An alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. For example, the color of the pea seeds Mendel worked with was yellow or green. Hence, yellow and green are alleles at the same genetic locus. Sometimes there are more than two alleles at a given locus (multiple alleles).
A prenatal test used to determine whether a fetus has a genetic defect. Following ultrasound to determine a safe place to insert a needle into the amniotic sac to withdraw amniotic fluid, a sample of amniotic fluid is collected. The whole procedure takes about 45 minutes. The amniotic fluid, containing fetal cells, is then subjected to laboratory analysis for chromosomal or genetic ...