Brent Cornell

Key Knowledge: Predicted genetic outcomes for a monohybrid cross and a monohybrid test cross

A genetic cross is a means of determining the traits of potential offspring based on the genetic characteristics of the prospective parents

• A monohybrid cross determines the genotypes (and resulting phenotypes) of potential offspring for one gene only
  

Monohybrid Cross

Monohybrid crosses can be calculated according to the following steps:

• Step 1:  Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)
• Step 2:  Write down the genotype and phenotype of the prospective parents (this is the P generation)
• Step 3:  Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)
• Step 4:  Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)
• Step 5:  Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F₁ generation)

Test Cross

A test cross involves mating an unknown genotypic individual with a known homozygous recessive

• This is because recessive alleles will always be masked by the presence of dominant alleles
• Hence the phenotype of any offspring will reflect the genotype of the unknown parent

Test crosses can be used to determine whether a dominant phenotype is homozygous or heterozygous

• If the unknown parent is homozygous dominant, all offspring will express the dominant phenotype
• If the unknown parent is heterozygous, half the offspring should be dominant and half recessive