Question

Performing Monohybrid Crosses

A monohybrid cross is addressing the inheritance of one gene. Answer the following questions:

Question 1:

Drosophila melongaster, more commonly known as the fruit fly, can have red or white eyes. You breed a red-eyed fly to a white-eyed fly and get 50 offspring that all have red eyes. Which trait is dominant?

Question 2:

Consider an offspring from the previous question. Its phenotype is red eyes.

What is its genotype? (Circle one of the choices below)

Homozygous dominant, Heterozygous, or Homozygous recessive

Question 3: (2pts)

Next, you perform a cross of two heterozygous, red-eyed flies. They have 100 offspring. How many of these offspring do you predict will have red eyes? Show your work using a Punnett square. [Use letters R and r to designate the alleles]

Question 4:

Suppose you found a fly that had escaped and was flying around your lab. He has red    eyes, but you do not know his genotype. To find out, you cross him with a white eyed fly (since you know her genotype). The result is 50% white eyed and 50% red eyed offspring. What is the escaped fly’s genotype? Explain how you know.

X-linked Inheritance

Consider this imaginary animal, the tree rat. They are usually black, but sometimes they can be purple. The allele for the purple color is X-linked. You manage to capture two of these animals from the same family, a black female and her purple male offspring.

Question 5:

What is the genotype of the black female mother?

Question 6

Do you know the phenotype of the father based on the male offspring? Why or why not?

Question 7: (2pts)

Next you breed the tree rats to each other (inbreeding is acceptable in this case). What percent of the offspring do you expect to be purple? Are any of them female? Use a Punnett square to show your work. [Use the letters B (black) and b (purple) to represent the alleles.]

Answer 1

A monohybrid cross is addressing the inheritance of one gene. Answer the following questions:

Question 1:

Drosophila melongaster, more commonly known as the fruit fly, can have red or white eyes. You breed a red-eyed fly to a white-eyed fly and get 50 offspring that all have red eyes. Which trait is dominant?

If all the F1 flies are red eyed then it should be that red trait is dominant

Red eyes=R

White eyes=r

Red eyes (RR) x white eyes (rr)

F1= Rr (all the flies are heterozygous with red eyes)

Question 2:

Consider an offspring from the previous question. Its phenotype is red eyes.

What is its genotype? (Circle one of the choices below)

Heterozygous

The genotype is heterozygous since it has genes for both red eyes and white eyes. Since the red eye gene is dominant it expresses over the white eye gene.

Question 3: (2pts)

Next, you perform a cross of two heterozygous, red-eyed flies. They have 100 offspring. How many of these offspring do you predict will have red eyes? Show your work using a Punnett square. [Use letters R and r to designate the alleles]

Rr x Rr

	R	r
R	RR (Red eyes)	Rr (Red eyes)
r	Rr (Red eyes)	rr (white eyes)

Phenotypic ratio obtained is 3 red eyes and 1 white eyed flies

For 100 offsprings we expect 75 red eyed flies and 25 white eyed flies

Question 4:

Suppose you found a fly that had escaped and was flying around your lab. He has red    eyes, but you do not know his genotype. To find out, you cross him with a white eyed fly (since you know her genotype). The result is 50% white eyed and 50% red eyed offspring. What is the escaped fly’s genotype? Explain how you know.

Rr (red eyed fly) x rr (white eyed fly)

= Rr, Rr, rr, rr (50% white eyed and 50% red eyed offspring)

the escaped fly’s genotype is Rr (heterozygous)

RR is not possible as it would give rise to all the red eyed offsprings when crossed to white eyed fly. To illustrate RR x rr = Rr (all the flies are heterozygous with red eyes).