Genetics Test Cross

​Genetics Test Cross

By Daryl Jones

​Understanding Genotypes Through Strategic Breeding

​What is a Test Cross?

​A test cross is a genetic cross between:

• An organism with an unknown genotype (showing dominant phenotype)

• An organism that is homozygous recessive for the trait

​Purpose: To determine whether the unknown organism is homozygous dominant or heterozygous

Why Use Test Crosses?

• Phenotypes can be deceiving! Dominant traits look the same whether the organism is homozygous or heterozygous

• Important in agriculture and breeding programs

• Helps predict offspring traits

• Reveals hidden recessive alleles

​Example: Tall Pea Plants

Given Information:

• T = Tall (dominant)

• t = short (recessive)

• Unknown plant is tall

• Genotype: TT or Tt?

​Test Cross Setup:

Cross the tall plant with a short plant (tt)

T? × tt

Scenario 1: If Unknown is TT

​Scenario 2: If Unknown is Tt

Multiple Choice

A black guinea pig (B is dominant for black, b is recessive for white) is test crossed with a white guinea pig. All offspring are black. What is the genotype of the black parent?

• BB

• Bb

• bb

• cannot be determined

Multiple Choice

In pea plants, tall (T) is dominant to short (t). A tall plant is crossed with a short plant, producing 52 tall and 48 short offspring. What is the genotype of the tall parent?

• TT

• Tt

• tt

• TTt

Multiple Choice

Why must a test cross always involve crossing with a homozygous recessive individual?

A

• Because recessive alleles are easier to observe

• Because the recessive parent can only contribute recessive alleles, making the unknown genotype visible in offspring ratios

• Because dominant alleles are harmful

• Because it produces more offspring

Multiple Choice

A brown rabbit (dominant) is test crossed. The offspring ratio is 1 brown : 1 white. What was the genotype of the brown parent? (B = brown, b = white)

• BB

• Bb

• bb

• BBb

Multiple Choice

In cattle, polled (hornless, P) is dominant to horned (p). A polled bull is test crossed with several horned cows. If the bull is heterozygous, what percentage of offspring would be horned?

• 0%

• 25%

• 50%

• 100%

Multiple Choice

In tomatoes, round fruit (R) is dominant to pear-shaped (r). A test cross produces 89 round and 91 pear-shaped tomatoes. What is the genotype of the round-fruited parent?

• RR

• Rr

• rr

• not able to be determined