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104 The Test Cross
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a particular trait. The principle is simple. The individual with Key Idea: If an individual's genotype is unknown it can be the unknown genotype is bred with a homozygous recessive determined using a test cross. individual for the trait(s) of interest. The homozygous recessive It is not always possible to determine an organism's genotype can produce only one type of allele (recessive), so the by its appearance because gene expression is complicated offspring phenotypes will reveal the genotype of the unknown by patterns of dominance and by gene interactions. The test cross was developed by Mendel as a way to establish the of aparent The basis Test (below). Cross A test cross can be used to determine the genotype of single or multiple genes in an individual. genotype of an organism with the dominant phenotype for Parent 1 Unknown genotype (but with dominant traits)
PR E O V N IE LY W
Parent 2 Homozygous recessive genotype (no dominant traits)
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X
a
b
a
b
The common fruit fly (Drosophila melanogaster) is often used to illustrate basic principles of inheritance because it has several genetic markers whose phenotypes are easily identified. Once such phenotype is body colour. Wild type (normal) Drosophila have yellow-brown bodies. The allele for yellow-brown body colour (E) is dominant. The allele for an ebony coloured body (e) is recessive. The test crosses below show the possible outcomes for an individual with homozygous and heterozygous alleles for ebony body colour. A. A homozygous recessive female (ee) with an ebony body is crossed with a homozyogous dominant male (EE).
E
B. A homozygous recessive female (ee) with an ebony body is crossed with a heterozygous male (Ee).
Female gametes
Female gametes
e
e
e
e
Ee
Ee
E
Ee
Ee
e
ee
ee
Male gametes
Male gametes
E
Cross A:
(a) Genotype frequency:
Ee
Ee
100% Ee
(b) Phenotype frequency: 100% yellow-brown
Cross B:
(a) Genotype frequency:
50% Ee, 50% ee
(b) Phenotype frequency: 50% yellow-brown, 50% ebony
1. In Drosophila, the allele for brown eyes (b) is recessive, while the red eye allele (B) is dominant. How would you set up a two gene test cross to determine the genotype of a male who has a normal body colour and red eyes?
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2. List all of the possible genotypes for the male Drosophila:
(a) What is the genotype of the male Drosophila? (b) Explain your answer:
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3. 50% of the resulting progeny are yellow-brown bodies with red eyes, and 50% have ebony bodies with red eyes.
A-3
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