Question

QUESTION 7 4 po In a hypothetical cross of ABX a b, the observed frequencies of the tetrad classes are 0.60 parental ditype. 0.38 tetratype, and 0.02 nonparental ditype. What is the corrected map distance between the a and b loci? 40 27.5 0.12 25 None of the above

Answer 1

When two loci are considered the tetrads can be classified as the parental ditype (PD), nonparental ditype (NPD) and tetrtype (TT). In PD, all cells produced are similar to the parents (parental type). In NPD, all cells produced are different from both the parent (recombinant). In TT, half the cells are like parent and half are different from parent. There are two conditions, either No crossing over (NCO) will takes place, or when crossing over occur. When crossing over occur, it can be of 4 type, Single cross over (SCO) between 2 strands, double corss over (DCO) between 2 strands, two DCO between 3 strands and DCO between 4 strands. Below figure shows the crossing over pattern and which cross over will produce what type of tetrad.

NCO > PD - SCO II | 2 Steamp t involved — DLO II/ 3 strand I l involved, Dco y I 2 Strand į involved / - DCO III 3 Strand I involved to * * TH(com 0603 ned further O Dco » NPD (4 strand involved

Map distance = (total recombinant events / total tetrads) x 100 = [(1/2[SCO] + DCO) / total tetrads] x 100
Map distance = ([1/2 (TT – 2 NPD) + 4 NPD]/ total tetrads) x 100
Map distance = (1/2 TT + 3 NPD) / total tetrads x 100

According to question,

TT = 38; NPD = 2; total tetrads = 100 (38 + 60 + 2)

So,

Map distance = [ (1/2 × 38) + 3 × 2 ]/ 100 × 100

= (19 + 6) / 100 × 100

= 25 map unit