Question

In maize colored endosperm [C] is dominant over colourless [c]; and full endosperm [R] is dominant over shrunken [r]. When a dihybrid of the ${ F }_{ 1 }$ generation was test crossed, it produced for phenotypes in the following percentage:
Coloured full - ${ 44 }\%$
Colorless full - ${ 7 }\%$
Colored shrunken - ${ 5 }\%$
Colorless shrunken - ${ 44 }\%$
From this data what will be the distance between two non-allelic genes:
(a) 44 units
(b) 5 units
(c) 7 units
(d) 12 units

Answer 1

Hint: During the dihybrid cross when the percentage of the recombinant phenotype is added this will give the overall percentage of recombinant and we know that one map unit is equal to ${ 1 }\%$ of recombinant.

Complete answer:
During a dihybrid cross in maize, the colored endosperm is dominant over colorless endosperm and also full endosperm is dominant over shrunken endosperm.
- Then the dihybrid of the ${ F }_{ 1 }$ generation was tested crossed it produces four phenotypes of the following percentage:
- Colored full (CR) - ${ 44 }\%$
- Colourless full (cR) - ${ 7 }\%$
- Coloured shrunken (Cr) - ${ 5 }\%$
- Colourless shrunken (cr) - ${ 44 }\%$
- The percentage of recombinant phenotype formed in ${ F }_{ 1 }$ generation shows the distance between genes under study.
- The higher the percentage of recombinant phenotype higher will be the distance between the two loci, that means that to loci present far apart
- Here, we have got two recombinant phenotypes:
Colourless full (cR) - ${ 7 }\%$
Coloured shrunken (Cr) - ${ 5 }\%$
- Therefore the overall percentage of the recombinant phenotype is ${ 7 }\%$
 + ${ 5 }\%$ = ${ 12 }\%$
- As we know that 1 map unit is equal to ${ 1 }\%$ of recombinants.
- Therefore, ${ 12 }\%$ of the recombinant is equal to 12 units.
So, the correct answer is '(d) 12 units'.

Note:
- The dihybrid cross is a cross of two separate traits with different alleles.
- The phenotype of the offspring tells us about the genetic makeup of the original parents.
- Recombinant phenotype is the phenotype of offspring that have different allele combinations of their parents.
- 1 map unit is equal to ${ 1 }\%$ of the recombinant.
- The higher the percentage of recombinant phenotype higher will be the distance between the two loci.