Question

Two genes are hypothesized to be linked. The map distance between the genes is thought to be 12 map units. Out of 1000 offspring produced by two heterozygous parents, predict how many would occur in each phenotypic category.

Answer 1

Answer:

Approximately the number of individuals with each phenotype is:

• RS-->0.75 x 1000 = 750
• rs--> 0.19 x 1000 = 190
• Rs--> 0.05 x 1000 = 50
• rS--> 0.05 x 1000 = 50

Explanation:

Available data:

• Two genes are hypothesized to be linked
• The map distance between the genes is thought to be 12 map units.
• Two heterozygous parents
• 1000 offspring produced

Let us say that the two diallelic genes are R (alleles R and r) and S (alleles S and s)

Heterozygous parents might have the following genotype RS / rs  

We know that there are 12mp between genes, which means that the recombination frequency equals 0.12 or 12%.

0.12 of the gametes are recombinants and 0.88 are parentals. There are two types of parentals and two types of recombinant, so 0.06 + 0.06 are recombinants while 0.44 + 0.44 are parentals. This is:

Cross:

Parentals)      RS/rs     x     RS/rs

Gametes)    RS (parental) 0.44

                   rs (parental) 0.44

                   Rs (Recombinant) 0.06

                   rS (Recombinant) 0.06

Punnet square)         RS           rs             Rs             rS

                      RS    RS/RS      RS/rs      RS/Rs        RS/rS

                      rs      RS/rs       rs/rs       Rs/rs          rS/rs

                      Rs     RS/Rs      rs/Rs       Rs/Rs        rS/Rs

                      rS     RS/rS        rs/rS       Rs/rS          rS/rS

F1) Genotypes:

• 1 RS/RS --> 0.44x 0.44 = 0.1936

• 2 RS/rs --> 0.44x 0.44 = 0.1936 x2 = 0.3872
• 2 RS/Rs --> 0.44 x 0.06 = 0.264 x 2 = 0.053
• 2 RS/rS --> 0.44 x 0.06 = 0.264 x 2 = 0.053
• 1 rs/rs --> 0.44x 0.44 = 0.1936
• 2 rs/Rs --> 0.44 x 0.06 = 0.264 x 2 = 0.053
• 2 rS/rs --> 0.44 x 0.06 = 0.264 x 2 = 0.053
• 1 rS/rS --> 0.06 x 0.06 = 0.0036
• 1  Rs/Rs --> 0.06 x 0.06 = 0.0036
• 2  Rs/rS --> 0.06 x 0.06 = 0.0036 x 2 = 0.0072

Phenotypes:

• RS-->0.7506 (RS/RS +  RS/rs + RS/Rs + RS/rS + Rs/rS)
• rs--> 0.1936 (rs/rs)
• Rs--> 0.0566 (rs/Rs + Rs/Rs)
• rS--> 0.0566 (rS/rS + rS/rs)

Now, to know how many individuals would occur in each phenotypic category, you just need to multiply it by 1000, that is the total number of individuals

• RS-->0.75 x 1000 = 750
• rs--> 0.19 x 1000 = 190
• Rs--> 0.05 x 1000 = 50
• rS--> 0.05 x 1000 = 50