test cross - a hybrid interbreeding individuals with special analyzer.The analyzer is chosen homozygous individuals, whose recessive trait was reflected in the phenotype.Test cross widely used in plant breeding and genetics to determine the genotype.
As is known, according to the laws of genetics, a recessive trait when monohybrid cross does not occur in the first generation.In the second generation, he had seen only one-sixteenth of the individuals.Recessive trait is stored in the genotype of individuals of the first generation, but they are not seen externally.Thus, individuals of the first generation having a recessive genotype phenotypically different from individuals Monohybrid for a dominant trait.Test cross is designed to obtain information about the presence or absence of a recessive trait in the genotype.
consider the mechanism for determining the genotype of the individual gray flies.Grey color is the dominant feature in relation to the black coloring of the insect's body.Biologist crosses gray fly with black and evaluates the resulting offspring.If all individuals have a gray color, then studied the fly had only dominant in the genotype.If half got flies have black color, it is possible to infer the presence of a recessive trait.
Incomplete dominance of each genotype corresponds to a phenotype.To find out, is there any special features observed in the result of incomplete dominance or independent feature, pledged Genetically, it is also necessary to conduct test cross.If the test result is an indication of incomplete dominance, then there will be further its "mixing" with the recessive trait analyzer.If the sign was an independent, he will be transferred intact to all individuals, if the test individual is homozygous or part of the animals, if it is heterozygous.For example, if crossed, pink and white flowers have turned light pink, there is incomplete dominance.If all the hybrids of the first order have pink flowers, then this feature is incorporated in allele as an independent and dominant over white.In this case, the individual is homozygous investigated.If part of the hybrids inherited the pink color of the petals, and the other part is transmitted white, the pink sign of an independent, dominant, studied individual carries the genotype both signs.
in practice to determine the genotype is not always enough to make a test cross.Example, falls into the category of exceptions, can be explained by one of three ways nonallelic gene interactions: epistasis, polymers or complementarity.
When epistasis gene expression of a pair of allelic genes suppressed outside the other allele pairs.Suppressors may incur as a dominant or recessive trait.As a result of epistasis by crossing homozygous individuals with red flowers (dominant) and homozygous individuals with white flowers (recessive) in the first generation of hybrids will only plants with red flowers, and the second - 3/4 hybrids will have a red color, 3 /16 - white and 1/16 inherits signs of other alleles (eg, yellow).
to explain the actions of the phenomenon of the polymer can give the following example.The flowers contain several heterozygous alleles carrying red signs (dominant) and white (recessive) colors of petals.The more hybrids of the second generation of recessive individuals, the whiter flower.In the presence of individuals with both alleles dominant trait, and without it, outwardly manifested something in between these traits.In determining the genotype through test cross polymer is easily confused with incomplete dominance.
When complementarity non-allelic genes complement each other and contribute to help form a new feature.
