Question

When two loci are completely linked, the offspring observed in a dihybrid testcross in the F2 are:

a.) all the recombinant types

b.) all the parental types

c.) 50% of recombinant, and parental types

d.) 25% of recombinant types, and 75% parental types

e.) None of the above

Answer 1

Let R and T the 2 loci and are linked. This means R and T are 2 genes present on same chromosomes. Linked means when 2 chromosomes are present on same chromosomes means there map distance or recombination frequency is <50% or < 50 cM.

Alleles == R -- round and dominant

r -- wrinkled and recessive

T -- tall and dominant

t -- short and recessive.

In F1, we cross Homozygous dominant with Homozygous recessive. So, it will be round and tall with wrinkled and short.

RRTT X rrtt ----------- F1

Gametes = RRTT = RT

rrtt = rt

On crossing these Gametes we will get,

RrTt = all progenies, round and tall.

Now, in F2 we have to perform the test cross, test cross is performed with the recessive genotype with unknown dominant individual to predict it's genotype and here used for knowing the recombinants.

So, cross performed with F1 progeny with Homozygous recessive individual.

RrTt X rryy

Gametes = RrTt = RT, Rt, rT and rt

rrtt = rt

On crossing these we get,

	rt	phenotye	type
RT	RrTt	round tall	parental
Rt	Rrtt	round short	Recombinant
rT	rrTt	wrinkled tall	Recombinant
rt	rrtt	wrinkled short	parental
		4 phenotyes

The RrTt, round and tall and rrtt, wrinkled and short are the Parental type they are similar to the parent. They will have highest number of progenies similar to them.

Rrtt, round and short and rrTt wrinkled and tall are recombinants as they are different from the parents. Hence, a new kind or recombinants obtained while this cross.

So, accordingly the answer to the above given question is Option D, 75% of Parental and 25% Recombinant types.

Option D is correct as we obtain progenies in which the maximum number is contributed by the parental types. They can't be 50-50% as the frequency be 50% the genes are unlinked. To have linked genes recombination frequency should be <50 or less than 50. So, accordingly option c is wrong.

Options a and b are wrong as we performed the cross we obtain both parental and recombinant types and not all parental or all recombinants. So, both these options are wrong. Option e is wrong as option D is the best answer and follow the correct conclusion for recombinants and parental types.

Thank you...

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