Shown here are two alleles of an SNP, one of which is associated with a higher r

Question

Shown here are two alleles of an SNP, one of which is associated with a higher risk of developing high blood pressure (hypertension). You amplify DNA from multiple patients at a cardiologist’s office and digest each with EcoRI in order to determine the patients’ genotypes for this SNP.

Which lane in the gel below represents an individual heterozygous for the hypertension allele? Which lane in the gel below represents a sample from an individual heterozygous for this SNP?

I know that the answer is lane B for both parts of the question, but I don't understand why that is.

Normal allele: 5' ATTCGCGGAATTCTGG 3' 3'TAAGCGCCTTAAGACC 5 ed with hypertension: 5A 3 TAAGCGCCCTAAGACC 5' A B C D E Biology how Life Work © 2014W.H. Freeman andCompany

Explanation / Answer

All mammals are diploid organisms which indicate that they have two copies of each chromosome. Nearly all the cells of an organism carry two similar copies of each chromosome. The only exception is the gametes (egg and sperm cells) have the haploid number of chromosomes. For example in the case of humans, all the cells have total 44 +XY/XX chromosomes depending on the type of sex. But gametes posses either 22+X /22+Y. All the genes are located on chromosomes (DNA) of every individual has two copies of each gene i.e. one on one chromosome (eg. First copy of p53 gene on chromosome 17 it is one copy) and other on other chromosome (eg. Second copy of p53 gene another chromosome 17 it is another copy).

An allele is an alternative form of a gene. We have two copies of each allele. The allele may be base (A or G or T or C) or character (tall character or dwarf character).

Case 1: If an individual is homozygous for an allele (in our case A in first strand & T in the second strand of DNA molecule) it means that same allele is present at same locus in both homologous chromosomes (in our case the A in first strand & T in second strand present in the same locus in both chromosomes). When we add the EcoRI restriction enzyme due to the absence of restriction site it will not make any cuts in both chromosomes/DNA strands. When we run this sample on the gel it produces single band. Which is represented by A Band.

Case 2: If an individual is heterozygous to an allele (in our case A in first strand & T in the second strand of DNA present one copy of chromosome/DNA strand and G in first strand & C in the second strand of DNA present second copy of chromosome/DNA strand). Here the first copy of chromosome/strand is carrying normal allele (A &T) and the second copy of chromosome/strand carrying mutant allele (G & C). Now When we add the EcoRI restriction enzyme to this sample due to mutation in DNA strand or chromosome a new restriction site is generated in second copy of chromosome/DNA strand and it will recognize the site & cut into two unequal DNA fragments/pieces (on gel they appear as two bands) but other copy of chromosome/DNA strand do not have any mutation so it does not have any restriction site so enzyme does not make cut in it (on gel it appear as single band). When we run this mixture on gel we will get total three bands (one band from first copy of chromosome/strand and two bands from second copy of chromosome/strand) which is represented by B Band.

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