(1) An introduction to sex-linked genetic disorders

This section will help you answer questions such as ...  Explain what a sex-linked genetic disorder is? What is the main difference between an X and Y chromosomes? What causes colour blindness?  How many alleles control your inherited blood group?   Can you draw a genetic diagram to show how a sex-linked disorder is inherited?

Reminders: A chromosome as a thread-like structure of DNA, carrying genetic information in the form of genes.

A gene is a length of DNA that codes for a protein. An allele as a version of a gene.

Reminder that in the biological science of genetics, inheritance is the transmission of genetic information from one generation to the next generation by chromosomes of DNA, but not all traits passed on are desirable.

A sex-linked characteristic as a characteristic in which the gene responsible is located on a sex chromosome and that this makes it more common in one sex than in the other.

If you are male, there are certain genetic disorders you are more likely to suffer from.

This is due to alleles linked to the male X and Y chromosomes.

A characteristic will be sex-linked if the allele that codes for it is located on the X or Y sex chromosomes.

The Y sex chromosome is smaller than the X chromosome and so carries fewer genes.

Therefore most genes on the sex chromosomes are located on the X chromosome.

Since male men have only one X sex chromosome, it often only carries one allele for a sex-linked gene.

Since men only have one allele, the characteristic of this allele is shown even if it is recessive.

Which means that men are more likely than women to show recessive characteristics for genes that are sex-linked (on the X chromosome).

Therefore sex-linked genetic disorders are caused by faulty alleles on the sex chromosomes, and usually due to faulty alleles on the X chromosome (male or female carriers).

e.g. colour blindness, haemophilia and muscular dystrophy are all caused by a recessive gene carried on the male or female X chromosome.

Some genetic sex-linked disorders are due to faulty alleles on the male Y chromosome too e.g. male infertility.

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Summary of learning objectives and key words or phrases

Be able to describe the genetics of sex-linked genetic disorders in terms of XX and XY chromosomes.

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