Question

1. What is a gene family? What mechanism generates them?

2.What is a SNP? What is a haplotype? How can SNP haplotypes be determined?

3. Assuming the PAV allele is the original one, what types of mutations (silent, missense, nonsense, etc.) have changed the PAV allele into the AVI allele? Explain. Based on the information given, how do these changes fit with what you have learned about the “functional enzyme hypothesis”?

4.You repeat Snyder’s experiment, but instead of determining the ability of each family member to taste PTC, you determine what alleles they carry: 2 PAV (PAV/PAV), 2 AVI (AVI/AVI), or 1 PAV and 1 AVI (PAV/AVI). Using the table provided below, what are the possible outcomes for offspring if the parents have the genotypes described in the first column? Below the genotype, write the phenotype(taster or non-taster) of each kind of individual

Answer 1

1. A set of genes which have similar biochemical functions belongs to a gene family. As such, members of a gene family arise through gene duplication and subsequent accumulation of mutations. Generally, they have more than 90% nucleotide similarity. Organization of genes into gene families is one level of organization of genome.Genes within a species that have similar nucleotide sequences are called paralogsGenes from different species that share similar nucleotide sequences are called orthologs

2. Single nucleotide polymorphisms (SNPs) are variations at a single nucleotide (base pair) in a DNA sequence. A single nucleotide polymorphism generally arises due to point mutations. On an average, they occur once in every 254 nucleotides. Most of the SNPs have no effect, since they are found in non-coding regions of genome.

On an average, at least one base pair difference occurs in every 50 individuals, at a specific locus. SNPs are useful as genetic markers.

3. PAV and AVI refer to the Taste receptor-2 member 38 protein haplotypes. The proteins are responsible for bitter taste perception. The alleles are responsible for differing ability to taste phenylthiocarbamide (PTC, a chemical used to analyze bitter taste perception).

The haplotype AVI is generally called nontaster, and PAV is called taster. The combinations of these alleles give different persons the varying ability to perceive taste. A missense mutation in the original PAV allele might have changed the properties of the protein which made them unable to bind to their ligands. As a result, individuals homozygous for AVI alleles cannot perceive bitterness much.

4.

Parent Genotype Combination	Possible Genotypes of the children	Possible phenotypes of the children
Both parents PAV/PAV	All the children are PAV/PAV	Good at perception of bitter taste
Both parents PAV/AVI	¼ PAV/PAV ½ PAV/AVI ¼ AVI/AVI	¼ Normal ½ Intermediate ¼ poor -Perception of bitter taste
One PAV/PAV, the other PAV/AVI	½ PAV/PAV ½ PAV/AVI	½ Normal ½ Intermediate At perception of bitter taste
One PAV/PAV, the other AVI/AVI	All PAV/AVI heterozygous	All progeny are intermediate at bitter taste perception
One PAV/AVI, the other AVI/AVI	½ AVI/AVI ½ PAV/AVI	½ of the progeny poor at bitter taste perception, the other half intermediate
Both parents AVI/AVI	All the progeny are AVI/AVI homozygotes	All the progeny are poor at the perception of bitter taste