Question

A rare, X-linked recessive allele, e, results in a cleft in the iris of the eye. Its dominant counterpart, E, controls normal iris development. An extremely rare autosomal dominant allele, M, is responsible for a particular type of migraine headache, and the expression of its recessive counterpart, m, results in the absence of this type of migraine. A migraine-free mother with normal irises has a daughter who suffers from migraines and has a cleft iris.

Answer 1

a. Based on this information, what could be concluded about the possible genotypes of the mother, father and daughter?
b. What is the probability of this couple having a child without migraines and with normal irises?

So migrain-free mother with normal irises - EEmm
Daughter who suffers from migraines has a cleft iris - eemm

Answer 2

oh so it's XEXE not EE and XeXe but I'm clueless after that D:

Answer 3

1. List the possible genotypes for the mother and the daughter. You started in the right place.

The mother has normal irises so you know that she is NOT homozygous recessive for the e allele. That means that she can be either EE or Ee - not EE as you have her. You also know that she is migraine free, so she has to be mm. If she had a single dominant allele MM or Mm, she'd have headaches and she does not.

By contrast, the daughter has cleft irises so you know with absolute certainty that she is homozygous recessive for the e allele, ee. You also know that she does have headaches which means she has at least one and possibly two dominant M alleles. So she's either MM or Mm.

To summarize:

Mom: EEmm or Eemm
Daughter: eeMM or eeMm.

Clear?

Answer 4

ohh.. yup.

Answer 5

but then how do I find the father's genotype?

Answer 6

That was the first step. Identifying which genotypes the mother and daughter could have. Now you use what you know about inheritance to make more concrete inferences.

Focus on the fact that the daughter has two recessive "e" alleles. That means that she got one from whichever of Mom's two X chromosomes she inherited and the other from her father's X chromosome. That means that Mom MUST be heterozygous for that allele - Ee and not EE - and that Dad MUST have one Y chromosome and one X chromosome with the recessive allele "e" on it.

Does that make sense?

Answer 7

ohhh, yeah it does! Thank you!!

Answer 8

Now do something similar for the migraine allele, M.

You know that the daughter is either MM or Mm because she does have headaches. You know that the mother does not have headaches and is mm. You know that Mom gave at least one chromosome with that small m allele on it to the daughter. It doesn't matter which chromosome she gave, they both have the same allele. That means the daughter cannot be MM - instead she is Mm. Moreover, you know that the recessive m came from Mom. Which means she had to get the dominant allele M from her father.

Does that make sense?

Answer 9

yeah, ohh.. I guess small steps eh? It does make sense. Thank you so much!

Answer 10

is it still XEXe? or just Ee? If you get what i mean..

Answer 11

oh, never mind it's a dihybrid cross.

Answer 12

XEXe and Ee are the same thing. That's just notation: XEXe indicates that the allele is on the X chromosome. I prefer using one letter E and e to denote one allele and trust myself to just remember that it's on the X chromosome.

So you conclude that the genotypes of the mother and daughter are CERTAINLY as follows:

Mom: Eemm
Daughter: eeMm

Dad is a little more complicated. The eye allele is fine: you know that he has one Y chromosome and one X chromosome with a recessive allele on it. The migraine allele is not quite so fine. He could be MM or Mm - in either case he would still be capable of donating the necessary dominant M to his daughter.

So you have Dad: YeMM or YeMm.

To calculate the probabilities at the end of the problem, you set up the appropriate dihybrid crosses for Mom and Dad (one for each of Dad's possible genotypes) and calculate the probabilities for their different kinds of offspring.

Answer 13

An X chromosome can have two genes for the dad? o.o I thought the Y holds nothing.

Answer 14

No. Dad has one Y and one X. The Y can be thought of as a blank. The X holds one allele.

The problem says that the headache allele is on a different autosomal chromosome. That means that both Mom and Dad carry two different copies of it, and both parents could pass either one or the other on to their daughter.

Answer 15

ohhh, I think I get it.. so it's the homologous chromosomes that can cary two different copies?

Answer 16

which are the autosomal?

Answer 17

Both Mom and Dad have two homologous of each autosomal chromosome. Each of these two homologous chromosomes carries one allele for each gene.

That's in contrast to sex chromosomes X and Y. They loosely be thought of as homologous too, in as much as each person has two of them - women two Xs and men an X and a Y. They're not homologous in that the Y contains very few genes and their alleles don't correspond to those on the X.

Answer 18

ohhh.. so in this case the migrain alleles are not in the sex chromosome? I'm so sorry for asking too much question D:

Answer 19

or eye allele

Answer 20

is it possible for you to draw it, because maybe I can understand it more fully..

Answer 21

You're right. The eye alleles are on the sex chromosomes - but the migraine alleles are not.

It's not a problem to ask questions until you understand it, but I think it should probably be clear to you by now. And you should be feeling more certain about how to solve this kind of problem.

Answer 22

ohhh, okay now I get it. The eye alleles are on the sex chromosomes. OHH. Okay! Yeah I get it now, thank you! :)