Biology 102: Genetics

DNA (Deoxyribonucleic Acid)

A double-stranded helical molecule composed of nucleotides that carries the genetic instructions for the development and functioning of living organisms.

Gene

A segment of DNA that encodes the instructions for making a specific protein or functional RNA molecule.

Chromosome

A structure of tightly coiled DNA and histone proteins found in the nucleus; humans have 23 pairs (46 total).

Allele

One of two or more alternative forms of a gene that arise by mutation and are found at the same locus on a chromosome.

Genotype

The genetic makeup of an organism, referring to the specific combination of alleles it carries for a particular trait.

Phenotype

The observable physical or biochemical characteristics of an organism, determined by its genotype and environmental influences.

Dominant Allele

An allele that expresses its phenotype even when paired with a different allele; typically represented by a capital letter.

Recessive Allele

An allele whose phenotype is only expressed when two copies are present (homozygous); masked by a dominant allele.

Homozygous

Having two identical alleles for a particular gene (e.g., AA or aa).

Heterozygous

Having two different alleles for a particular gene (e.g., Aa).