Why is having an extra chromosome 21 tolerable to the point that someone with this condition can survive to maturity?
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Hemophilia is a recessive X-linked condition and is inherited from parents. . The probability that Natalie and Daniel’s first child has hemophilia is<u> 1/4</u>, not 1/8. The statement is<u> false.</u>
<h3>What is an X-linked gene?</h3>When talking about sex-linked genes, we refer to all those genes located in one of the sex-chromosomes and not in the somatic ones.
X-linked genes are the genes located specifically in the X sex chromosome.
Traits coded by these genes are inherited. This is, X-linked genes are transmitted from parents to children through generations.
According to this, X-linked genetic disorders will be inherited as well.
- Men have a high probability of being affected since they only have one copy of the X chromosome.
- In women, the effect of the mutation might be musk in heterozygous state if the mutation is recessive.
Hemophilia is a recessive X-linked disorder, which means it is coded by the recessive allele.
- X+ ⇒ X chrosomosome with Dominant allele
- X- ⇒ X chrosomosome with Recessive allele
<u>Genotype Phenotype</u>
X+X+ Healthy woman
X+X- Healthy carrier woman
X-X- Hemophilic woman
X+Y Healthy man
X-Y Hemophilic man
Let us make a pedigree of these families,
<u>Natalie's family</u>:
Mother healthy carrier X+X-
Father healthy X+Y
Brother hemophilic X-Y
Natalie Healthy X+X- or X+X+
Natalie has 1/2 chances of being carrier X+X- and 1/2 of being X+X+.
<u>Daniel's family</u>:
Mother healthy carrier X+X-
Father hemophilic X-Y
Sister hemophilic X-X-
Daniel Healthy X+Y
He has 1/2 chances of being healthy X+Y and 1/2 of being affected X-Y.
According to the product probability rule, if events A and B are independent from each other, the probability of occurrence of both events together is
P(A∩B) = P(A) x P(B)
In this case,
- event A is Daniel being affected ⇒ 1/2
- event B is Natalie being carrier ⇒ 1/2
P(A∩B) = P(A) x P(B) = 1/2 x 1/2 = 1/4
The probability that Natalie and Daniel’s first child has hemophilia is 1/4, not 1/8. The statement is<u> false.</u>
You can learn more about X-linked genes at
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