Question

In: Biology

2. A trihybrid testcross yielded the following progeny: (10pts) a+c / abc 21

2. A trihybrid testcross yielded the following progeny: (10pts)

a+c / abc 21 +bc / abc 77

abc / abc 400 a++ / abc 71

ab+ / abc 1 +++ / abc 402

+b+ / abc 27 ++c / abc 1

Total = 1000

Which genes if any are linked?

b. If the genes are linked what is the gene order?

c. If any genes are linked what are the map units between the them? Include coefficient of coincidence and interference.

Expert Solution

The cross mentioned is a trihybrid test cross. This means that one of the parents is homozygous recessive for all the three loci. A typical test cross should produce all possible genotypes in equal proportion, if the genes are not linked. But here, there is a huge difference in number of individuals between each type of genotype. Hence, we can confirm that all the three genes are linked. The genotype of recessive parent is abc/abc.

From the data, we know that there are 8 types of offsprings produced. Of the 8 types, 2 stands out as the most dominant ones. These two are called parental types. The parental types are :

abc/abc (with 400 individuals) and +++/abc (with 402 individuals).

From this observation, we can conclude that the genotype of the second parent is +++/abc (i.e. all the three genes are in coupling phase).

Now, let's analyse the genotype with the least number of individuals. This genotype is called double recombinants. The double recombinants in this case are:

ab+/abc and ++c/abc (both with 1 individual each).

The speciality of a double recombinant is that it will have all the genes in place similar to the parental types except for the genes that exist in between the points of double crossing over or double recombination. With this idea, we can conclude that the gene 'c' is the one that is in between genes 'a' and 'b'.

The gene order is acb

Now let's rewrite all the offspring types according to the gene order.

acb/acb = 400 - Parental type

+++/acb = 402 - Parental type

+cb/acb = 77 - Recombinant type with recombination between a and c.

a++/acb = 71 - Recombinant type with recombination between a and c.

ac+/acb = 21 - Recombinant type with recombination between c and b.

a+b/acb and +c+/acb are double recombinants with one individual each.

From the above observations:

(A). All the three genes are linked.

(B). The genes are in the order : acb

(C) Map units between the genes:

Map units = Percentage of recombination

Map units between a and c :

Map units between a and c = percentage of individuals with recombination between a and c + percentage of double recombinants

= percentage of '+cb/acb' + percentage of 'a++/acb' + percentage of double recombinants

= [(77÷1000)+(71÷1000)+(1÷1000)+(1÷1000)]*100 = 15%

(Note: Double recombinants are considered as during the event of double crossing over also, there will be recombination in between a and c)

Hence map units between a and c = 15cM(centi morgan or map units)

Similarly,

Map units between c and b = percentage of individuals with ac+/acb + percentage of individuals with genotype ++b/acb + percentage of double recombinants

= 2.1%+2.7%+ 0.2% = 5%

Hence, map units between c and b = 5cM.

Map units between a and b:

Recombination occurs between a and b in three cases viz. crossing over between a and c, crossing over between c and b and the double crossing over producing double recombinants.

Hence map units between a and b = percentage of all the recombinant types including two times the double recombinants (because between a and b, there are two points for double recombination )

= (71+77+21+27+ 2+2)÷1000*100

= 20%

Map units between a and b = 20cM.

Coefficient of coincidence :

Coefficient of coincidence = observed double recombinants ÷ expected double recombinants

Observed double recombinants = 2

Expected double recombinants = (recombination percentage between a and c) * ( recombination percentage between c and b) = 5% * 15% = 0.75

Coefficient of coincidence = 0.2÷0.75 = 0.267

Coefficient of interference = 1 - coefficient of coincidence = 1 - 267 = 0.733

Coefficient of interference = 0.733

gladiator answered 2 years ago

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