Suppose a geneticist is using a three-point test cross to map three linked rabbi

Question

Suppose a geneticist is using a three-point test cross to map three linked rabbit morphology and behavioral mutations called si, sf, and Le. si is associated with the silky fur phenotype, and sf is associated with the short-footed phenotype. Both si and sf are recessive mutations with respect to wild type. lessthanorequalto is a dominant mutation that confers the lethargic phenotype. He first crosses true-breeding lethargic rabbits to true-breeding silky fur, short-footed rabbits. Next, he backcrosses the F1 progeny to silky fur, short-footed parents, and obtains the following results: The order of these genes is: first. (Write the genes as si, sf, and Le. Failing to do so will result in no points from this question.)

Explanation / Answer

Solution) Let's consider a random order of the given gene and try to understand the information given in the question;

We have the information that

"sisi" would give silky fur phenotype (recessive mode)

"sfsf" would give short footed phenotype (recessive mode)

"Le_" would give lethargic phenotype (dominant mode)

First cross true-breeding lethargic rabbits x true-breeding silky fur, short-footed rabbits

= si+si+ sf +sf+ LeLe / x sisi sfsf lele

F1 progenies would be (si+si sf +si Lele)

now, backcrossing F1 progenies to the true-breeding silky fur, short-footed rabbits

second cross = si+si sf +si Lele x sisi sfsf lele

now, there will be recombination in the first parent only, the second parent will always provide (si sf le gametes).

We see the highest number of offsprings from the Lethargy (si+ sf + Le ) and silky fur, short-footed (si sf le)

and the least number of offsprings from the short-footed lethargic (si+ sf Le) and silky fur (si sf+ le)

we know that the double crossover recombinant class of offspring is the least and differs in genotype only for the gene in middle.

now comparing the parental ( si+ sf + Le and si sf le) to the double crossover recombinants ( si+ sf Le and si sf+ le ), we observe that only the gene for short-footed (sf) is different. so the middle gene must be sf. rest two genes can be present on either side.

so the possible gene order would be "si sf Le" or "Le sf si"

Such that first gene- si/Le, second gene - sf, third gene- si/Le.

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