Phenotypically normal?

[fataliz]

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Came across this scenario and i couldn't understand why.

Male with hemophilia XhY crossed with female with color blindess AND hemophilia. XcXh.
What is the probability their daughter is PHENOTYPICALLY NORMAL?

i assumed it would be 0% since the daughter would either be XhXh or XhXc. EIther way, she has hemophilia. But apparently the model answer is 50% and the reasoning is that XhXc, is considered a normal Phenotype??? Thank your so much for reading.

 

[FoundAWay]

Both are x linked recessive.
XhXc is phenotypically normal, the Xh carries the good copy for color vision and the Xc carries the good factor VIII

 

[NextStepTutor_3]

Exactly. For X-linked recessive disorders, female heterozygotes will be carriers, but they will not actually display the disease phenotype. This is a tricky one because there are two disorders involved, but XhXc daughters technically have one allele that is "normal" at the hemophilia locus (even though it's associated with colorblindness) and one allele that is normal at the color vision locus (even though it's associated with hemophilia).

 

[sazerac]

Note that mom is a carrier for colorblindness and hemophilia. She is phenotypically normal too.

 

[fataliz]

Both are x linked recessive.
XhXc is phenotypically normal, the Xh carries the good copy for color vision and the Xc carries the good factor VIII

Hi. sorry i lost you. why is XhXc considered phenotypically normal? doesn't it contain 2 recessive alleles for 2 different traits?

i don't get your 2nd part too. you seem to be saying that they cancel each other out since they carry the trait that the other is lacking?

thank you for your time!

 

[fataliz]

Exactly. For X-linked recessive disorders, female heterozygotes will be carriers, but they will not actually display the disease phenotype. This is a tricky one because there are two disorders involved, but XhXc daughters technically have one allele that is "normal" at the hemophilia locus (even though it's associated with colorblindness) and one allele that is normal at the color vision locus (even though it's associated with hemophilia).

Hi. i understand that femal geterozygotes can be silent carriers without displaying the trait if they have a dominant allele in the corresponding homologous chromosome. i don't get the part where u say the hemophilia locus is considered normal when it is recessive? likewise for the color vision locus.

i realise i may have some wrong assumptions. do Xh and Xc share the same locus on the homologous chromosome? thanks for taking the time to reply me

 

[fataliz]

Note that mom is a carrier for colorblindness and hemophilia. She is phenotypically normal too.

hi. if the mom is XhXc, why wouldn't she display any traits since there are no dominant genes? wouldn't she have to express both recessive alleles? thanks!

 

[fataliz]

ok after reading a little more, i seem to realise where my problem is. females need 2 recessive genes of the SAME allele to display that recessive trait? if you have 2 different recessive alleles on the same locus of the homologous chromosome, the traits won't usually display, is that right?

 

[NextStepTutor_3]

Yes! The two traits are at different loci, which you can assume unless they tell you otherwise. (Specifically, a locus in broad terms just refers to the location of a specific gene. Since two genes can't be located at the same position, they exist at different loci.) But more importantly, I just wanted to clear up some confusion from one of your previous posts - the hemophilia locus isn't normal when it is recessive, it's normal when the other trait is recessive.

Here's how I like to think of it. Of course, the notation doesn't quite match what they give in the passage, but see if this helps.

Take this XhXc female. Those two "X's" refer to her two separate X chromosomes. The first, Xh, contains the disease allele for hemophilia but not that for colorblindness. We could therefore think of that first allele as "XhC" - recessive for hemophilia, dominant for color vision.

Similarly, the second allele (Xc) contains the disease allele for colorblindness but not that for hemophilia. We can think of this one as "XHc" - dominant for hemophilia, recessive for color vision.

Now we can see why the female is a carrier for both traits, but doesn't display them - she has that "normal" allele to compensate for each recessive one.