Tuesday, March 1, 2011

Genetic Counseling

To learn about the field of genetic counseling, I went through the various problems here. Although I tend to prefer finding more creative ways of going about this, this time, I think I'll simply answer the questions given on the site itself.

First off: here's the pedigree described in the story:

(I apologize for the spots that are somewhat confusing. Progeny has some design problems that I eventually became tired of circumventing.)

Part II--Autosomal Dominant Traits

1. Do autosomal dominant disorders skip generations?

No. Simply because, by definition, they are dominant, then if their presence in the genotype will be reflected in the phenotype.

2.  Could Greg or his mother be carriers of the gene that causes myotonic dystrophy?

Because myotonic dystrophy is an autosomal dominant disorder, Greg or his mother could not be carriers of the gene that causes it. If they had the gene, they would suffer from the disease.

3. Is there a possibility that Greg’s aunt or uncle is homozygous for the myotonic dystrophy (MD) gene?

There is no possibility that Greg's family members are homozygous for the MD gene because one of their parents did not have the MD gene. Because they did not have it, they could not pass it on.

4. Symptoms of myotonic dystrophy sometimes don’t show up until after age fifty. What is the possibility that Greg’s cousin has inherited the MD gene?

There is a fifty percent chance that his cousin has the MD gene, because the cousin's mother was heterozygous (her father did not suffer) and the father does not have the gene. A Punnett square for this situation reveals a fifty percent chance of inheriting the MD gene.

5. What is the possibility that Greg and Olga’s children could inherit the MD gene?

There is no possibility of Greg and Olga's children having MD. Because their parents did not have the gene, they cannot have the gene themselves. Therefore, their children cannot get the gene from them.

Part III--Autosomal Recessive Traits

1.  What are the hallmarks of an autosomal recessive trait?

An autosomal recessive trait can skip generations, but requires two copies of the mutant gene to be apparent.

2.  What does consanguineous mean? Why is this concept especially important when discussing recessive genetic disorders?

If two people are said to be consanguineous, that means that they are descended from a common ancestor. This is important in recessive traits because if two people are consanguineous, then they often have somewhat similar genotypes--including recessive genes.

3.  What is it about the inheritance pattern of factor VIII deficiency seen in Greg and Olga’s pedigree that point toward it not being an autosomal recessive trait?

Looking at Greg and Olga's pedigree, we can see that it appears mainly in boys and only rarely is apparent. This would point towards this disorder being an X-linked gene.

Part IV--Sex-Linked Inheritance

1. What are the characteristics of X-linked recessive inheritance?

X-linked recessive inheritance is typically primarily apparent in boys because males only have one copy of the X-chromosome. This means that mutations on this chromosome are typically masked in girls by a good copy of the X-chromosome.

2.  Why does a son never inherit his father’s defective X chromosome?

A son cannot inherit his father's X chromosome because he always receives the Y chromosome from his father. This is what makes him a son and not a daughter.

3.  What is required for a woman to display a sex-linked recessive trait?

For a woman to display an X-linked recessive trait, she would need two mutant copies of the X-chromsome--one from both parents.

4.  Return to the pedigree drawn earlier for Greg and Olga; mark those persons who are carriers of the factor VIII deficiency gene.

On Greg's side of the family, his mother and maternal grandmother are carriers. On Olga's side, her maternal grandmother and her mother are carriers--and potentially, her.

5.  What is the chance that Olga carries the gene for factor VIII deficiency? Calculate the probability that she will pass it to her offspring. Will male children be affected in a different way than female children?

There is a  1/4 chance that Olga has this gene, and a 1/4 chance that she will pass this gene to her children. If she does, there is a 1/8 chance that the child will suffer from the disease (if male) and a 1/8 chance that the child will be a carrier.

6. What is the chance that Greg carries the factor VIII gene? Can he pass the gene on to his sons? His daughters? How will each be affected?

Because the factor VIII gene is on the X chromosome and he is not a sufferer, there is no chance that he has the gene. He cannot pass the gene on to his sons, because he will give a Y chromosome to them. He will give an X chromosome to his daughters, but the mutant gene will not be on it.

I will stop here--this one's already gone on long enough, especially for one as mundane as this one. I will probably include the rest of the tutorial in a Part II, which should be up within a few days.

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