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
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.