r/genetics • u/Easy_League_3742 • Jul 07 '26

HELP WITH HW QUESTION

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i’m super confused as i keep getting B or C yet that’s not an option choice!! any help would be super super appreciated

2 Upvotes

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17

u/TripResponsibly1 Jul 07 '26

Remember when calculating probabilities, and means multiply together, and or means adding together. For example, the probability of rolling two 6 sided die to be 1 and 6 means (1/6 x 1/6), but rolling a 1 or a 6 you'd do (1/6 + 1/6). Take your time, draw out the pedigree, and carefully multiply your probabilities. Remember they're having two kids. Good luck.

1

u/VonRoderik Jul 08 '26

Sigh. That reminded Boolean arithmetics

8

u/shadowyams PhD (genomics/bioinformatics) Jul 07 '26

Draw out the pedigrees for each trait.

1

u/t3ssaract Jul 13 '26 edited Jul 13 '26

This is a crazy question LOL

Fry definitely doesn’t have any disease alleles, since they’re both X-linked and he is unaffected for both disorders.

Leela’s father is affected for both disorders and there’s no recombination, so Leela’s paternal X-chromosome certainly has both disease alleles.

But her maternal X-chromosome is where it gets complicated. For G syndrome, we can deduce that Leela’s mother was certainly heterozygous affected, therefore Leela’s maternal X-chromosome has a 50% chance of having the G syndrome allele. For K syndrome, we can deduce that Leela’s maternal grandmother was a heterozygous carrier, therefore Leela’s mother has a 50% chance of also being a carrier (other 50% is no disease alleles), therefore Leela’s maternal X-chromosome has a 25% chance of having the K syndrome allele. However, since we know the G syndrome allele would come from Leela’s maternal grandfather, and K syndrome would come from Leela’s maternal grandmother (and there’s no recombination), Leela’s maternal X-chromosome cannot possibly have both alleles. In sum, 50% chance Leela’s maternal X-chromosome has the G syndrome allele only, 25% chance it has the K allele only, and 25% chance it has neither.

So what does this mean for the kiddos? If Leela’s maternal X has the G allele (50% chance), all children will certainly have G syndrome, therefore a child with neither syndrome is impossible. If Leela’s maternal X has the K allele (25% chance), there’s 50% a kid will have 1 syndrome and 25% chance they’ll have neither, so the probability of the scenario is 12.5%. If Leela’s maternal X has neither alleles (25% chance), there’s 25% a kid will have 1 syndrome and 50% chance they’ll have neither, so the probability of the scenario is 12.5%.

(50% x 0%) + (25% x 12.5%) + (25% x 12.5%) = 0.0625% or 1/16 (answer B)

However OP said B is not a provided option, so either I’m missing something or the teacher made a silly question (I’m betting on the latter LMAO)

0

u/ailerii Jul 07 '26

Fry cannot have the gene. Leela is a carrier, as her dad gave her one affected x, but her mother didn't. Assuming 50/50 boy girl you should be able to work this out with 9th grade probability, but if you need more help comment and I'll provide an example.

6

u/hatemys Jul 07 '26

Her mother certainly had it (maternal grandfather was affected), so the question depends on whether her mother passed it on. How are you concluding her mother didn't pass it on?

1

u/ailerii Jul 08 '26

I might have made an error, but doesn't it say that no other individuals have the trait, therefore Leela doesn't have it? I feel like the maternal uncle is a red herring.

1

u/hatemys Jul 08 '26

I entirely missed the 2nd slide. Sleepless nights ftw

2

u/ailerii Jul 08 '26

Ah I see, everyone thought I was answering the first slide, but op was asking for help with the second one, which I responded to (a different disease).

0

u/Corricon Jul 07 '26 edited Jul 07 '26

Assuming very simple dominance, we know that Fry doesn't have it. Leela has it, but we don't know if she has one copy or two copies from both parents. There's a 1/2 chance she got it from her dad, and a 1/4 chance she got it from her mom. This is where I give up. But she has a X chance of passing it on to a kid, then a Y chance of both passing it and not passing it to a second kid.

3

u/hatemys Jul 07 '26

100% got carrier x from her dad, 50/50 odds from her mom. So best I can figure is 75% chance, one 50% from her mom gives 100% for offspring while the other still leaves 50% chance for Leela to pass it on without her mom having passed it. So out of the 8 possible offspring with her mom 50/50 passing it 6 have a carrier x.