In a diploid species of plant, the genes for plant height and fruit shape are syntenic and separated by 18 m.u. Allele D produces tall plants and is dominant to d for short plant, and allele R produces round fruit and is dominant to r for oval fruit.

a. Identify the gametes for a plant with the genotype DR/dr, label parental and recombinants and give the frequency of each gamete type.
b. Give the same information for a plant with genotype Dr/dR

Answer and Explanation:

Crossing-over between two genes that are very close to each other in the chromosome is not that common. Crossing-over during meiosis occurs random positions all along the chromosome. Then, crossing-over frequency between two genes depends on the distance between them. A short distance between genes is a very little target for crossing-over to occur, which means that only a few of them will happen, compared with the number of events between genes that are more separated between each other.

To calculate the recombination frequency, we have to know that 1% of recombinations = 1 map unit = 1cm. And that the maximum recombination frequency is always 50%.  

The map unit is the distance between the pair of genes for which the recombination frequency is 1 in 100 meiotic products.

So, en the exposed example:

a.

Identify the gametes for a plant with the genotype DR/dr and label parental and recombinants

Gametes     DR parental type

                   Dr  recombinant type

                   dr  parental type

                  dR recombinant type

Give the frequency of each gamete type.

               18 map units = 18 % of recombination in total

                                    = % Dr + % dR

                                     = 9% + 9%

                100% - 18% = 82% of parental in total =

                                    = % of DR + % dr

                                    = 41% + 41%

The frequency for each gamete is:

DR 41%

dr 41%

Dr 9%

dR 9%

b.

Identify the gametes for a plant with the genotype Dr/dR

Gametes     Dr parental type

                   DR  recombinant type

                   dR  parental type

                   dr recombinant type

Give the frequency of each gamete type.

              18 map units = 18 % of recombination in total  

                                     = % DR + % dr

                                     = 9% + 9%

                100% - 18% = 82% of parental in total =

                                    = % of Dr + % dR

                                    = 41% + 41%

The frequency for each gamete is:

Dr 41%

dR 41%

dr 9%

DR 9%