BI111 Lecture Notes - Lecture 2: Color Blindness, Chromosome, Melanin

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29 Mar 2018

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Males have 1 x chromosome whereas females have 2 x chromosomes; there are several consequences: I(cid:374) (cid:373)ale it does(cid:374)"t (cid:373)atter (cid:449)hether the allele o(cid:374) x chro(cid:373)oso(cid:373)e is recessive or dominant, since there is nothing else to be suppressive hence it will be expressed no matter what. So all recessive alleles just like all dominant alleles are expressed under all circumstances if located on the x found in x y main. In females whether it will be expressed or not depends on whether there is one or two copies of recessive allele. Homogametic sex there are two copies of sex chromosomes: Heterozygote not expressing their recessive allele, often known as carriers. Example: in humans, colour blindness is determined by a gene on x chromosome. Males have higher chances of being colour blind than females because the gene responsible for colour blindness is recessive. Females can only be colour blind if they have two recessive alleles.

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. A recessive allele on the X chromosome is responsible for red-green color blindness in humans. This means that females have two copies (since human females are XX) and males have one copy (since human males are XY). This is called a sex-linked trait. A red-green color blind male is genotype Xn Y. A normal vision male is genotype XN Y. For this problem, a normal vision woman whose father is color-blind marries a color-blind male. N= not color blind allele, n = color blind allele. Y = Y chromosome.

a. What is the genotype of the color-blind father? Why?

b. What is the genotype of the color-blind husband? Why?

c. What is the genotype of the normal vision woman? Why?

d. What is the probability that this couple will have a son that is color-blind?

e. What is the probability that this couple will have a daughter that is color-blind?

Complete the following paragraph to learn about x-linked inheritance. expressed, men, sex-linked, ... Question: Complete the following paragraph to learn about X-linked inheritance. expressed, men, sex-linked,... Complete the following paragraph to learn about X-linked inheritance. expressed, men, sex-linked, repressed, X, Y, autosomal, two, one Sex-linked alleles have a different pattern of inheritance compared to those found on ____________ chromosomes. This is because on ____________ alleles, the allele is only carried on the X chromosome and not the Y chromosome. That means that females can pass down diseased alleles to offspring through ____________ ____________ chromosomes, while males can pass down alleles through ____________ ____________ chromosome. For this reason, it is more likely that ____________ are afflicted with sex-linked diseases, because whatever allele is carried on their ____________ ____________ chromosome is ____________ .

We have discussed how recessive phenotypes are more likely to be expressed in males if the corresponding gene is found on the X-chromosome. For example, one-type of colorblindness is caused by a recessive X linked allele, and this trait is much more common inmales than females. But we have also discussed how, in placental mammals, one copy of each X chromosome is randomly silenced in female cells (X inactivation). This raisessomewhat of a paradox. Why donât half of heterozygous females exhibit colorblindness just like hemizygous males that carry the recessive allele?

Based on your under standing of Xinactivation in placental mammals, provide an explanation for why heterozygous females generally do not exhibit colorblindness despite the random silencing of one X chromosome.

b.How would your answer change if we were talking about a marsupial mammal (inwhich the paternally inherited copy of the X chromosome is inactivated in every cell)? Would you expect half of heterozygous females in a marsupial species toexhibit color blindness? Why or why not?

BI111 Lecture Notes - Lecture 2: Color Blindness, Chromosome, Melanin

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