Genetic Screening

Genetic screening is the testing of an individual for the presence or absence of a gene. This is especially useful for individuals which show the phenotype of a dominant trait. The genotypes of such individuals may either be homozygous dominant or heterozygous.

1. Test Cross: A mating to determine the exact genotype of an individual who shows a dominant trait. Individuals are suspected of being heterozygous are mated with an individual who is homozygous recessive. If the individual of unknown genotype is indeed heterozygous, the offspring should be 50% dominant, 50% recessive. If the individual in question is really homozygous dominant, 100% of the offspring will show the dominant trait.

2. Karyotyping

3. DNA Profiling (DNA fingerprinting): Humans have short base sequences that are repeated many times. Such sequences are known as satellite DNA. The satellite DNA varies greatly from person to person in the number of repeats. Such DNA can thus be useful for identifying individuals.

DNA from an individual is extracted from their cells and the quantity of DNA is increased by a process known as PCR. The DNA is then cut by restriction enzymes (these enzymes only cut between certain base sequences). The cut DNA is then run through a process known as electrophoresis which uses current to separate the DNA fragments based on charge and size. The chance that 2 persons would have the same DNA fingerprints is less than a million, so this process is useful in identifying human remains as well as forensic (crime) investigations.