A test cross is a way to determine whether an organism that expressed a dominant trait was homozygous or heterozygous; backcross is the mating between parent and offspring to preserve the parental genotype; P represents parent, F1 (filial 1) represents the children of the parent and F2 represents the children of the F1.

In genetics, dominant alleles are assigned capital letters (e.g., AA), and recessive alleles are assigned lowercase letters (aa). If both copies of the allele are the same, that individual is said to be homozygous. If they are different, the individual is heterozygous (Aa, aA). The parent or P generation refers to the individuals being crossed. The offspring are the filial or F generation; F₁ or the first filial represents the children of the parents while the F₂ represents children of the F1 or grandchildren of the parents.

Homozygous and Heterozygous Genotypes. Two dominant alleles (BB) or two recessive alleles (bb) are homozygous. One dominant allele and one recessive allele (Bb or bB) are heterozygous.

A test cross can be performed to determine whether an organism expressing a dominant trait is homozygous or heterozygous. Meaning, this is a way to tell what hidden genes are carried, but not shown. In a test cross, if you want to find out if the dominant trait is homozygous or heterozygous, it must be crossed with the homozygous recessive (bb). If the offspring is Bb, half the offspring will express the recessive trait. If it is BB, all offspring will express the dominant trait, and none will express the recessive trait. For instance, if the parent has blue eyes, and one of her children has brown eyes, then the parent is a heterozygous dominant. 

Because a test cross is used to determine the genotype of the parent based on the phenotypes of its offspring, test crosses are sometimes called backcrosses. The backcross is mating between the offspring and the parent to preserve the parental genotype.

 

Practice Questions

MCAT Official Prep (AAMC)

Biology Question Pack, Vol 2. Question 82

Sample Test B/B Section Passage 7 Question 38

Section Bank B/B Section Question 30

Practice Exam 4 B/B Section Question 58

Key Points

• A test cross is a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote.

• In a test cross, the dominant trait must be crossed with the homozygous recessive to know whether it is homozygous or heterozygous it. If the offspring is recessive, half the offspring will express the recessive trait. If it is dominant, all offspring will express the dominant trait, and none will express the recessive trait.

• The parent or P generation refers to the individuals being crossed; the offspring are the filial or F generation.

• F₁ or the first filial represents the children of the parents; F₂ represents children of the F₁ or grandchildren of the parents.

• The backcross is mating between the offspring and the parent to preserve the parental genotype.

Key Terms

Testcross: a cross between an organism showing a dominant trait and an organism showing a recessive trait to determine whether the former organism is homozygous or heterozygous for that trait

Filial: offspring, daughter or son

Phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

Genotype: the part of the genetic makeup of a cell or an individual which determines one of its characteristics (phenotype)

Dominant: a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the same locus

Homozygous: of an organism in which both copies of a given gene have the same allele

Heterozygous: of an organism which has two different alleles of a given gene

Recessive: an allele that can be covered up by a dominant trait, represented by a lower case letter