Question

In a plant, leaf color and leaf shape are controlled by two linked genes Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed, and a recessive mutation in this gene causes them to be smooth. The following crosses were performed
Cross 1: pure breeding white, smooth X pure breeding red, pointed gives: all red, pointed
Cross 2: red, pointed F1 X pure breeding white, smooth (test cross gives: 40 white, smooth 36 red, pointed 10 white, pointed 14 red, smooth hape?
What is the recombination frequency between the gene for color and for shape?

Answer 1

Solution :

Red is dominant over the white leaves and pointed is dominant over smooth.

Red = R ,    white = r

Pointed = P ,   smooth = p

Red, pointed x white,smooth ------ Parents

RRPP                 rrpp

RP                      rp  ----------------- Gametes

         RrPp    -------- $F_1$

  (red, pointed)

When this $F_1$ is test crossed,

RrPp  x  rrpp     ----  $F_1$ test cross

Gametes   → rp

RP ---------- RrPp - red, pointed - 36 parental type

Rp ---------- Rprp - red, smooth - 14 re- combination

rP ----------- rrPp - white pointed - 10 re- combination

rp ----------- rrpp - white, smooth - 40 - parental type

                                    Total =   100

As this ratio is deviating from 1:1:1:1, it indicated the two genes are linked.

Linkage strength = percentage of crossing over = map distance between the genes.

Percentage crossing = recombination frequency = percentage of recombination.

Recombination frequency $=\frac{\text{total no. of recombination}}{\text{total no. of progeny}}\times 100$

                                         $=\frac{14+10}{100} \times 100$

                                         = 24 %

                                        = 24 map units