as "carriers" of sex-linked disorders. Males only have one X chromosome, so they can never be carriers; they either have the disorder or they do not.
In 3-mark "Explain" questions asking why males suffer more from sex-linked disorders, examiners explicitly look for you to state that the Y chromosome is smaller/shorter and lacks the masking allele.
Always write sex-linked alleles as superscripts on the X chromosome (e.g., ) and leave the Y chromosome completely blank (e.g., ). Writing an allele on the Y chromosome is a guaranteed lost mark.
Read probability questions very carefully: there is a big difference between calculating the "probability of the next child being affected" (out of all offspring) versus the "probability of a son being affected" (out of just the males).
X chromosome
The larger of the two sex chromosomes, present in two copies in females and one copy in males, which carries many genes including those for sex-linked disorders.
Y chromosome
The smaller sex chromosome found only in males, which carries very few genes and lacks alleles for most traits found on the X chromosome.
Sex-linked disorder
A genetic condition caused by an allele located on a sex chromosome, making the characteristic more common in one sex than the other.
Recessive allele
A version of a gene that is only expressed in the phenotype if two copies are present, or if no dominant allele is available to mask it.
Phenotype
The observable physical characteristics of an organism, determined by its genetic makeup.
Genotype
The specific combination of alleles an individual possesses for a particular gene.
Carrier
A female who has inherited one recessive allele for a genetic disorder but does not display symptoms because she also has a dominant functioning allele.
Put your knowledge into practice — try past paper questions for Biology
X chromosome
The larger of the two sex chromosomes, present in two copies in females and one copy in males, which carries many genes including those for sex-linked disorders.
Y chromosome
The smaller sex chromosome found only in males, which carries very few genes and lacks alleles for most traits found on the X chromosome.
Sex-linked disorder
A genetic condition caused by an allele located on a sex chromosome, making the characteristic more common in one sex than the other.
Recessive allele
A version of a gene that is only expressed in the phenotype if two copies are present, or if no dominant allele is available to mask it.
Phenotype
The observable physical characteristics of an organism, determined by its genetic makeup.
Genotype
The specific combination of alleles an individual possesses for a particular gene.
Carrier
A female who has inherited one recessive allele for a genetic disorder but does not display symptoms because she also has a dominant functioning allele.