Q15-7TYU - Campbell Biology

Question

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossover? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

Step-by-Step Solution

Answer

Fifty percent of the offspring obtained from crossover will exhibit recombinant phenotypes because A and B are located far from each other.

It is inferred from the results that the genes are unlinked as the number of recombinants produced is high.

Step 1: Recombinant frequency

The offspring that result from the crossing over of DNA (deoxyribonucleic acid) during meiosis is called recombinants. The frequency of recombinants is less than non-recombinants because crossing-over is a random process that occurs during meiosis.

The frequency of recombinants depends on the distance between the linked genes. The farther the genes are on the chromosome, the higher the chances of recombination frequency. 

Recombination frequency is a measure of recombinants considering the total offspring produced in a cross.

Step 2: Percentage of recombinants produced from crossover

The genotype of animal heterozygous for both A and B genes is AaBb. The gametes produced by this animal are AB, Ab, aB, and ab. The genotype of homozygous recessive is aabb. The gamete produced by this animal is ab.

When a heterozygous and homozygous animal is crossed (AaBb × aabb), four types of offspring are produced. The genotypes of the offspring produced are AaBb, Aabb, aaBb, and aabb. 

Out of the four types of progeny, AaBb and aabb are parental types, whereas Aabb and aaBb are recombinants. Thus, fifty percent of the offspring resulting from crossover are recombinants.

Step 3: Interpretation of the result

A and B genes are the same chromosome, and they are 50 map units apart which means they have a recombination frequency of 50 percent. A recombination frequency of 50 percent means that the two genes are unlinked. 

If A and B were on different chromosomes, then also the results would have been the same. This is because crossing-over between genes that are located far from each other would have occurred during meiosis. 

Fifty of the results obtained are recombinants. This confirms that the genes are unlinked as the percentage of recombination is high.