PUNNETT SQUARE CHEAT SHEET
Below is a sampling of Punnett Square problems that you will be expected to solve. In
order to do this, you will also have to understand the meaning of the terms below.
๎ Genotype: The letters that make up the individual. E.g. TT or Tt
๎ Phenotype: The physical characteristics of the particular trait. E.g. Tall or short
๎ Dominant trait: Signified by capital letter-E.g. T. If the traits you are using are dominant or recessive, this
trait will "overpower" the recessive trait and will be expressed. E.g. Tt
๎ Recessive trait: Signified by small case letter-e.g. t. An organism with a recessive allele for a particular
form of a trait will have that form only when the dominant allele for the trait is not present
๎ Homozygous: Has same letters. E.g. TT or tt (same alleles for trait)
๎ Heterozygous: Has different letters. E.g. Tt (different alleles for trait)
๎ Purebred trait: Also known as true breeding. Individuals genotype is homozygous and will only make one
type of gamete. E.g TT will always produces T, and T. tt will always produce t, and t.
๎ Gamete: sex cells. Represented by letter N (meaning they are haploid-contain half the chromosomes
๎ P generation: The parental generation (Usually the first one in a genetic cross)
๎ F1 generation: The first generation of offspring from P generation (means first filial: Latin for "son")
๎ F2 generation: The second generation of offspring from P generation (means first filial: Latin for "son")
๎ Monohybrid Cross: Also known as a Single-Factor Cross. Only one trait is used in the genetic cross.
E.g. T=Tall, t=short. Example: Tt x Tt
๎ Dihybrid Cross: Also known as a Two-factor Cross. Two trait are used in the genetic cross. E.g. T=Tall,
t=short & B=Black fur, b=white fur. Example TtBb x TTBB
๎ Incomplete Dominance: One allele is not completely dominant over the other. There is a blending with
the heterozygous offspring. E.g. RR=Red, Rr=Pink, and rr=white
๎ Co-dominance: Both alleles contribute to the phenotype. Offspring will have combination of two alleles.
E.g. RR=Red hair, Rr=Roan (mix of red and white hairs-almost looks pink), and rr=white
๎ Sex-linked trait: Genes located on the sex-chromosomes called sex-linked genes. Usually found on the
X chromosome. X-linked alleles are always expressed in males because males have only one X
chromosome.
๎ Multiple Alleles: There are more than two-choices for the allele. Example is human blood group genes.
There are three possible alleles for this gene. IA, IB, and i. IA and IB are co-dominant. There are four
possible phenotypes: A, B, AB, and O.
๎ Genotypic ratios: The ratio of different genotype in the offspring from a genetic cross. E.g 1:2:1
๎ Phenotypic ratios: The ratio of different phenotypes in the offspring from a genetic cross. E.g 3:1
