Q15 of 53 Page 33

It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population. Why?

Sickle cell anaemia is persisting in human population. It is a good disease example of a balancing selection, with affected individuals carrying mutations in both the parental and maternal inherited haemoglobin gene. As a result their red blood cells are less efficient in carrying oxygen throughout the body. But there is a biological advantage associated with sickle cell anaemia that the patients are better protected against malaria. This resistance offered against malaria disease could be the reason for the harmful alleles of sickle cell anaemia to persist in human population despite of the elimination of harmful genes over time.


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What is artificial selection? Do you think it affects the process of natural selection? How?

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With the help of an example differentiate between incomplete dominance and co-dominance.

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In a plant tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents work out a dihybrid cross. What is standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?

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a. In humans, males are heterogametic and females are homogametic. Explain. Are there any examples where males are homogametic and females heterogametic?

b. Also describe as to, who determines the sex of an unborn child? Mention whether temperature has a role in sex determination.