A woman, Melanie, and a man, Allan, who are both carriers for phenylketonuria (a rare autosomal...

A woman, Melanie, and a man, Allan, who are both carriers for phenylketonuria (a rare autosomal recessive disease) have two daughters, Jessica and Andrea, and two sons, Kyle and James. None of Melanie and Allan's parents, or their siblings, had phenylketonuria. However, both of their sons have the disease.
Answer questions 1 to 8 based on this scenario. Please use P to indicate the dominant allele for this condition and p to represent the recessive allele.

1) What are Melanie and Allan's genotypes?

2) Based on the scenario given, what are the most likely genotypes of Melanie's parents and of Allan's parents?

3) What are the genotypes of each of their daughters?

4) What are the genotypes of their sons?

5) What percentage of Melanie's eggs will carry the phenylketonuria allele?

6) What percentage of Allan’s sperm will carry the phenylketonuria allele?

7) Melanie is pregnant and she and Allan have just found out the sex of the fetus is male.

8) What is the probability the fetus is carrying two copies of the recessive phenylketonuria allele, and thus will be affected with the disease?

9) Among Melanie and Allan's four children, how does the observed frequency of affected children compare to the expected frequency? Does the observed result go against the concepts of Mendelian genetics?

Solutions

Expert Solution

It is given that both Melanie and Allan are carriers of Phenylketonuria (PKU), which is an autosomal recessive disorder. Therefore, they both are heterozygous for the condition with a genotype Pp. This is so because, if they were homozygous for P, they would not be carriers as they do not have even one copy of the recessive allele and if they were homozygous for p, they would have had the condition.

The genotype of Melanie and Allen are Pp

2) Since both Melanie and Allen are carriers, but none of their parents or siblings have PKU, this means that one parent of each Allen and Melanie have to be carriers. However, the probability of both parents being carriers is extremely low. This is so because if both parents of either Melanie or Allen were carriers, there is a 50% probability that they would have had affected children, which is not the case here.

Therefore, the genotypes are

Melanie's parents: Pp and PP

Allen's parents: Pp and PP.

However, which parent, the mother, or the father was the carrier cannot be determined since this is an autosomal recessive disorder. Also, there is a very slight possibility that both father and mother of Melanie as well as both the parents of Allen were carriers (Pp). But the posterior probability for this is extremely low.

3) The daughters of Melanie and Allen (having no PKU) have a 0.5 probability for being carriers (Pp) and a 0.5 probability for being homozygous dominant (PP) since their parents are carriers and so have one copy each of the dominant and recessive allele, either of which could be randomly passed on to their offspring. However, this cannot be confirmed without the data of their offspring being available.

4) Since it is given that both Kyle and James have PKU, this means that they both have two copies of the recessive allele p and so are recessive homozygotes (pp)

5 & 6) the genotypes of Melanie and Allen are Pp. So, they can have gametes ( egg or sperm) of either P or p genotype only. Therefore, there is a 50% or 0. probability for them having gametes of P genotype and 0.5 probability for a gamete of p genotype.

7&8) PKU is an autosomal recessive disorder and so does not depend on the sex of the fetus. Therefore, there is a 0.25 probability for the next child being PP ( not a carrier and not affected), 0.5 for being a carrier (Pp), and 0.25 for being affected (pp), irrespective of its sex. (please see the Punnet square below)

Melanie's gametes

Allan's gametes

PP (1/4)

pp( affected, 1/4)

9) From the Punnet square above, it can be seen that the expected Mendelian percentage of affected children is 25% or 1/4. However, two children out of 4, i.e. 50% were affected. But, since the number of offspring is very less, this cannot be taken as proof that Mendelian laws of inheritance were not followed.

gladiator answered 1 year ago

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