Monohybrid Crosses

These crosses involve one pair of contrasting traits. There are 5

different types which we will discuss.

1. Homozygous x Homozygous

• Ex. 2 pea plants are pure for the characteristic of plant height. One is short (homozygous recessive) and one is tall (homozygous dominant). What would be the genotype and phenotype ratios of the resulting offspring?

Givens:

• Alleles T = tall t = short

• “male” parent: TT

• “female” parent: tt

2. Homozygous x Heterozygous

• Ex. One pea plant is homozygous for the tall trait and another is heterozygous. What would you expect the genotype and phenotype ratios to be for the resulting offspring?

Givens:

• Alleles: T = tall t = short

• “male” parent: TT

• “female” parent: Tt

3. Heterozygous x Heterozygous

• Ex. Now cross 2 pea plant that are heterozygous for the height trait.

• Givens:

–Alleles: T = tall t = short

–“male” parent: Tt

–“female” parent: Tt

4. Test Cross

• This is a procedure in which an individual of unknown genotype and a dominant phenotype is crossed with a homozygous recessive individual. There are 2 possibilities…

• Alleles G = green g= red• A. One unknown possibility of

“male” parent: GG• “female” parent: ggOR

B. One unknown possibility of “male” parent: Gg

“female” parent: gg

5. Co dominance (Incomplete dominance)

• When neither of the 2 alleles act as a dominant trait but rather result in a phenotype that is intermediate between the 2.

• Ex. In a flowering plant called the Japanese Four O'clock, the color of the flower can be red, white, or pink. The 2 alleles present are R= red and R’= white. Neither of these alleles are dominant. What would the result of crossing 2 pink-flowered plants be?

Givens:

• RR= red

• RR’= pink

• R’R’= white

• “male” and “female” parents are both RR’