Both Duchenne muscular dystrophy and color blindness are caused by recessive alleles. DMD, unlike color blindness, nearly always occurs in males. Explain why.

Although both are X-linked recessive conditions, and therefore more likely in males, with the single X-chromosome. The recessive allele in colour blindness occurs at a higher frequency in the population and is a mild condition. Thus colour blindness does occur to a lesser extent in females because it needs the double recessive condition. DMD is a severe, disabling condition with a limited lifespan, and recessive allele frequency much lower, so the double recessive condition in females is very rare.

Explanation: DMD is an X-linked recessive, “nearly always in males” suggest that it also occurs due to a new mutation or some rare condition e.g. double recessive from an affected father and carrier mother, or inactivation of the normal gene in a heterozygote. It is also found that the defective allele is not completely recessive and that female carriers may exhibit mild to moderate effects.

colour blindness is polygenic, although the genes are all X-linked. It is more common in males than females. Females can carry two recessive alleles and so express the phenotype, but this is uncommon because the frequency of the recessive gene is low.

There are similarities in that both are X-linked recessives, therefore commonly expressed in males, who only have one X chromosome. The gene frequency of the colour blindness recessive is much higher than that of DMD, so the double recessive condition, which affects females, is more likely to be seen with colour blindness. In addition, DMD is a severe condition associated with disability and limited lifespan, which reduces the probability of mating between an affected male and carrier female

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