Analyzing Pedigrees

[lilchu22]

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Hey everyone, I have a quick question on how to analyze pedigrees in determining if a particular disease is autosomal/sex-linked and dominant/recessive. I have two "strategies": First, I look at a mom who has the disease, and if a son doesn't, then the disease IS NOT sex-linked. Also, I'll see if the disease skips a generation. If it does, then the disease is recessive. Is there anything else that I'm missing? How can you be for certain that a hypothetical disease is sex-linked recessive?? I usually spend a lot of time drawing tons of punnett squares and would be willing to take on some advice! Thanks 🙂

 

[Doctor D]

If the mother is sex linked recessive (xx) and shows the phenotype then of coarse she will always pass it to a son (xY). The dad always determines the sex, Y means boy and X for girl. So if it is the son of an affected mother he also MUST show the disease. Yes, skipping a generation usually means recessive.

I think you have the concept down, just remember to think about where sex chromosomes come from. The Y chromosome is basically worthless and doesnt carry many genes. Therefore any abnormalities are usually associated with the X chromosome. So a girl whose mother shows a sex linked recessive disease, but whose father is normal will be Heterozygous (Xx) and not be affected. If the mom and dad had a son however he is screwed (xY). This is known as monozygosity I think... Since boys only have one X chromosome, it is always expressed whether dominant or recessive. Hope this helps 😀

 

[lilchu22]

If the mother is sex linked recessive (xx) and shows the phenotype then of coarse she will always pass it to a son (xY). The dad always determines the sex, Y means boy and X for girl. So if it is the son of an affected mother he also MUST show the disease. Yes, skipping a generation usually means recessive.

I think you have the concept down, just remember to think about where sex chromosomes come from. The Y chromosome is basically worthless and doesnt carry many genes. Therefore any abnormalities are usually associated with the X chromosome. So a girl whose mother shows a sex linked recessive disease, but whose father is normal will be Heterozygous (Xx) and not be affected. If the mom and dad had a son however he is screwed (xY). This is known as monozygosity I think... Since boys only have one X chromosome, it is always expressed whether dominant or recessive. Hope this helps 😀

ahhh, that last sentence makes everything so much easier to wrap my head around. thanks!