Model Answer:

A genotype refers to the genetic composition of a cell or organism, which can vary based on the nucleotide sequence in the region(s) of interest. Although there can be many different forms (alleles) of a gene or sequence, each cell or individual normally has only two copies of each sequence, and they can either be identical to one another (homozygous) or different from one another (heterozygous). A phenotype of a cell or organism refers to its observable features, which are determined in part by its genotype, but can also be influenced by environmental factors. Some phenotypes are easily seen and vary widely among individuals, like natural hair color or height, but a phenotype may also involve less easily observable characteristics like blood type, which fall into a few distinct groups.

Model Answer:

Beneficial mutations are those that provide some sort of advantage to the organism in its environment, such as the Δ32 allele of the CCR5 gene which can reduce HIV infection rates in exposed individuals and delay progression to AIDS after HIV infection has occurred. The PiZ allele of the gene encoding α1AT is an example of a harmful mutation because it results in reduced levels of α1AT which, in the homozygous state, can lead to the development of emphysema due to increased breakdown of elastin in the lungs. Neutral mutations provide neither benefit nor harm to the individual, such as the AVI allele present in PTC non-tasters since the inability to taste this chemical does not affect the survival or reproductive capacity of the individual.

Model Answer:

Designating a mutation as simply beneficial, harmful or neutral does not account for the influence of the environment on the organism, which may determine how the mutation ultimately affects the individual’s fitness. For example, when an individual is heterozygous for the S allele of β-globin, they may have mild symptoms of blood disease (which would be considered harmful), but these individuals are also partially protected from malaria, which is highly beneficial for individuals in regions where malaria is widespread.

Model Answer:

Differences in the amount of DNA or point mutations that alter restriction enzyme recognition sites are two types of differences in DNA sequences that can be used to uniquely identify an individual. Variable number tandem repeats (VNTRs) are generated by the repetition of short stretches of DNA one after another in a particular region of a given chromosome. The size of the fragment generated by PCR amplification of these regions can reveal differences in the number of repeats within the region of interest. Point mutations in a DNA sequence can also introduce or disrupt a recognition site for a restriction endonuclease, so the pattern of restriction fragments that are generated by digestion of the region of interest can be used to determine whether a polymorphism exists in that region. Since each of these types of polymorphisms occur at many different regions within the genome, the combination of the alleles at several positions can be used to distinguish the DNA of one individual from another.

Model Answer:

A SNP is a single nucleotide polymorphism, or in other words, a difference between two sequences at a single nucleotide position within a DNA sequence (point mutation). By definition, SNPs must be common enough to be found at least once in a random sample of 50 individuals, but the effect of a given SNP on phenotype can vary widely. Some SNPs are neutral, whereas others are beneficial or can increase disease risk. SNPs that correlate with disease risk or incidence are of particular interest to researchers as a potential way to identify those that are at-risk for particular diseases before the onset of symptoms so that preventative treatment or early medical interventions can be initiated.

Model Answer:

DNA microarrays can be used to detect both SNPs and copy number variation (CNV). This technology relies on the ability of complimentary DNA sequences to hybridize to one another under favorable conditions. To detect SNPs, a microarray chip is covered with short, single-stranded DNA oligonucleotides that, in different regions of the chip, differ from one another by a single nucleotide to represent all four possible SNPs that might be present at one particular position within the sequence. The chip is then incubated with single-stranded, fluorescently-labeled DNA from one individual, which will hybridize to the chip only where the sequence is exactly complimentary. The pattern of fluorescence on the chip can then be used to determine which allele(s) of the SNP are present in individual samples. CNVs are detected in a similar manner except that the oligonucleotides adhered to the chip represent different regions of a larger stretch of DNA and the intensity of the signal for each region reflects the number of copies of that sequence that are present in the sample DNA.

Model Answer:

Model Answer:

Individuals with Down syndrome have three copies of chromosome 21, so they have an additional copy of the genes and elements present on this chromosome. Although the exact gene(s) responsible for this syndrome are still unknown, the increased dosage of chromosome 21 results in a group of traits which are characteristic of Down syndrome including mild to moderate mental disability, short stature, unique facial features and shorter life expectancy.