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Genotypes and Punnett Squares

1. You have decided to breed Labrador dogs. You have just purchased several young adult dogs, and have made your first rounds of matings. Your dogs come in two colors, black and yellow. After the first litters of puppies have been born, you make the following observations:
Whenever you matted a yellow male to yellow female, all of the puppies came out yellow. When ever you mated a black male to a black female, in most cases all of the puppies came out black, but from one mating you got four black puppies and one yellow puppy.

1. Based upon these results, and assuming the simplest genetic interpretation, which of these two colors is dominant, and which is recessive?

2. Assign letter symbols for the two alleles of this color gene (yellow and black).

3. Explain why you came to this conclusion. Be explicit and specific about the clue (s) that you used, and what they mean.

4. Assuming that your hypothesis about the nature of the black and yellow color is correct, if the two black dogs with the yellow puppy have a lot of liters of puppies, what percentage of those puppies do you predict will be yellow? show your work.

In gerbils there are several genes which affect fur color. One of these genes has a wild type dominant allele for brown fur (C) and a recessive allele for a light reddish copper color fur (c). A second gerbil gene has two alleles, one (W) which causes gerbils to have white patterns in their fur (e.g. a white collar, white tip on the tail, white feet etc) and one (w) which causes gerbils to be a solid color all over. These two alleles have a pseudo-dominance relationship. Ww gerbils have white markings:ww gerbils are solid colored. WW gerbils die as embryos and are never born. There is a pedigree for a family of gerbils, showing the phenotypes for these two traits. As far as possible, fill out genotypes for all members of this family.

C> Tay Sachs disease is a human genetic disorder. This condition is caused by the recessive allele of a single gene. We'll use T for the dominant normal allele and t for the Tay Sachs allele.
1. Ellen and Edward have three children. The first two were fine, but the third child suffered from Tay Sachs disease, and died before the age of two. What is Ellen's genotype for this gene? What about Edwards's?
2. If Ellen and Edward decide to have a fourth child, what is the chance that this child will be normal? Show your work on this part.

3. Babies who suffer from Tay Sachs disease always die in the first few years of life; they neber reach reproductive age. So no person with Tay Sachs disease is ever a parent. Why doesn't this genetic disorder disappear from the human species?

d. Two pairs of twins were born that night within a few moments of each other, but somehow the staff forgot to put name tags on the babies, and now they don't know which babies belong to which parents. You need to help them out by analyzing the blood types of these four babies and the two couples who are anxiously waiting to take their new babies home with them.

Mrs Smith is blood type A and her husband is blood type B.

Mrs Abernathy is blood type O and her husband is blood type AB.

all four babies are girls.
Baby 1 is type O
Baby 2 is type A
Baby 3 is type B
Baby 4 is AB
Which two babies go home with Mrs Smith and which two go home with Mrs Abernathy. Explain how you figured it out exactly.

e. During all of the above another baby girl was born to Suzy Q. Suzy Q is not sure who the father of her child is. Look at blood types and color vision.
SuziQ has blood type A and normal color vision.
The baby girl has blood type O and is colorblind.
Man 1 has blood type O and normal color vision.
Man 2 has blood type AB and is color blind.
Man 3 has blood type B and is colorblind.
Are any of these men a reasonable candidate for this baby's father. Explain for each why he is or is not a possible father.

f. Ethyl and Egbert are starting their family. Ethyl has free earlobes, but her mother has attached earlobes. Ethyl is not able to roll her tongue. Egbert also has free earlobes, but he is able to roll his tongue. Egberts son from a previous marriage has attached earlobes and is a non-roller.
1. What are Ethyl's and Egberts genotypes for these two genes?
2. If Ethyl Egbert have a large family, what fraction of their children do you expect to have a phenotype (for these two traits) just like their mother Ethyl?
show your work.

g. Fruit flies normally have dull red eyes, but geneticists have identified quite a few genes which cause them to have different colored eyes. The very first of hese discovered was a gene with a recessive allele for white eyes. This gene is X linked. X linkage works the same in fruit flies as it does in mamals. We'll use the symbols W (dominant red eyes) and w (recessive white eyes for these two alleles.
1. you mate a red eyed female fly (whose male parent had white eyes) to a male with red eyes. What are the genotypes of these two flies?

2. Fruit flies have hundreds of offspring at a time. When you look at the offspring from this mating, what phenotypic ration do you expect to find? Don't forget that you are looking at the effects of an X linked gene, and don't forget to include your work.

h. Again in fruitflies, there is another eye color gene which is not sex linked. The recessive allele for this gene causes the flies to have brilliant red eyes (called cinnabar). The dominant normal allele is Cn, the recessive cinnabar allele is cn.
You perform a classic monohybrid cross with this gene. This means that you begin by mating ahomozygous normal fly with a homozygous cinnabar eyed fly (your parental generation). Their offspring are F1 generation. You allow the F1 flies to mate with each other to produce an F2 generation.
1. What is the genotype of the F1 flies? What is their phenotype?
2. When you examine the F2 flies, what phenotypic ratio do you expect to find for the cinnabar trait? show your work.
3. Compare the phenotypic ratio for this cross to the one you figured out from problem g. How are they different from each other.

i In snapdragons, one flower color gene has two alleles, one for red flowers (CR) and the other for white flowers (CW). These two alleles have an incomplete dominance relationship: the heterozygous plant has pink flowers. As you did in problem #h you set up a classic monohybrid cross for this trait. You mate two homozygous parents (parental generation) to produce an F1 generation, then you mate the F1 to each other to make an F2 generation.
1 what color flowers do the two parental plants have?
2 What color flowers do the F1 plants have?
3 Predict the phenotypic ratio for the F2 plants. Include your work.
4. Suppose that you planted an entire garden of pink, red and white flowering snap dragons. Every year, before the plants are mature enough to mate, your walk through your garden and pick every red flower and every white flower leaving only the pink ones to make pollen and mate with each other. For 10 years, no snapdragon with white or red flowers has been allowed to mate in your garden - only the pink ones. After the 10th year of mating, what phenotypic ratio do you predict that you'll find as the flowers emerge in the spring?

j. As was mentioned in the earlier problem, there are several genes in gerbils which affect their fur color. It gets pretty interesting to look at how these genes interact, since they all affect the same trait. When one trait is influenced by a number of different genes, that trait is called quantitative.

one of the genes which affects gerbil fur color has a dominant allele for brown fur (B) and a recessive allele for black fur (b)
Another fur color causes a condition called "dilute". This gene acts a bit like a partial albino - color is produced, but in much smaller than normal amounts. The full color allele (D) is completely dominant and the dilute allele (d) is recessive. A gerbil which would normally be brown, but who has the dd genotype will be tan in color. A dilute black gerbil is gray.

1. You mate a BBdd gerbil to a bbDD gerbil. What color are these gerbils?

2 What color are the hybrid offspring of this mating?

3. To your dismay, two of these BbDd gerbils escape from you and hide in the closet. By the time you recapture them the female is pregnant. What fraction of her babies do you predict will be tan in color? Show your work.

Solution Preview

A. Labs:
<br>1. The crosses between the black labs are more informative here, since a yellow puppy was produced by black parents. This would suggest that the yellow trait is recessive, since a recessive trait is "over-ruled" by the dominant trait.
<br>2. Since black is dominant, you can use:
<br>3. You never saw a black puppy from yellow parents, but you did get a yellow puppy from black parents. Develop this idea more thoroughly to answer this question.
<br>4. This question requires a Punnet square. If yellow is recessive, then both parents must have at least one yellow allele (y). But both parents are black, so they each must also have the black allele (Y).
<br>__| Y | y
<br>Y | YY | Yy
<br>y | Yy | yy
<br>1 out of every 4 puppies (25%) would be yellow (yy).
<br>B. Gerbils:
<br>You did not download the pedigree, but I can explain how it would be filled out. There should never be a WW genotype-these die before birth. All gerbils with white markings are Ww, all solids are ww.
<br>Brown fur is dominant and so can be CC or Cc, depending on the parents. Reddish copper fur will always be cc.
<br>C. Tay Sachs:
<br>1. This is similar to the lab question: no parent can be tt because no one with this genotype survives long enough to have children. Therefore, each parent must have at least one normal (T) allele. However, the third child must have the tt genotype. So both parents must also carry the mutant (t) allele.
<br>2. This requires a Punnet square again. As above, there is a 25% chance that the offspring will be homozygous for the ...