Test cross

A test cross is the mating of an organism whose genetic makeup is unknown with an organism whose entire genetic makeup for a trait is known to determine which genes are carried by the former. A dog with a black-coloured coat may be pure breeding, with two genes for black coat colour, or hybrid, with one gene for black coat colour and one for red coat colour in a breed where the gene for black coat colour is dominant over (suppresses the effect of) the gene for red coat colour. 

To determine which genetic endowment the dog possesses, it is mated to a red-coated dog, which invariably possesses only two genes for red coat colour. If all of the offspring are black, the black-coated parent must be pure breeding and have both genes for black coat colour; if some of the children are red, the parent must be hybrid and have one gene for black coat colour and one gene for red coat colour.

What is a Test cross?

The test cross was initially used as an experimental mating test to determine which alleles are present in the genotype. The genotype of an organism is its genetic makeup, and it reflects all of the alleles, or forms of the gene, that the organism carries. As a result, a test cross can aid in determining whether a dominant phenotype is homozygous or heterozygous for a particular allele.

Diploid organisms, such as humans, have two alleles for each genetic locus, or position, with one allele inherited from each parent. Different alleles do not always have the same external effects or phenotypes. In a heterozygous organism with two different alleles at a specific locus, one allele can be dominant and mask the effect of a second recessive allele. Recessive alleles only manifest their phenotype if an organism carries two identical copies of the recessive allele, indicating that the organism is homozygous for the recessive allele. This means that an organism with a dominant phenotype’s genotype can be either homozygous or heterozygous for the dominant allele. As a result, visually inspecting an organism with a dominant trait will not reveal its genotype.

Outcomes of the Test cross

An organism that exhibits the dominant trait may be homozygous or heterozygous for that trait. A test cross can be performed with an individual who exhibits the recessive trait because its genotype is known – homozygous recessive – to determine which is the genotype.

The cross can result in one of two outcomes:

• The dominant trait is present in all of the offspring. As a result, the unknown had to be pure or homozygous for the dominant trait.
• Half of the offspring have the dominant trait, while the other half have the recessive trait. As a result, the unknown had to be hybrid or heterozygous.

Crossing of Monohybrids

The origin experiment Mendel conducted to determine the genotype of a yellow pea is a typical example of a test cross. The alleles Y and y are used for the yellow and green versions of the allele, as shown in the image below. 

The yellow allele, Y, outnumbers the y allele. As a result, only the yellow allele is seen in the phenotype of an organism with the genotype Yy. Mendel had a yellow pea and was curious whether it was YY or Yy.

This was important to Mendel, and it is still important to many seed producers and farmers today. The plant that a seed produces determines its quality. If a YY plant is self-fertilized, all of its offspring would be yellow peas. 

There are many desirable traits to reproduce, and a homozygous plant is an obvious choice to do so with. In a dominant/recessive relationship, however, it is impossible to tell the difference between a homozygous dominant plant (YY) and a hybrid, or heterozygous plant (Yy). Both would yield yellow seeds. However, if the Yy plant self-fertilizes, there is a chance that an offspring with the (yy) genotype will be produced, resulting in green peas. Mendel wanted to get to the bottom of this once and for all, so he devised the following test cross.

Mendel crossed an unknown yellow pea, with a green pea, both of which were homozygous recessive (yy). The chart below depicts the test’s two possible outcomes.

Conclusion 

A test cross is the mating of an organism whose genetic makeup is unknown with an organism whose entire genetic makeup for a trait is known to determine which genes are carried by the former. The genotype of an organism is its genetic makeup, and it reflects all of the alleles, or forms of the gene, that the organism carries. As a result, a test cross can aid in determining whether a dominant phenotype is homozygous or heterozygous for a particular allele. A test cross can be performed with an individual who exhibits the recessive trait because its genotype is known – homozygous recessive – to determine which is the genotype.