(6.1) 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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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Sex-Linked Genetic Disorders: XX and XY Chromosomes

Understanding sex-linked genetic disorders is crucial for grasping human genetics.

These disorders arise due to genes located on the sex chromosomes (X and Y), affecting inheritance patterns.

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1. The Role of Sex Chromosomes

Humans have 23 pairs of chromosomes, with the 23rd pair determining biological sex:

• Females (XX): Two X chromosomes.

• Males (XY): One X and one Y chromosome.

Since the Y chromosome is much smaller and carries fewer genes, most sex-linked disorders are associated with the X chromosome.

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2. X-Linked Inheritance

Sex-linked disorders are often X-linked, meaning the faulty gene is carried on the X chromosome. Since males have only one X chromosome, they are more likely to express recessive disorders because they lack a second X chromosome to compensate.

Key Features of X-Linked Inheritance:

• Males (XY): If they inherit a faulty X chromosome, they will have the disorder.

• Females (XX): They need two faulty X chromosomes to express the disorder. If they inherit only one, they become carriers.

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3. Examples of X-Linked Disorders

1. Red-Green Colour Blindness:

   • Affects the ability to distinguish between red and green shades.

   • More common in males due to X-linked inheritance.

   • See (6.2) Colour blindness - a sex linked genetic disorder

2. Haemophilia:

   • A disorder where blood does not clot properly.

   • Historically known for affecting royal families due to inheritance patterns.

   • See (6.3) Haemophilia - a sex linked genetic disorder

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4. Importance in Human Genetics

Studying sex-linked disorders helps scientists:

• Understand inheritance patterns.

• Develop genetic screening techniques.

• Improve treatment options for genetic conditions.

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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.

You should be familiar with Punnett squares to predict inheritance probabilities and understand why males are more affected by X-linked disorders.

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