Question

Two-Point Mapping

The general strategy used to determine the genetic distance between linked genes is to: 1. Make all of the genes of interest heterozygous in an individual or strain 2. Examine the gametes produced in the heterozygous individual(s) Using our example above. You have two pure-breeding lines of plants, one with green seeds and short height (GG tt) the other with yellow seeds and tall height (gg TT). You cross the two strains to make the genes heterozygous. What type of a cross would you perform to determine the types of gametes being produced by the F1 heterozygous individuals?   

1. In all of the crosses below, draw two horizontal lines representing the homologous chromosomes of each individual and the genes on each chromosome. Show the P generation cross. Show the gametes produced by each parent strain (show the chromosome in the gamete)

2. Show the F1 double heterozygotes (show the chromosome lines) and the tester stock that you would use in the testcross.

3. In your F1 double heterozygotes. a) Which gametes will be formed if crossing-over does not occur? These gametes are referred to as parental or non-recombinant gametes. Why? b) Which gametes will be formed if crossing-over does occur between the genes? These gametes are referred to as non-parental or recombinant gametes. Why?

Answer 1

Two pure breeding lines are-

GGtt and ggTT

Gametes are- Gt and gT

F1 - GgTt.

to determine the types of gametes produced by heterozygous individual they are crossed with pure recessive for these two genes ggtt (testcross).

Q.1 (see figure 1)

Figure1

Q.2 (see figure 2)

Figure 2

Q.3 (See figure 3)

figure 3

If crossing over does not occur then the gamets are called parental type because it has a similar arrangemnt of the genes like the parents.

But is crossing over takes place then the gamets called recombinant type. In this case the chromosome having the gene arrangenment which is not similar to the parents (non parental). This new arrangement is formed by recombination(crossing over) and hence they called recombinant type.