5. Tischkoff et al. (1996) chose to genotype the modern human CD4 gene, specifically in terms...

5. Tischkoff et al. (1996) chose to genotype the modern human CD4 gene, specifically in terms of the length of a short tandem repeat sequence (STR) in an intron, along with a closely-linked Alu element that was partially deleted in some individuals. Define these terms and explain how their genotyping was performed. Why was the rate of mutation in these particular genetic markers expected to be appropriate for studying recent events in human evolution? Why was natural selection expected to have a minimal impact on these genetic markers? Why (how) did the results support the “out of Africa” model, both in terms of the genetic diversity of various modern human populations, and also in terms of the inferred ages of those populations?

Tishkoff, S. A. et al. (1996) Global patterns of linkage disequilibrium at the CD4 locus and modern human origins. Science 271, 1380-1387.

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Expert Solution

STR or short tandem repeats are DNA sequences within intronic regions that are 2-13 nucleotides long. Alu elements are considered to be transposable elements that have millions of copy within the genome. Blood sequence from 1600 individuals spanned across different continents were taken and CD4 STRP analysis using PCR was done. CD4 STR started as a small repeat but over the course of 4-6 million years of evolution have developed a high level of mutations. Due to variation in STR lengths the sequences varied from 80 to 135 bp. Since STRP have moderate to high rates of mutation (as previously studied) The series of mutation can help a researcher to understand the level of polymorphism and human evolution.

The sequences selected are non coding regions. Linkage disequilbrium was found between Alu sequence and STRP. Genetic drift might have played a role in evolution/ variation in sequences/polymorphisms rather than process of natural selection.

An out of Africa model is one in which ancestors have evolved from Africa and moved to various places during course of evolution. Decreased variation in terms of sequence in non-African population further suggests linkage disequilibrium compared to African population. Also mitochondrial gene data also suggests the same.

gladiator answered 1 year ago

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