Question

1. Genes A, B, and C are each located on a different chromosome type. Predict the phenotypic ratio in the progeny of the test cross: AaBbCc X aabbcc. Assume that capital letters represent dominant alleles and lower case letters represent recessive alleles. Use the symbols “A”, “B”, & “C” to represent the dominant phenotype specified by genes A, B, & C, and the symbol “a”, “b”, & “c” to represent the recessive phenotypes.

2. If genes A & B are located on the same chromosome type, what process is it that allows an individual of genotype AaBb to produce four different gamete types?

Answer 1

1. The trihybrid testcorss will involve crossing of a trihybrid (AaBbCc) with triple recessive (aabbcc). Eight phenotypes will be formed as a result of test cross in the ratio 1:1:1:1:1:1:1:1

2. Linkage and crossing over

T.H.Morgan proposed that two genes are found in coupling pahse or in repulsion phase, because these were present on same chromosome (Coupling) or on two different, but homologous chromosomes (repulsion). Such genes are then called linked and the phenomenon is called linkage. The linkage is broken down due to the phenomenon of crossing over occurring during meiosis, which involves the exchange of chromosome segments.

The above illustration describes a cross between AB/AB x ab/ab that would give rise to a F1 hybrid (AB/ab). This dihybrid will then be crossed wiht double recessive parent ab/ab to get the testcross progeny. The formation of gametes from AB/ab in presence and in absence of crossing over is depicted in the example.