Chapters 13-14

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Biological Science
BSC 1010
Sat Gassava

1. A) Explain what is sex linkage, and B) Apply the concept of sex linkage to explain why color blindness is more prevalent in men than in women. A sex linkage is when a gene is located on one of the sex chromosomes instead of the other chromosomes. Color blindness is a sex-linked disease. This is caused by recessive mutant alleles on the X chromosome. The Y chromosome contains fewer functional alleles than the X chromosome and it does not include those involved in this disease. In every cell, only 1 of the set of 2 chromosomes is used, which is switched off at a random process and is independent in every cell. In females this principle applies to the X chromosomes, as both are fully functional. If the mutant allele is present on one of the X chromosomes, it is expressed in 50% of cells; however, it is still preformed because the other 50% of cells easily compensate for the mutant allele. There is still an expression of the ability to distinguish between colors and the woman had no disease, but is a carrier. In males, there is only 1 X chromosome, so it codes for it what every cell does so if the mutant allele is present, then the disease manifests, as there is no copy of the normal allele to compensate. In contrast to autosomal cells, in which both sexes have equal probability of inheritance, sex linked alleles are those which are located on the X or Y chromosome. This explains why color blindness is more prevalent in men than in women, because the allele responsible is located on the X chromosome. Since color blindness is a recessive trait, women who have the gene are more likely to have inherited the dominant gene, which would be located on the other X chromosome. Men, however, only have one X- chromosome, so that if they have the recessive gene then it is automatically expressed phenotypically. 2. Explain why the distance between 2 genes located on the same chromosome affects the likelihood of those alleles being inherited together. Relate your answer to the events that occur during meiosis. Genetic linkage is the tendency of genes that are located on the same chromosome to be inherited together during meiosis. The amount of crossing over depends on how close the genes are to each other on the chromosome. During prophase, non-sister homologous chromosomes cross over. The greater the distance between alleles on the chromatids makes the probability of a crossover point between them greater. Thus, if they are proximal, they have a smaller probability of being separated. Hence, greater distance implies lesser chance of being inherited together. If crossing over does not occur, then it produces parental gametes. If crossing over occurs, then it produces recombinant gametes. When a crossover occurs between the two genes, then each homologue would carry
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