Describe red-green colour blindness as an example of sex linkage
Describe red-green colour blindness as an example of sex linkage
Answer
Red-green color blindness is a common example of a sex-linked characteristic that illustrates how certain traits are inherited through genes located on the sex chromosomes, particularly the X chromosome. This condition is primarily associated with the inability to distinguish between red and green hues, affecting many individuals worldwide.
1. Genetic Basis of Red-Green Color Blindness
Red-green color blindness is caused by mutations in the genes responsible for producing photopigments in the cone cells of the retina, specifically the OPN1LW and OPN1MW genes, which are located on the X chromosome. These genes encode for the long-wavelength (red) and medium-wavelength (green) photopigments, respectively.
2. Inheritance Pattern
X-Linked Recessive Inheritance
- Males (XY): Males have only one X chromosome. If this X chromosome carries the allele for red-green color blindness (denoted as Xc), they will express the trait because there is no corresponding allele on the Y chromosome to mask it. Thus, a male with genotype XcY will be color blind.
- Females (XX): Females have two X chromosomes. For a female to express red-green color blindness, both of her X chromosomes must carry the allele for color blindness (genotype XcXc). If she has one normal allele and one affected allele (genotype XcX), she will be a carrier but will not exhibit symptoms of color blindness herself.
3. Prevalence of Red-Green Color Blindness
Red-green color blindness is significantly more common in males than females due to its X-linked recessive nature:
- Approximately 8% of males are affected by red-green color blindness.
- In contrast, only about 0.5% of females are affected because they would need to inherit two copies of the recessive allele, which is less likely.
4. Transmission of the Trait
The inheritance pattern can be illustrated through a simple genetic cross:
- If a color-blind male (XcY) has children with a carrier female (XcX), their offspring could inherit the following genotypes:
| Parent Genotypes | Offspring Genotypes | Possible Outcomes |
|---|---|---|
| XcY (father) | XcX (daughter) | Carrier female |
| XX (daughter) | Normal vision female | |
| XcY (son) | Color-blind male | |
| XY (son) | Normal vision male |
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