x-linked recessive disorders?

Question

the ratio affected male: affected females increase in population as the disease mutation frequency decreases

Answer 1

Is that a question or a statement?

X-linked diseases where female affected individuals survive are rare. Incontinentia pigmenti springs to mind

Answer 2

This is true.

Consider an x-linked allele with frequency 0.1

Probability P (male carries this allele) =0.1

P(female has a copy of this allele) = P((on 1st X) + P(on 2nd X))-P(she has 2 copies) = 0.1 + 0.1-(0.1 x 0.1) =0.199

For simplicity, I'm going to ignore the correction and say P(female carries this allele) = 0.2

P(of affected son) = P(mother has allele) = 0.2
P(affected daughter) =P (both parents have allele) = 0.1 x 0.2 = 0.02

Ratio affected males to females is 10:1

If the frequency of the recessive allele is only 0.01, then using the same calculations:

P(affected son) = 0.02
P(affected daughter) = 0.0002

ratio of affected sons to daughters is 100:1

so as the frequency of the allele declines, the probability of both parents carrying it is very rare and there are hardly any affected daughters.

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Answer 6

These genetic diseases are diseases caused by an error in a single DNA gene. X-linked means the error occurs on the X chromosome which is the 23rd sex-linked X chromosome. Such diseases are sometimes called "sex-linked" rather than X-linked.

Some examples of X-linked recessive disorders are Hemophilia and Duchenne muscular dystrophy. These occurs only in boys, which is what we expected from an X-linked recessive disorder, as discussed below. For a list of this type of disorders, see x-linked recessive disorders.

Recessive means that disease only occurs when a person has two copies of the bad gene. For autosomal recessive diseases, this usually means they must inherit the disease from both parents, but this is not the case for X-linked recessive diseases. Men have only a single X chromosome, so they have only one copy of any gene on the X chromosome. Thus, a gene error on X definitely caused disease in men (who are XY), but women are XX, and have two copies of the gene. X-linked recessive disorders are more likely to occur than autosomal recessive disorders, because men have only one X chromosome, whereas all people have 2 copies of each autosome.

Recessive diseases often occur in genes that produce an enzyme. In a carrier, who has only one bad copy, there is often no disease, because the second gene can pull up the slack, and maintain health. In some recessive diseases, a carrier gets a mild form of the disease. For example, in X-linked recessive hemophilia, a female carrier has one bad gene on chromosome X, but the good gene on the other X chromosome produces enough of the good clotting enzyme to maintain health. The recessive disease only arises when the male has no good gene on the other chromosome (because they get a Y instead of a second good X).

X-linked recessive inheritance: These diseases arise from an error on the X chromosome, which causes disease only when there is no corresponding paired X chromosome with a good gene. However, since men are XY a man with the bad gene on the X chromosome must get the disease, because there is no second X chromosome. Since women are XX, they usually have a second good X chromosome which suppresses the bad X gene, leaving them disease-free, but as carriers. The following patterns of inheritance are typical:

* Gender bias: Typically, males are the ones who get the disease, whereas females are carriers. Men cannot be carriers because they cannot have a bad X chromosome gene without getting the disease. Women cannot get the disease, because they typically have a second good X chromosome. However, in the rare case of a daughter of an affected father and a carrier mother, then the daughter might have two bad X genes and get the disease itself (like an autosomal recessive disease).
* Father to son transmission: 0% chance of disease and 100% chance of disease-free unless the mother is also a carrier (males always get their single X from the mother not father and cannot get a bad gene from the father), 0% chance of carrier (males cannot be carriers).
* Father to daughter transmission: 0% chance of disease (females can only be carriers), 100% chance the female child is a carrier (because the father gives a bad X gene as there is only the bad one to give). If the mother is also a carrier, the female can be fully afflicted with the rare double-recessive female version of the disease.
* Mother (carrier) to son transmission: 50% chance of disease, 50% chance disease-free, 0% chance of carrier (males cannot be carriers).
* Mother to daughter:: 0% chance of disease (females can only be carriers), 50% chance of female carrier, 50% chance neither affected nor carrier.
* Other children: If a couple has one (male) child with the disease, what are the odds for another child having it. Usually this means the mother is a carrier, because the father cannot transmit the disease to a child (and the father would probably have noticable disease). So the risk for a second child of the same couple is probably the mother-to-son transmission risk, 50% chance of disease, and a female child cannot have the disease but has a 50% chance of being a carrier.

Mild disease in female carriers: Female carriers can have a mild form of disease, because they have a bad gene on one of their two X chromosomes, and a good gene on the other. If the disease is not totally recessive, a partial disease can result even though the woman has one good gene. In other words, if the second gene copy is not a good enough "backup", a partial level of mild disease can still result in carriers. However, most X-linked recessive diseases have symptom-free female carriers.

Sporadic cases: A genetic disease that occurs when neither parent has any genetic defect is called a sporadic genetic disease. These cases arise via random genetic mutations in the DNA. A sporadic mutation can be the cause of an x-linked recessive disease (whereas it is unlikely for an autosomal recessive disorder) because only a single mutation is required (in males) to cause the disease. Females can also become carriers owing to random mutations.

Answer 7

This would be expected as women are carriers of fragile X, and men are prone to it. Women have two X genes, which makes us less genetically susceptible to numerous problems, not only to fragile X, but to color blindness and other genetic traits found in the X chromosome.

"Because males normally have only one copy of the X chromosome, those males with significant trinucleotide expansion at the FMR1 locus are symptomatic. They are intellectally disabled and may show various physical features of the fragile X syndrome.

Females have two X chromosomes and thus have double the chance of having a working FMR1 allele. Females carrying one X chromosome with an expanded FMR1 gene can have some signs and symptoms of the disorder or be normal.

Males with the fragile X cannot transmit it to any of their sons (since males contribute a Y chromosome, not an X, to their male offspring), but will transmit it to all of their daughters, as males contribute their X to all of their daughters.

Females carrying one copy of the fragile X can transmit it to their sons or daughters. Sons who receive the fragile X are at high risk of intellectual disability. Daughters who receive the fragile X may appear normal or they may be intellectually disabled, usually to a lesser degree than boys with the syndrome."

Answer 8

The male would be XmY and the female would be XMXm So, the punnet square would give you 4 different genotypes: XMXm XMY XmXm and XmY

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Answer 9

what is the question here?