Question

In a species of tropical fish, e colorful orange end black variety called montezuma occurs. When two montezu- mas, are crossed, 2/3 of the progeny are montezuma, and 1/3 are the wild-type, dark grayish green color. Montezuma is e single- gene trait, end montezuma fish are never true-breeding. a. Explain the inheritance pattern seen here end show how your explanation accounts for the phenotypic ratios given. b. In this same species, the morphology of the dorsal fin is altered from normal to ruffled by homozygos- ity for a recessive allele designated f Whet progeny would you expect to obtain, end in what proportions, from the cross of e montezuma fish homozy- gous for normal fins to a green, ruffled fish? c. Whet phenotypic ratios of progeny would be expected from the crossing of two of the montezuma progeny from part b?

Answer 1

Answer:

a. Montezuma phenotype is given by a dominant allele.

b. 8 MmFf : Montezuma and normal fins (1/2) and 8 mmFf:  green body and normal fins (1/2)

c. 9/16 Montezuma and normal fin, 3/16 Montezuma and ruffled fin, 3/16 Green body and normal fin, 1/16 Green body and ruffled fin.

Explanation:

a. Montezuma is a single-gene trait, and according to the description,  after a cross of two heterozygous fishes offspring showed these phenotypic ratios:

- 2/3 montezuma  

- 1/3 wild-type

If parentals are heterozygous and the vast majority of the progeny has the montezuma phenotype, it is probable that montezuma phenotype is related to a dominant allele. If M is montezuma allele and m wild-type, this cross can be seen in the first Punnett Square, and the progeny is:

- 1/4 MM Montezuma

- 2/4 Mm Montezuma

- 1/4 mm wild-type

These proportions are the same that reported ones. Therefore, montezuma is inherited in a dominant way.

b. The fin phenotype is given by F/f gene,  where F: normal fin, f: ruffled fin. If a montezuma fish homozygous for normal fins (MmFF) is crossed with a green, ruffled fish (mmff), the progeny will be:

- 8 MmFf : Montezuma and normal fins (1/2)

- 8 mmFf:  green body and normal fins (1/2)

This cross is explained in the Punnett Square 2.

c. Montezuma fishes from part b are MmFf. They have montezuma body color and normal fins.

If two individuals MmFf are crossed, it will produce the following offspring (as it is shown in the final Punnett Square):

-  9/16 Montezuma and normal fin

-  3/16 Montezuma and ruffled fin

- 3/16 Green body and normal fin  

- 1/16 Green body and ruffled fin.