International A Level Biology / Gene expression and genetics
Sex linkage and red-green colour blindness
Edexcel International A Level Biology (YBI11), chapter 6: Gene expression and genetics, specification point 2.15. Free lesson.
Read the revision notes · International A Level Biology past papers
Red-green colour blindness is an X-linked recessive condition, so males are affected more often because they have only one X chromosome. Knowing how to write genotypes and trace inheritance helps you interpret crosses and pedigrees.
Why males are affected more often
The X chromosome is large and carries many genes, including genes for the red and green cone pigments. The Y chromosome is much smaller and has no matching allele for most genes on the X. Females are XX, so they have two alleles for each X-linked gene. Males are XY, so their single X allele is expressed without a matching allele to mask it. The recessive allele is on the X chromosome, not the disorder itself, and never on the Y. An affected father passes his Y chromosome to his sons, not his X.
How to write colour-blindness genotypes
Use a superscript on the X to show the allele: X^B gives normal colour vision and X^b gives red-green colour blindness. Females can be carriers because they have two X chromosomes. Males cannot be carriers because they have only one X. A female with X^BX^B has normal vision and is not a carrier. A female with X^BX^b has normal vision and is a carrier. A female with X^bX^b is colour blind. She is homozygous recessive, meaning both alleles are recessive. A male with X^BY has normal vision. A male with X^bY is colour blind.
How to cross a carrier mother with a normal father
A genetic cross shows which allele each parent can pass on. The father produces sperm carrying X^B or Y. The carrier mother produces eggs carrying X^B or X^b. Each offspring has an equal chance of receiving each of the four combinations.
How to read X-linked recessive pedigrees
A pedigree can help you decide whether a recessive condition is sex-linked or autosomal. A pattern with affected males and no father-to-son transmission can suggest X-linkage. In a family with an affected father, each daughter receives his X^b. If she has normal vision, she must also have X^B and is a carrier. An affected father passes his X^b to every daughter, but passes his Y to every son. An affected son with unaffected parents may have received X^b from a carrier mother. An unaffected daughter of an affected father can be X^BX^B or X^BX^b if her mother's allele is unknown. An…
Check what the probability refers to
Sons get their X chromosome from their mother, while daughters get an X from each parent. A probability can be calculated just for sons or for all children, so read the question carefully. For a carrier mother and normal father, half the sons are expected to be colour blind. For the same cross, the chance that any child is a colour-blind son is 1 in 4.