Patterns of inheritance 5
Joanna George-Johnson, Contributor

So far we have been looking at various ways traits or characteristics are carried to the next generation. This week, we will be looking at genes that are found on the 'X' chromosome. These traits that are carried by genes on the 'X' chromosome are called sex-linked traits.

From previous lessons we learnt about dominant and recessive alleles. Today, the traits that we will look at will also follow the same dominant recessive rules.

So what makes these genes so special? one might ask. Let's take a closer look at them.

A closer look at the sex chromosomes

• The nuclei of human cells contain 22 types of autosomes and two sex-determining chromosomes.
  
  
• Just as we learned before, each chromosome has a copy. (One from your mom and the other from your father) called homologous pairs.
  
  
• Males are different from females mainly because of the type of sex chromosomes each gender gets.
  
  
• In females, the sex chromosomes are very similar and relatively long. They are called X chromosomes.
  
  
• In males the sex chromosomes are different. They have one X chromosome (long one) and one Y chromosome (a shorter one).
  
  
• The presence of the Y chromosome is decisive for unleashing the developmental programme that leads to a baby boy.
  
  
• Therefore in a woman, all of her eggs will have one copy of the X chromosome. (Remember that gametes have only one copy of each chromosome. See first lesson in series.)
  
  
• Some of the sperm in man will have one X chromosome and others will have Y chromosome.
  
  
• Thus at fertilisation, depending on which sperm fertilise the egg will determine whether it will be a boy or a girl! (See diagram above)

Sex-linked inheritance

Colour blindness is a known sex-linked disease in humans. It is caused by a recessive allele that causes a person to have abnormal colour vision.

What are the possible offspring that a colour blind man and a heterozygous woman would have if they mated?

Let us use the punnett square to figure this out.

Let: N represents normal vision n represents colour blindness

See diagram below

Random fertilisation

Possible genotype of offspring (2n)

Possible phenotype

	carrier female
	colour blind female
	Normal male
	Colour blind male

Observations from sex-linked crosses

• A man can never pass a sex-linked trait to his son
  
  
• Males are never carriers of a sex-linked disease; they either have the disease or not.
  
  
• Women might be carriers of the disease. (They have the gene, but not the sickness since they can have the dominant normal allele )

All of the daughters of a man who have a sex-linked disease will be carriers except if the mother has the gene too; then they may get the full blown disease.

Misconception Alert!!

Please note that despite what you may hear on the street, such as, 'all man colour blind', is not really true. All men are not colour blind. In fact, a very small percentage of the population has this disease. Also you can see it is possible for women to also get colour blindness.

What is true is that sex-linked traits are more prevalent in males than in females.

Questions

1. Haemophilia is a sex-linked disease in humans characterised by the inability for the victim's blood to clot. It occurs from a mutated gene carried on the X chromosome.

a. Using appropriate symbols and a genetic diagram, explain how two parents with the disease could end up with a son with haemophilia.

b. Explain why women are less likely to have haemophilia.

c. Describe why this disease will be especially dangerous for females to have.

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.