Colour Blindness
Colour blindness is an X-linked recessive condition. Crossing a carrier mother (XN Xn) with a normal-vision father (XN Y) gives sons a 1:1 chance of being colour blind, while all daughters have normal vision.
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Why colour blindness is X-linked recessive
The gene for red-green colour blindness is carried on the X chromosome. The allele for normal vision (XN) is dominant, and the allele for colour blindness (Xn) is recessive.
Because it is X-linked, the condition follows the same inheritance pattern as other X-linked recessive traits, affecting males more often than females.
Worked problem 2 (medium): carrier mother x normal-vision father
A mother who is a carrier (XN Xn, normal vision but carries the allele) has children with a father who has normal vision (XN Y).
Parental genotypes: XN Xn x XN Y.
Gametes: mother produces XN and Xn; father produces XN and Y.
| XN | Y | |
|---|---|---|
| XN | XN XN | XN Y |
| Xn | XN Xn | Xn Y |
Reading the result
Among daughters: 1 XN XN (normal vision) : 1 XN Xn (carrier, normal vision), all daughters have normal vision, but half are carriers.
Among sons: 1 XN Y (normal vision) : 1 Xn Y (colour blind), sons have a 1 in 2 chance of being colour blind.
Overall, the offspring phenotype ratio is 3 normal vision : 1 colour blind, but the colour-blind individual is always a son, never a daughter, in this particular cross.
How it is examined
SPM questions typically give the vision status of two parents and ask you to state genotypes, complete the Punnett square, and give the probability that a son or daughter will be colour blind.
Worked problem 1 (easy): normal-vision mother x colour-blind father
A mother with normal vision who is not a carrier (XN XN) has children with a colour-blind father (Xn Y). The mother passes on only XN, while the father passes Xn to daughters and Y to sons.
- No child is colour blind in this cross.
- Every daughter is a carrier (XN Xn) with normal vision, because she receives Xn from her father and XN from her mother.
- Every son has normal vision (XN Y), because he receives his single X (carrying XN) from his mother and the Y from his father.
| Xn | Y | |
|---|---|---|
| XN | XN Xn | XN Y |
| XN | XN Xn | XN Y |
Worked problem 3 (SPM level): carrier mother x colour-blind father
A carrier mother (XN Xn) has children with a colour-blind father (Xn Y). The mother produces XN and Xn gametes, and the father produces Xn and Y gametes.
Here a daughter can be colour blind. Among daughters: 1 XN Xn (carrier, normal vision) : 1 Xn Xn (colour blind).
Among sons: 1 XN Y (normal vision) : 1 Xn Y (colour blind). The overall ratio is 1 colour blind : 1 normal vision, and both sons and daughters can be affected.
| Xn | Y | |
|---|---|---|
| XN | XN Xn | XN Y |
| Xn | Xn Xn | Xn Y |
Practice problems
Draw each Punnett square yourself, then check your answers below.
- 1. A colour-blind man has children with a woman who has normal vision and is not a carrier. State the proportion of sons and daughters expected to be colour blind.
- 2. A carrier woman has children with a man who has normal vision. What is the probability that their first child is a colour-blind son?
- 3. A woman is colour blind. What can you deduce about her father's vision, and why?
Answers
Common mistakes
Source:SRC-DSKP-EN
Frequently asked questions
Is colour blindness dominant or recessive, and where is the gene located?
If a carrier mother has children with a normal-vision father, what are the chances of colour blindness?
Why is colour blindness more common in males?
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