Genetic Terms You Need To Know
Gene: a sequence of DNA that encodes for a certain trait
Allele: one of two (or more) alternative forms of a gene
Dominant Allele: an allele that dictates the expression of a trait
(capital letter, ex: A)
Recessive Allele: an allele whose trait is masked by the presence
of a dominant allele (lower case letter, ex: a)
Genotype: genetic make-up of an organism (letter combination)
Phenotype: physical appearance of an organism (its outward
appearance)
Homozygous: both alleles in a gene pair code for the same trait
(ex: AA or aa)
Heterozygous: the two alleles in a gene pair that do not code for
the same trait (ex: Aa)
Sex Chromosome: the chromosome that determines the sex of
an organism (the X and Y chromosome)
 Autosome: any chromosome that is not a sex chromosome
Punnett Square: a chart which shows all possible gene
combinations in a cross of parents
Monohybrid cross: a cross between two individuals for one trait
(ex: Aa x Aa)
Dihybrid cross: crossing two different characteristics at the
same time (AaBb x AaBb)
 Genotypic Ratio: the number of times each genotype appears in
the offspring. Written from most dominant trait to the recessive.
 Phenotypic Ratio: the number of times each phenotype appears
in the offspring. Written from the dominant trait to the
recessive.