There are three recessive phenotypes in lizards red scales b

There are three recessive phenotypes in lizards (red scales, barbed tongue, and dull claws), each encoded by a different gene. You perform a three-point test cross with a heterozygous, wild-type lizard and recover the data below Complete the right-most column, identifying each phenotypic class as Parental, SCO, or DCO. Which gene is in the middle? How can you tell?

Solution

During the process of meiosis, the replication of chromosomes occur and form a tetrad. The chromosomes interact in the tetrad state and the DNA can have no cross over/parental, one cross over or two cross over. Three point test cross can be used to determine the order of genes on a chromosome and also can determine the distance between two each pair of genes.Three point test cross is a cross between triple heterozygote to triply recessive tester.

SCO is single cross over. A single cross over between linked genes generates recombinant gametes.

DCO is double cross over. A double cross over between linked genes generates parental gametes.

The genotype with highest number of progeny are no cross over, lowest number of progeny are double cross over.

In three point test cross mutatnt alleles are designated by small letters, wild type is designated by + sign.

Recombination frequency = recombinant progeny/ total progeny %

Parental

Observed progeny

Parental, SCO or DCO

Red,barbed,dull

145

SCO

Red,dull

99

SCO

Red, barbed

5

DCO

Dull, barbed

312

Parental (NCO)

Wild-type

134

SCO

Dull

2

DCO

Red

332

Parental (NCO)

barbed

103

SCO

Parental	Observed progeny	Parental, SCO or DCO
Red,barbed,dull	145	SCO
Red,dull	99	SCO
Red, barbed	5	DCO
Dull, barbed	312	Parental (NCO)
Wild-type	134	SCO
Dull	2	DCO
Red	332	Parental (NCO)
barbed	103	SCO