Why do sex-linked traits follow different patterns for inheritance than other traits?

Answer 1

Short Answer:

There are just over a thousand genes on the X chromosome and about 26 on the Y. Of the 26 on the Y, 9 relate to sperm and one (SRY) relates to male anatomy, the others involved in general tasks.

The fact that there are a hundred times more genes on the X chromosome means that traits related to the X, and variations in those traits, are far far more common.

Sex-linked traits are those that can only be passed along from the father and not the mother or from the mother and not the father.

Obviously, males do carry and pass on sex-linked traits, just not very many.

Gender can only be passed along from the father, along with genes related to sperm.

Females pass on to the male offspring a larger number of traits, (such as some color blindness and male pattern baldness) which have no counterpart in the Y chromosome.

More:

Males have an X and a Y chromosome and females have two X chromosomes.

Because the Y chromosome is smaller than the X with far fewer genes, there are genes carried only on the X and only gotten from the mother. That does not mean that all males have Y chromosomes that are the same.

Overwhelmingly though, the absence of the set of genes on the Y chromosome results in the corresponding genes on the X chromosome being expressed, in particular all of the X-linked disorders of a gene of an X chromosome in a male do not have a second X gene that can be expressed to compensate. Thus, there are a set of sex-linked traits that can only come from the mother.

(Color blindness and male pattern baldness are the two most discussed examples.)

A male offspring (son) has the same Y chromosome as the father and hence any traits influenced by that chromosome.

A female offspring (daughter) has the same X chromosome as the father and hence any traits influenced by that chromosome. A recessive gene on the X from the father will not usually be expressed in a daughter, but a dominant gene on the X from the father will be expressed in the daughter.

For a female to inherit a trait associated with an X-chromosome recessive gene, there must be two copies, one from each parent.

Aside:

The issue of gender-connected inherited traits has much more complexity than this simple discussion can address. For example, not all of the so-called male characteristics are carried on the male (Y) chromosome. The expression of genes on other chromosomes, from both the mother and father, is influenced by genes on the Y chromosome or simply by the absence of two X chromosomes. The reader should investigate further.

Caveat: This discussion applies to humans and other mammals with the XX/XY sex determination system. See related links.

Fun Fact: A woman discovered that the Y chromosome was male. While studying worms, in 1905, Nettie Stevens at Bryn Mawr College identified the Y chromosome as the determining factor in gender.

Answer 2

The sex-linked traits always go to the offsprings depending up on the gender where as the autosomal traits have freedom to move either way in the male or female offspringabiding bytheMensel's law of independent assortment.

Answer 3

Sexlinked traits follow a different pattern of inheritance than a non-sex linked trait because of the size difference between the X and y chromosomes.

Think about non-sex chormosomes as being the same size, each having the exact same number of genes in the same postions (loci). This means that a person has to have two copies of the recessive genes (one on each homolog of that chromosome) before the trait will be expressed.

Lets say A is normal (dominant) and a is abnormal (recessive).

People who are aa have the abnormal condition and people who are AA or Aa are normal. The normal ratio is 1 AA:2 Aa:1 aa if both parents are Aa. There is a 25% chance that a child will inherit two abnormal genes and the chance of any sex child will be so affected is exactly the same.

When a recessive trait is located on the X chromosome only a female with two X chromosomes has the same number of genes on each X chromosome (the two X chromosomes have the same inheritance behavior as a homolog chromosome in females). In males who inherit the much smaller y chromosome there are many genes on the X chromosome that do not have a matching gene on the y chromosome. This means that recessive traits on the X chromosome that have no matching genetic material on the y chromosome will always be expressed.

So, lets say that there is a family where the mother is Aa and the father (who only has one allele on the y chromosome is A. (A is normal and a is abnormal).

None of the daughters produced can be aa, because the father will always pass A. Daughters will only be Aa or AA.

Sons on the other hand, will get either A or a from the mother and, since the y chromosome has no genetic material at this gene locus the boys will be A normal or a affected at in a 1:1 ratio.

If the father is a on his X chromosome, and the mother is AA 100% of the daughters will be carriers (Aa) and all the sons will be normal (A-).

Answer 4

Sex-linked traits are a special case. They are controlled by genes on the sex chromosomes, X and Y. The female has two X chromosomes and the male has one X and one Y.Only one X gene is active in the female. The other is inactivated and shows as a Barr Body.

Answer 5

Females and males have different sex chromosomes. :)

Answer 6

Because the Y chromosome is so small and has relatively few genes, most sex-linked traits are governed by genes of the X sex chromosome, and are therefore X-linked.

Answer 7

Because the sex of the subjects is critical to inheritance.

Answer 8

Males only have one X chromosome.