TEST CROSS AND THE BACKCROSS

TEST CROSS AND THE BACKCROSS

7.  TEST CROSS AND THE BACKCROSS

A test cross is a cross between an organism that is heterozygous for one or more genes(for example, Ww) and an organism that is homozygous for the recessive alleles (for example, ww). Because the heterozygous parent is expected to produce W gametes and w gametes in equal numbers, where as the homozygous recessive parent produces only w gametes,  progeny are 1:1 with the genotype Ww and genotype ww. The former have the dominant phenotype because W is dominant over w, and the latter have the recessive phenotype. A test cross is often extremely useful for genetic analysis. Test crosses are used to test an individual's genotype by crossing it with an individual of a known genotype.
7.1.    Monohybrid test cross 
Monohybrid test cross is a cross between an organism that is heterozygous for one gene and a second organism that is homozygous recessive for one gene.


7.2.    Dihybrid test cross: Dihybrid test cross is a cross between an organism that is heterozygous for two genes and a second organism that is homozygous recessive for two genes.


A test cross is a way to explore the genotype of an organism. Earlier use of the test cross was as an experimental mating used to determine which allele are present in the genotype. To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross is examined. If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele. If the test cross results in only phenotypically dominant offspring, then the parent organism is homozygous dominant for the allele.


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