Question

What are the characteristics of X-linked recessive traits and X-linked dominant traits? Explain this by using examples. How do human cells compensate for the X-linked gene dosage difference in XX and XY nuclei? How are autosomal genes are influenced to sexual dimorphism?

Answer 1

X-linked recessive traits also are known as Sex-linked recessive traits

• In X-linked recessive traits, most affected individuals are male
• Affected males result from mother who are affected or who are known to be carriers (Heterozygous)
• Affected females come from father and affected or carrier mothers
• The sons of affected females should be affected
• Approximately half the sons of the carrier should be affected
• Examples of genetic disorders are Duchenne muscular dystrophy, Hemophilia A and B

X-linked dominant traits also are known as Sex-linked dominant traits

• The trait does not skip generations
• Affected males come from affected mothers
• Approximately half the children of an affected heterozygous female are affected
• Affected females come from affected mothers or fathers
• All the daughters, but none of the sons, of an affected father are affected
• Examples of genetic disorders are Vit D resistant rickets

There are three main mechanisms for dosage compensation which are

1. Random inactivation of one female X chromosomes (also called X -inactivation )
2. a two-fold increase in the transcription of a single male X chromosome
3. decreased transcription by half in both of the X chromosomes of a hermaphroditic organism
4. For example, in humans, females (XX) silence the transcription of one X chromosome of each pair, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes as do human males (XY), both sexes having essentially one X chromosome per cell, from which to transcribe and express genes.

Sex limited genes are responsible for sexual dimorphism.  Sex limited genes are usually determined by autosomal genes and primarily concerned with the secondary sexual characters. In human forexamples breast development is a trait the is normally limited to female, whereas beard growth is limited to males.