Question

What is criss cross inheritance?Explain the inheritance of one sex-linked recessive character in human being?

Answer 1

Hint:The process in which the genes or genetic information is passed on from one generation to another is called Inheritance.The sex is determined by the X chromosomes and Y chromosomes.Different disorders have different type of inheritance.

Complete answer:
The inheritance of sex linked characters is criss cross inheritance.They are transmitted from mother to son and father to daughter.If the father is affected then it is passed on to the daughter who is the carrier.If mother is affected it is passed on to the son who is the carrier.Different results are produced due to reciprocal cross.Sex linkage of the gene controls the trait if different results occur from two pairs of cross.Sex linked characters are classified into two types
          1.sex-linkage traits
          2.sex limited traits.
Both the matching genes should be abnormal to cause disease in recessive inheritance.It is similar to X-linked recessive.This is more common in males than females as the male have single x chromosome and this single recessive gene is responsible for the occurrence of disease.The other half of the gene from Y chromosome does not have genes similar to x chromosome and so they do not protect the males.Hemophilia is an example for this.

 Typical Scenario:
 In a pregnancy if the mother has a single abnormal x chromosome and if she is the carrier of disease,the possible outcomes are
> 25 percent healthy boy
>25 percent boy with disease
>25 percent healthy girl
>25 percent carrier girl without disease

If the father is the carrier,the possible outcomes are
         1.50 percent it is a healthy boy
         2.50 percent it is a carrier girl without disease
This states that none of the children might have the disease but the traits will be passed on to the grandson.

X-linked Recessive in females
An abnormal gene from both the parents will be needed to get this disorder as the female has two x chromosome.This is a very rare case.
If the father has the disease and if the mother is the carrier,the possible outcomes are
         1.25 percent it is a healthy boy
         2.25 percent it is a boy with disease
         3.25 percent it is a carrier gir
        4.25 percent is a girl with disease.
If both the mother and father have the disease,the possible outcome is 100 percent chance of a child with disease be it boy or girl.

Note:A normal X chromosome which is inactive can be present in the female carrier.This is skewed X-inactivation.This female carriers may have mild symptoms or symptoms similar to those of males.