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An allele is a specific version or variation of a gene. While a single gene may provide the code for a general trait, such as hair color, the different alleles of that gene determine the specific variation of the trait that an individual expresses.

Alleles

A dominant allele is a variant of a gene that expresses its associated phenotype even when only one copy is present. An individual with at least one dominant allele, whether in a heterozygous (e.g., Bb) or homozygous (e.g., BB) state, will exhibit the dominant trait.

Dominant Alleles

A recessive allele is a variant of a gene that only expresses its associated phenotype when an individual is homozygous for that allele, meaning they have two copies (e.g., bb). The presence of a dominant allele will mask the expression of the recessive trait.

Recessive Alleles

The Punnett square shows different combinations of alleles that can be inherited from parents. Capitalized letters used in Punnet squares denote dominant alleles, while the lowercase letters denote recessive alleles. In this case "B" is a dominant allele and "b" is a recessive allele.

Punnett Square

The relationship between genes, alleles, and traits can be illustrated with hair color. A specific gene provides the code for the general trait of hair color, while the different alleles, or variations of that gene, determine the specific hair color an individual actually has.

Gene-Allele Interaction for Hair Color

An individual is described as homozygous for a particular gene when they possess two identical alleles for that gene. For instance, in the case of the cleft chin trait, genotypes BB (homozygous dominant) and bb (homozygous recessive) are both considered homozygous.

Homozygous

An individual is described as heterozygous for a particular gene when they have a combination of two different alleles for that gene. For example, if 'B' is the allele for a cleft chin and 'b' is for a smooth chin, an individual with the genotype Bb is heterozygous.

Heterozygous

While superficial traits like skin tone, eye color, and hair texture are determined by an individual's genetic makeup, these visible differences do not reflect the full scope of human genetic variation. In fact, scientific analysis of the human genome reveals that there is more genetic diversity within any single racial group than there is between different racial groups.

Genetic Diversity Within and Between Human Races

A single gene is responsible for determining whether a pea plant's flowers are purple or white. What accounts for the difference in flower color between two different pea plants?

A specific gene in a species of plant is responsible for determining the color of its flowers. Within this plant species, some flowers are red, while others are white. What term best describes the different variations of this single gene that result in either red or white flowers?

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