Crosses Involving Single Genes Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 21, 2011


A testcross is used to determine the genotype of an individual exhibiting a dominant phenotype because this individual could have either a homozygous or heterozygous genotype. The testcross parent is always homozygous recessive for all of the genes under consideration. The purpose of a testcross is to discover how many different kinds of gametes are being produced by the individual whose genotype is in question. A homozygous dominant individual will produce only one kind of gamete; a heterozygous individual will produce two kinds of gametes with equal frequency.


EXAMPLE 2.10 Consider the case in which a testcross is performed with a white-coat male guinea pig and a black-coat female of unknown genotype.

Conclusion: The black female parent must be producing only two kinds of gametes and therefore she is heterozygous dominant Bb.


In the backcross, the F1 progeny are mated back to one of their parents (or to individuals with a genotype identical to that of their parents). Sometimes "back-cross" is used synonymously with "testcross" in genetic literature. The testcross is different in that a recessive homozygote is always used as the testcross parent; this is not necessarily true in a backcross.

EXAMPLE 2.11 A homozygous black female guinea pig is crossed to a white male. An F1 son is backcrossed to his mother. This cross and backcross is diagrammed as follows, using the symbols ♀ for female and ♂ for male.
P: BB x bb
  black-coat female   white-coat male
F1:   Bb  
    black males and females  
F1 backcross: Bb x BB
  black-coat son   black-coat mother
Backcross progeny: 1/2 BB : 1/2 Bb   All-black offspring

Practice problems for these concepts can be found at: 

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