[GHS IB Genetics]

Theoretical Genetics


Allele: One gene of a gene pair for a trait.

In the gene pair Bb for hair color, both B & b are alleles .

Genotype: The 2 alleles an organism possesses for a trait. (Its genetic makeup.)

Phenotype: The external appearance of the organism for a trait. (blue eyes, black hair, hitchhiker's thumb, etc...)

Dominant Allele: An allele that masks the presence of another allele for the same characteristic. Usually shown as a capital letter (Ex......A, B, G, T, etc....)

Recessive Allele: An allele that is hidden by the presence of a dominant allele for the same characteristic. Usually shown as a lower case letter (Ex......a, b, c, t, etc....)


(In eye color, brown eyes (B) is dominant to blue eyes (b). Individuals who are Bb have brown eyes. Blue eyed people can only be bb.)

Codominant Alleles: Pairs of alleles that both affect the phenotype (appearance) of the individual. Neither allele is completely dominant.

Example: In hair, straight hair (S) is codominant to curly hair (s). Individuals with the genotype Ss show the effects of both traits, and thus have wavy hair.

Locus: The particular position on a chromosome where a gene is.

Homozygous: Having 2 identical alleles of a gene (Ex: AA, bb, DD.....)

Heterozygous: Having 2 different alleles of a gene (Ex: Aa, Bb, Dd.....)

Carrier: an individual that has a recessive alele of a gene that does not have an effect on their phenotype.

Test cross: Testing a suspected heterozygote by crossing it with a known homozygous recessive.