Mendellian Genetics

Posted: August 12th, 2013

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Mendellian Genetics

Question 1: Cystic Fibrosis inheritance

After using the Punett square to predict the probability of a married couple where both partners are heterozygous to the cystic fibrosis to having a child with cystic fibrosis, I found the probability to be one out of four or 25%. When both partners are heterozygous, it means that they are both carrying the ‘f’ and ‘F’ alleles. Therefore, both are Ff since they are heterozygous. In this case, ‘f’ is the cystic fibrosis allele and for one to have fibrosis it is necessary to have two of them denoted as ‘ff’. In order for this couple to have a child with the disease, the child must inherit ff alleles. The chance for that is 25% according to the punett square. All the possible outcomes are FF where a child inherits the F allele from both parents, Ff where a child could inherit the f from the mother, another Ff where the child could inherit the f from the father and finally, the ff where the child could inherit the cystic fibrosis carrying allele from both parents. The fourth outcome represents the child that would inherit cystic fibrosis, in this case, homozygous to cystic fibrosis. On the other hand, there is a 50% probability that the child could be a carrier of the allele. A carrier is a person that carries only one of the recessive allele. Additionally, there is a 25% probability that the child could be born without any of the recessive alleles in case the child inherits the F alleles from both mother and father.

Question 2: Hemophilia inheritance

Hemophilia is a sex trait that is linked to human beings. It is inherited the same way that drosophila inherits white-eye color. With hemophilia, females are heterozygous where they inherit the X chromosome from both parents. On the other hand, males are hemizygous meaning they only have one X chromosome that they inherit from their mother. Hemophilia is carried by the X chromosome. Therefore, when a man has an X chromosome that is hemophiliac, they become hemophiliac. On the other hand, for a woman to be hemophiliac she has to inherit two X chromosomes that are hemophiliac. When they inherit one, the other one acts to protect them from the disease but they become carriers. Men have higher chances since they only got one X chromosome and a Y chromosome. When the X is hemophiliac, there is no other chromosome to protect them.

Although hemophilia is mostly inherited, a third of the cases are found in people without any family connection to the condition. This could be through mutation happening to the male fetus or female as well. In this case, a male fetus will be born with hemophilia while a female child will become a carrier. This kind of case is referred to as spontaneous mutation where the gene mutates during growth in the womb.

The inheritance of hemophilia is different between male and female because men only have one X chromosome while women have two. One acts as a compensation for the other in women. For men, the Y chromosome cannot compensate for the X chromosome. In this case, where the man is hemophiliac and the woman a carrier, there are several likely outcomes to the offspring. For every pregnancy, there are four possible outcomes. The outcomes or probability will be dependent on the X chromosome that the male child receives from the mother. If they receive the chromosome that is mutated they become hemophilic. For female child, it is dependent on the X chromosomes she receives from both parents. If one of the chromosomes is hemophilic, she becomes a carrier but only gets affected by hemophilia in case she receives both that are hemophilic.

The first outcome could be a male child without the hemophilic condition. In this case, the child will inherit the X chromosome of the mother that is normal. The probability of having a male child will be one out of four or 25%. The other possible outcome is having a female child who is a carrier. In this case, the child will inherit the normal X chromosome from the mother and the hemophilic X chromosome from the father. The probability of a female child who is a carrier is 25% as well. The third probability is a male child who is hemophiliac like the father. In this case, the male child inherits the hemophilic X chromosome from the mother since she has one mutated chromosome. This probability is at 25%. The fourth possibility is a daughter with hemophiliac condition. For this to happen, the female child would have to inherit both of the hemophiliac chromosomes from both parents. In this case, the daughter will have no normal Chromosome to protect her. Therefore, she will have both of her X chromosomes carrying the condition.

In other cases such as where both parents are hemophiliac, all the children inherit the disease. The girls will inherit one of the infected X chromosomes from the father and the other one from the mother. The sons will inherit the affected X chromosome from the mother. In cases where the father and mother are normal, the children are born normal unless a spontaneous mutation occurs where the male child is born with the condition while the female child becomes a carrier. Another possible case is the man being hemophiliac while the woman is normal. In this case, daughters will become carriers through the X chromosome they inherit from the father while the sons will be normal through the X chromosome they inherit from the mother. Another possible scenario is the woman being hemophiliac and the man is normal. In such a case, a female child will be a carrier while the male child would be hemophilic. Finally, in a case where the father is normal and the mother a carrier, the male child could be either normal or hemophilic on a 25% probability while a daughter would be either a carrier or normal on a 25% probability as well.