What is a testcross?

It is not always possible to determine what genes an organism is carrying by simply looking at its appearance. After all, gene expression is a complex process that is dependant on many environmental and hereditary factors. For example, Gregor Mendel’s experiments with pea plants showed how dominant traits can mask recessive ones, thus causing him to muse how “rash it must be... to draw from the external resemblances of hybrids conclusions as to their internal nature“ (Mendel, 1866). Today, scientists use the word “phenotype“ to refer to what Mendel termed “external resemblance“ and the word “genotype“ to refer to an organism’s “internal nature.“ Thus, to restate Mendel’s musing in modern terms, we cannot infer an organism’s genotype by simply observing its phenotype. Indeed, Mendel showed that phenotypic traits can be hidden in one generation, yet reemerge in subsequent generations. This occurs because some alleles are dominant over others, which means that their phenotype will mask the phenotype associated with the recessive alleles. Because of dominance, there is not a one-to-one correspondence between the alleles that an organism possesses (i.e., its genotype) and the organism’s observed phenotype. Consider, for instance, the genes that code for eye and body color in the fruit fly Drosophila melanogaster. In these flies, the brown-eye allele (b) is recessive to the normal red-eye allele (B). Similarly, the ebony body color allele (e) is recessive to the normal (yellow-brown) body color allele (E). Because ebony has 100% penetrance, a fly that has dark black body color has the homozygous genotype ee. However, a fly that has a normal body color may have the homozygous genotype EE or the heterozygous genotype Ee. Things get slightly more complex when considering two genes. For instance, a wild-type fly (with red eyes and a yellow body) has one of four possible genotypes: EEBB, EEBb, EeBB, and EeBb. There is no way to tell these genotypes apart visually, but there is a well-established experimental technique to determine the fly’s genetic makeup. Specifically, to detect the underlying genotype of an organism with a dominant phenotype, one must do a type of breeding analysis called a test cross. The test cross is another fundamental tool devised by Gregor Mendel. In its simplest form, a test cross is an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype). In order to understand how test crosses work, it helps to consider several examples, including those that involve just one gene of interest, as well as those that involve multiple genes. #testCross #phenotype #genes #genotypes #breeding #gregorMendel #homozygous #recessiveGenotype #Allele