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%  

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."