Autosomal Recessive Disease & Sex Cells?!

[SaintJude]

Premise: Metabolic disorder A is inherited by an autosomal recessive pattern.

Question: Suppose that a mutation occurs in the DNA of a sperm cell that inactivates the gene for an enzyme involved in metabolic disorder A. Assuming the sperm successfully fertilizes an egg, which of the following events must occur to produce a child with metabolic disorder A?

The answer: A similar mutation occurs during oogenesis.

If this is this disorder is an autosomal recessive pattern, why does the mutation have to occur in oogenesis also?

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[MedPR]

Premise: Metabolic disorder A is inherited by an autosomal recessive pattern.

Question: Suppose that a mutation occurs in the DNA of a sperm cell that inactivates the gene for an enzyme involved in metabolic disorder A. Assuming the sperm successfully fertilizes an egg, which of the following events must occur to produce a child with metabolic disorder A?

The answer: A similar mutation occurs during oogenesis.

If this is this disorder is an autosomal recessive pattern, why does the mutation have to occur in oogenesis also?

What were the other answers?

 

[BA11]

If this is this disorder is an autosomal recessive pattern, why does the mutation have to occur in oogenesis also?

Because of the premise that it is recessive. If the fertilized egg has only one mutated copy of the gene, the dominant wild type will be expressed. Both copies of the offspring's genome must have the mutation for the disorder to be inherited. Thus, the mutation must occur both in oogenisis and spermatogenesis.

 

[MedPR]

Because of the premise that it is recessive. If the fertilized egg has only one mutated copy of the gene, the dominant wild type will be expressed. Both copies of the offspring's genome must have the mutation for the disorder to be inherited. Thus, the mutation must occur both in oogenisis and spermatogenesis.

But it's an autosomal disease.

 

[SaintJude]

Oh , right. I am appalled at my own lack of understanding. I think I didn't realize what an egg cell contained exactly. So, it actually contains all of the genetic information from one parent. It contains half of the diploid genome (which will form the basis of the autosomal genetic information), right?

Oh my gosh I'm so sorry if this is a question people think I should know, but I seriously never actually covered this in detail although I was a Biology major ?!

 

[chiddler]

But it's an autosomal disease.

why do you see conflict?

yes it is autosomal. that means there must be "aa" for the disease to occur. When sperm has mutation, it is "Aa" so the dominant wild type is still expressed. The only way for "aa" to develop is if both sperm and oocyte have the mutation.

this seems very simple. am i misunderstanding the question?

 

[MedPR]

Oh , right. I am appalled at my own lack of understanding. I think I didn't realize what an egg cell contained exactly. So, it actually contains all of the genetic information from one parent. It contains half of the diploid genome (which will form the basis of the autosomal genetic information), right?

Oh my gosh I'm so sorry if this is a question people think I should know, but I seriously never actually covered this in detail although I was a Biology major ?!

I guess I'm worse off than you. I'm still confused.

 

[BA11]

But it's an autosomal disease.

Each sperm and egg contains 22 autosomes and 1 sex chromosome (assuming they formed correctly during meiosis, of course). They combine to give 22 SETS of autosomes and 1 SET of sex chromosomes for a total of 46 individual chromosomes. With two copies of each numbered chromosome, you need two of the same mutation (one on each of the two chromosomes) in order to inherit an autosomal recessive disorder.

Don't get hung up on the fact that sperm and egg are germ cells, they still contain autosomal chromosomes. Otherwise, sexual reproduction would not work.

 

[SaintJude]

Nothing a good picture can't fix!
Say that the pink part of a chromosome shown in scenario A transcribes for a gene responsible for a the metabolic disorder, an autosomal recessive pattern. Notice that Meiosis II (which will occur in oogenesis) will lead to an egg with the pink part in it. Since the disease is an autosomal recessive pattern, you'll need 2 copies of the "pink part" one from the dad, and one from the mother. Although this is not shown in the picture, you'll need 2 "pink parts of the chromosome"

The fertilized egg (containing both mutated genes) will turn embryo (scenario B)-> blastula --> gastrulation--> human being w/ autosomal recessive disorder.

At least, that's how I understand it now. Ba11, please correct me if I'm wrong.

 

[BA11]

Oh , right. I am appalled at my own lack of understanding. I think I didn't realize what an egg cell contained exactly. So, it actually contains all of the genetic information from one parent. It contains half of the diploid genome (which will form the basis of the autosomal genetic information), right?

Oh my gosh I'm so sorry if this is a question people think I should know, but I seriously never actually covered this in detail although I was a Biology major ?!

I graduated as a biology major and have tutored people for the MCAT since I took it last year and trust me when I say it isnt uncommon to get confused by questions like this. If a professor asked this question in a senior level genetics class, 25% of the class would get it wrong not because they don't know it but because it's the kind of question that produces brain farts.

It isnt anywhere near the most obvious MCAT question I've asked or been asked.

 

[BA11]

Nothing a good picture can't fix!
Say that the pink part of a chromosome shown in scenario A transcribes for a gene responsible for a the metabolic disorder, an autosomal recessive pattern. Notice that Meiosis II (which will occur in oogenesis) will lead to an egg with the pink part in it. Since the disease is an autosomal recessive pattern, you'll need 2 copies of the "pink part" one from the dad, and one from the mother. Although this is not shown in the picture, you'll need 2 "pink parts of the chromosome"

The fertilized egg (containing both mutated genes) will turn embryo (scenario B)-> blastula --> gastrulation--> human being w/ autosomal recessive disorder.

At least, that's how I understand it now. Ba11, please correct me if I'm wrong.

Exactly, and the fact that you can explain it to yourself in this fashion means you really knew it all along.

 

[MedPR]

Each sperm and egg contains 22 autosomes and 1 sex chromosome (assuming they formed correctly during meiosis, of course). They combine to give 22 SETS of autosomes and 1 SET of sex chromosomes for a total of 46 individual chromosomes. With two copies of each numbered chromosome, you need two of the same mutation (one on each of the two chromosomes) in order to inherit an autosomal recessive disorder.

Don't get hung up on the fact that sperm and egg are germ cells, they still contain autosomal chromosomes. Otherwise, sexual reproduction would not work.

This is exactly what I was doing. Your original answer makes sense now, thank you!