Pedigree question

Question

What type of pedigree is it (autosomal dominant, x-linked dominant, autosomal recessive, or x-linked recessive)? Explain.

Answer 1

Hello, thank you for your question. I think we can approach this question in a process of elimination format.

In generation 1, a female has the phenotype and her son in generation 2 does not. If the trait were x-linked recessive, her son would have to have the phenotype because she would be carrying two copies in order for her to have the phenotype and she would have to pass this down to her son. Therefore, we can eliminate x-linked recessive.

In generation 1 we also see a male with the phenotype who has a son in generation 2 who also has the phenotype and a daughter who does not. If the trait were x-linked dominant, his daughter would have the trait and his son would not, since the son gets his x chromosome from his mother and the daughter would have to get an X-chromosome carrying the dominant gene from her father. We can therefore eliminate x-linked dominant.

In generation 2 we have two people with the phenotype having a child who does not have the phenotype. If the trait were autosomal recessive, then both members of generation 2 would have to be homozygous for the trait in order to be displaying the phenotype (this is by definition of a trait being autosomal recessive). Therefore, every child they would have would also have to display the phenotype because the only trait being crossed here is the trait of the phenotype. Since their child does not have the phenotype, the trait cannot be autosomal recessive.

The people in generation 2 are themselves both children of one person with the phenotype and one without. and therefore both members of generation 2 are likely heterozygous for the trait. This is why their child does not have the phenotype, despite the odds (3/4) being in favor of their child having the phenotype. By process of elimination and by this fact, we know that this pedigree must be reflecting an autosomal dominant trait.

I hope this helps you understand the answer to the question! If you need more assistance in understanding pedigrees or any other biology topic, please feel free to reach out and I'd be happy to help you with whatever questions you have in a private session.

Best of luck!

Brandon