What genotype do you cross the unknown genotype with in a test cross?

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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

Testing an Unknown Dominant Phenotype

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

Testing for Gene Linkage

Test crosses can also be used to determine if two genes are linked or unlinked by mating with a known heterozygote

• If there is an equal ratio of the four potential phenotypes, the two genes are likely unlinked (independent assortment)
• If there are two phenotypes in high amounts and two phenotypes in low amounts (recombinants), the two genes are likely linked
• A chi-squared test for association can be used to determine the statistical likelihood of each scenario

Test crosses require large numbers of offspring to produce reliable data for meaningful conclusions

• With the advent of genetic screening and genome mapping, test crosses have become less commonly used

Test crosses operate under the same principle no matter whether you are considering one gene or multiple genes; in all cases, you are crossing an individual of a dominant phenotype but unknown genotype to an individual that is homozygous recessive for all relevant genes. Because the "tester" individual makes one known type of gamete, the ratios of phenotypes among the progeny of the cross indicate the type and frequencies of gametes made by the individual with the unknown genotype. Once you know the gametes that this individual produces, you can "reconstruct" the individual's genotype.

Consider again the fruit fly Drosophila melanogaster, and recall that the ebony-body allele (e) is recessive to the normal yellow-body allele (E), while the brown-eye allele (b) is recessive to the normal red-eye allele (B). If you are given a male with a yellow body and red eyes, how can you determine its genotype?

In this example, there are now four possible genotypes that are associated with the dominant phenotype of yellow body/red eyes. These four genotypes can produce one, two, two, and four different gametes, respectively (Table 3). Moreover, in combination with the single gamete from the "tester" parent, these gametes will produce one, two, two, or four progeny phenotypes.

Table 3: Possible Male Gametes and Their Frequency

Now, say you carry out the test cross and obtain 400 progeny. You sort these progeny by phenotype and discover that you have 200 flies with a yellow body and red eyes, as well as 200 progeny with a yellow body and brown eyes. These progeny must have the genotypes described in Table 4.

Table 4: Offspring Phenotype and Genotype and Corresponding Parental Gametes

You know that the homozygous recessive tester parent produces only one type of gamete (eb). Thus, the yellow-bodied, red-eyed progeny must be heterozygous at both loci (EeBb) due to the receipt of an EB allele from the unknown parent. Meanwhile, the yellow-bodied, brown-eyed progeny must be heterozygous at the body color locus but homozygous recessive at the eye color locus (Eebb). This could only happen if the progeny received an Eb gamete from the individual with the unknown genotype. Thus, you can deduce that the fly with the unknown genotype produced two types of gametes, EB and Eb, in equal frequencies. This means that you can reconstruct the fly's genotype as EEBb (case 2 in Table 3).

In sum, a test cross is a device that can be used to infer the Mendelian alleles present in parental gametes based on the observation of offspring phenotypes. Specifically, the ratio of phenotypes in a set of offspring reveals missing information about one of the parent's genotypes. Test crosses may also be used to determine whether two genes are linked, as well as to determine the underlying genotype if an allele's penetrance is less than 100%.