Question

AIDS (acquired immunodeficiency syndrome), caused by the HIV virus is very common in Zaire. The virus infects T cells of individuals by binding to a protein on the surface of T cells, named CCR5. Scientists observed that some people in Zaire never developed AIDS, even when they were exposed to the virus. They thought this could be due to the fact that CCR5 gene has two alleles in the Zairean population (CCR5-S (S); sensitive to AIDS; and CCR-R (R): resistant to AIDS). The image shown below is the partial sequence of the CCR5-S gene. The R allele of CCR5 differs from the below sequence by only one nucleotide: nt. #40 (C) is an (A) in people who can resist the infection. But it is not known if the protective mutation is dominant or recessive in its activity. You think you can answer this question by performing RFLP analysis. You, therefore, collect DNA from 6 patients who you know have been exposed to HIV. Three have the disease and three don't You amplify the region shown below by PCR from the genomic DNA from these patients which will result in a 60bp product You then select the appropriate enzyme from the list shown below to digest this DNA. Based on this information please answer the following questions: You have 4 enzymes in your laboratory. The recognition sites of these enzymes are listed below and the sites they cut in the sequence are indicated by an arrow in the above figure. (Please note that the recognition site and the digestion site might not be identical for some enzymes, and not all enzymes recognize palindromic sequences). AvaI: 5-GGTCC-3'; MmeI: 5-GTCGGA-3; SmaI: 5'-CCCGGG-3'; XmnI: 5'GAANNNNTCC You select the correct enzyme and you perform RFLP analysis on the PCR product you have obtained. When you run your gel, you get the results shown in the next page (the first three lanes are from patients who developed AIDS, and the last 3 are from patients who resisted the disease. Please answer the following questions based on these results: Which individuals are homozygotes and which are heterozygotes for which allele? Please explain briefly.

Answer 1

Alleles are the specific genes, for example, brown hair. There are DOMINANT alleles which are represented by capital letters, (B) and RECESSIVE alleles which are represented by lower case letters. (b)

When the genes are part of a creature they become GENOTYPES (BB, Bb, or bb)
BB is homozygous DOMINANT, and it means that the creature has inherited a B allele from both parents.

Bb is heterozygous, and means that the creature has inherited both alleles, one from mom, one from dad. The dominant allele, (B), will ALWAYS be the one that is shown by the creature. In this case, the creature does have the recessive gene, so if Brown hair was dominant to blonde hair, while the creature SHOWS brown, they also CARRY the blonde gene. They are called a carrier.

bb is homozygous RESSESIVE, and means that the creature inherited the recessive allele from both parents. This is the only case in which a creature would show the recessive trait, because in any other time the dominant would take over.