When Gregor Mendel conducted his genetic experiments with pea plants, he observe

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When Gregor Mendel conducted his genetic experiments with pea plants, he observed that a trait’s inheritance pattern was the same regardless of whether the trait was inherited from the maternal or paternal parent. Mendel made these observations by carrying out reciprocal crosses: For example, he first crossed a female plant homozygous for yellow seeds with a male plant homozygous for green seeds, and then crossed a female plant homozygous for green seeds with a male plant homozygous for yellow seeds.

Unlike Mendel, however, Morgan obtained very different results when he carried out reciprocal crosses involving eye color in his fruit flies. The diagram below shows Morgan’s reciprocal cross: He first crossed a homozygous red-eyed female with a white-eyed male, and then crossed a homozygous white-eyed female with a red-eyed male.

Drag the labels to their appropriate locations to complete the Punnett squares for Morgan’s reciprocal cross.

Drag blue labels onto the blue targets to indicate the genotypes of the parents and offspring.

Drag pink labels onto the pink targets to indicate the genetic makeup of the gametes (sperm and egg).

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Part B - Experimental results: The F2 generation

In one of Morgan’s experiments, he crossed his newly discovered white-eyed male with a red-eyed female. (Note that all of the females at that time were homozygous for red eyes because the allele for white eyes had not yet propagated through Morgan’s flies.) All of the F1 flies produced by this cross (both males and females) had red eyes.

Next, Morgan crossed the red-eyed F1 males with the red-eyed F1 females to produce an F2 generation. The Punnett square below shows Morgan’s cross of the F1 males with the F1 females.

Drag the labels to their appropriate locations to complete the Punnett square for Morgan’s F1 x F1 cross.

Drag pink labels onto the pink targets to indicate the alleles carried by the gametes (sperm and egg).

Drag blue labels onto the blue targets to indicate the possible genotypes of the offspring.

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Part C - Experimental prediction: Comparing autosomal and sex-linked inheritance

You now know that inheritance of eye color in fruit flies is sex-linked: The gene encoding eye color is located on the X chromosome, and there is no corresponding gene on the Y chromosome.

Case 1: Eye color exhibits sex-linked inheritance.

Case 2: Eye color exhibits autosomal (non-sex-linked) inheritance. (Note: In this case, assume that the red-eyed males are homozygous.)

For each case, predict how many of the male and female offspring would have red eyes and white eyes.

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Case 1: Eye color exhibits sex-linked inheritance

1. If there were 100 female offspring, would have red eyes and would have white eyes.

2. If there were 100 male offspring, would have red eyes and would have white eyes.

Case 2: Eye color exhibits autosomal (non-sex-linked) inheritance

3. If there were 100 female offspring, would have red eyes and would have white eyes.

4. If there were 100 male offspring, would have red eyes and would have white eyes.

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First half of reciprocal cross Second half of reciprocal cross female male female male

Explanation / Answer

ans)

Part -a)

When Morgan crossed a homozygous red-eyed female with a white-eyed male, all offspring had red-eyes. However, in the reciprocal cross (homozygous white-eyed female with a red-eyed male.

All female - red-eyes

All male - white-eyes

w+ w+ (female) X   wY  (male) ---------->    w+ w (female)  , w+ y (male)

w w (female)   X w+ Y (male) ---------------> w+ w (female) , w y (male)

Part - b)

When a homozygous red-eyed female was crossed with the white-eyed male, the resulting F1 females : w+ w, Males : w+ Y.

Crossing the F1 Males with F1 females would yield these results:

All the F2 females would have red-eyes, although some would be homozygous (w+ w+) and others would be heterozygous (w+ w)

Half the F2 males would have red eyes ( w+ Y), and half would have white eyes (w Y)

Part -c)

Case 1: Eye color exhibits sex-linked inheritance

Females : 100% red eyes                                                   Males: 0% red eyes

                 0 % White eyes                                                          100% White eyes

Case 2: Eye color exhibits autosomal (non-sex) inheritance

Females & Males : 100% red eyes, 0 % white eyes

w+ Y w w+ w w Y w w+ w w Y

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