A breeder wants to set up a program to produce zebrafish with long fins. Having long fins is a dominant trait compared to the short fin wild type. Before she can market her zebrafish, she needs to perform a test cross on the long-finned fish she plans to use as a breeder. How can she tell whether her fish are homozygous or heterozygous for the long-finned trait?

Question

Emilia Bass

Biology

13 March, 04:04

A breeder wants to set up a program to produce zebrafish with long fins. Having long fins is a dominant trait compared to the short fin wild type. Before she can market her zebrafish, she needs to perform a test cross on the long-finned fish she plans to use as a breeder. How can she tell whether her fish are homozygous or heterozygous for the long-finned trait?

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Kieran Kaufman · Answer 1

The breeder needs to perform a cross test and observe the offspring. If she finds zebrafish with short fins in the offspring, then her fish is heterozygous, while if all the offspring have long fins, then her fish is homozygous.

Explanation:

The phenotype (observable trait) is determined by the combination of two alleles. Individuals are homozygous if both alleles are the same, while heterozygous if both alleles are different. Alleles can be dominant or recessive. Dominant alleles determine the phenotype when they are present while recessive alleles determine the phenotype only in a homozygous individual. For example, if A represents the dominant alleles (long fins) and a represent the recessive alleles (short fins):

An individual with AA will have long fins.

An individual with Aa will have long fins.

An individual with aa will have short fins.

A test cross is performed to determine the zygosity for a specific trait. The test consists of breeding the interested individuals (that have long fins; AA or Aa) with homozygous recessive individuals (that have short fins; aa) and compare their offspring.

If the recessive trait is observed in the offspring, then the individual was heterozygous (Aa). On the other hand, if the recessive trait is not observed in the offspring, then the individual was homozygous (AA)

The probabilities of the cross are;

Heterozygous with recessive homozygous

Aa with aa = Aa, Aa, aa, aa. This means that half of the offspring will have long fins (Aa), and the other half will have short fins (aa).

Dominant homozygous with recessive homozygous

AA with aa = Aa, Aa, Aa, Aa. This means that all of the offspring will have long fins (Aa).