Joy just had twin babies (a boy and a girl). She is delighted, but can't decide who to share her happiness with as she doesn't know who the father of the babies is! She has had blood typing done on the children and on the men she suspects could be the father. Here are the results:

Joy--Type A

Baby boy--Type O

Baby girl--Type A

 

Ben--Type A Bob--Type B

Jerry--Type AB Frank--Type O

 

1. What is Joy's genotype? How do you know?

AO, because her son is OO and he must have gotten an O allele from mom.

2. Who of these men can you rule out as a potential father? Show with diagrams who you can and cannot rule out.

The daughter is not very helpful in this problem, because she is type A (could be AA or AO). We don't know if she got the A from mom or maybe she got an O from mom. Therefore, we don't know what she got from dad.

 

The boy's genotype is helpful. He must have gotten one O allele from mom, and if he is blood type O, we know that he has two O alleles. Therefore, he got the other O allele from dad. This means that any potential father must have an O allele.

 

Jerry is AB, so he has no O allele. Therefore, he can't be the father of the twins.

The other men all may have an O allele, so they are potential fathers.

3. Ben is worried about the potential ramifications of being named the father of these two babies, so he submits information that both of his parents have type AB blood. Does this get Ben off the hook? ExplainÉ

Yes, if Ben's parents were both AB, the only way Ben could be Type A is to be an AA genotype. So Ben doesn't have the O allele that we are looking for.

4. Bob thinks Ben has a good angle, so Bob submits information that both of his parents have type B blood. Does this get Bob off the hook? ExplainÉ

No, Bob's parents are B, but could be either of them could be BB or BO, leaving open the possibility that Bob is BO. He can't be eliminated as the father this way.

5. Of this group of guys, who might be the father?

Bob, Frank

6. Five years later, Joy's boy is in kindergarten and is diagnosed as being color blind! Colorblindness is due to an X-linked recessive allele. Joy thinks maybe she should check with the men again to see if any of them are color blind? She thinks that maybe one of them passed a gene on to her son that caused him to be colorblind and she could finally know who the father is. Should she check? ExplainÉ

No reason to bother checking the men. All her boy's X-linked alleles come from his mom.

7. After learning that the little boy was colorblind, the school nurse checked his sister. Surprise! The little girl was colorblind, too. Could this information be helpful to Joy in any way? ExplainÉ

Yes, this information is helpful. The girl must have two X chromosomes both with the colorblindness allele. One of these came from her mom (who was a carrier). The other came from her dad. Since her dad only has one X chromosome, he would be colorblind if his X chromosome carried this allele.

8. If both of your parents are farsighted (the "farsighted allele" is a dominant, autosomal allele), is there any chance that you could have normal vision? ExplainÉ

Yes, if both of your parents are heterozygous, there would be a 25% chance that you would be homozygous recessive (normal sighted). If either of your parents are homozygous at this gene, there is no chance that you would be normal sighted.

9. If both of your parents have very thin lips (the "thin-lip allele" is a recessive, autosomal allele), is there any chance that you could have broad lips? ExplainÉ

No, because both of your parents have the recessive phenotype, which means that they have two recessive alleles. They are both homozygous (ll) and could only have thin-lipped babies together!

10. John has six fingers (the polydactly allele is a dominant, autosomal allele). His wife has five fingers. They have already had one daughter with six fingers. Now baby #2 is on the way. What are the chances that their second child (a boy) will have the normal number of fingers?

I didn't mean to make this one quite so tricky, but since I did:

If John is Pp, there is a 50% chance that the next child will have more than 5 fingers (and the odds will be 50:50 every time this couple has a child). If John is PP, there is a 100% chance that the next child will have more than 5 fingers. The sex of the children doesn't matter because this is an autosomal gene, not a sex-linked gene.

11. Marcus' father has recently shown signs of Huntington's disease, which is due to the presence of an autosomal, dominant allele. His mother does not have Huntington's disease. What is the chance that Marcus will have Huntington's disease? Hint: there may be more than one answer here. Do you know everyone's genotype?

If Marcus' father has only one HD allele (Hh), then there is a 50% chance that he will pass this allele on to each of his children. If the father has two of these alleles (HH), then there is a 100% chance that he will pass one on to his children. If any child inherits a single Huntington's allele (H), they should develop the disease.

12. Margaret's brother was diagnosed with sickle cell disease (a double dose of the sickle-cell allele is an autosomal, recessive allele). Margaret is healthy, but wonders what the chances are that she is a carrier. Can you help her with this question? If Margaret is a carrier of the sickle cell allele (she has the sickle cell "trait"), what does she need to know about her fiancé?

The trick here is seeing that if a couple has a child with the sickle cell disease, the child is homozygous for the recessive allele. Since one of these alleles came from each parent, then each parent has at least one sickle allele. There is no information here to suggest otherwise (my bad), so it is safe to assume that Margaret's parents are both healthy. This means that they must both be heterozygous at this gene. So, do a cross between two people with these genotypes: Ss x Ss. There is a 50% chance that any of this couple's children would be a carrier.

 

"Extra for experts"--There are 4 possibile combinations of parental alleles (S from mom and S from dad, S from mom and s from dad, s from mom and S from dad, or s from mom and s from dad). Margaret is healthy. That means that one of these 4 possibilities can be eliminated. So what are the odds that she is a carrier now? 2/3 (66%)

 

It would be nice for Margaret to know if her fiancé carries this allele also, because two carriers can produce children with the disease. A very simple blood test could answer this question.

13. What is the genotype of a girl with normal color vision whose father was colorblind? If she marries a man with normal color vision, what types of vision do you expect their children to have (and in what proportions)?

If N = normal and n = colorblind, the girl is XN Xn (because she is a girl, she must have received an X chromosome from her father, who only had an Xn as a colorblind man). She has normal color vision, so her mom must have donated an XN to the egg.

 

If this girl marries a man with normal color vision, all of the daughters will have normal color vision (although half of them are carriers). Half of the sons will have normal vision and half will be color blind. Do this kind of problem using a square and remember to keep track of X and Y.

14. Suppose that you are a marriage counselor and a young woman comes to you for advice. She tells you that her brother has hemophilia, which is an X-linked, recessive trait. Her parents are both normal. She wishes to marry a man who had an uncle (his mother's brother) with hemophilia and wants to know the probability of the disease in possible children that she may have. What would you tell her, and how would you explain your conclusions.

This young woman's mother must have been a carrier of hemophilia to have a son with the disease. If both of the parents are normal, then they are XNY and XNXn. If you do a square with X's and Y's, you will see that half of the daughters should be carriers. So this woman has a 50% chance of being a carrier.

 

Now for the fiancé: He does not have hemophilia (I didn't say that explicitly, but I should have and this would have been a better question). If he carried the allele for hemophilia, I would have told you that he had hemophilia, having only one X chromosome. Your friend may not realize that sex-linked traits work this way, so you could explain that to her.

 

Finally, since the fiancé is XNY and she MIGHT be XNXn, should they worry about their future children? If she really is a carrier, there is a 50% chance that her sons will have hemophilia. Daughters will be fine, but might be carriers (50% chance).