Think about how learnt sex-linked disorders are passed onto offspring.


Nope, that is what we call **sexually transmitted diseases** or STDs.


If you don't remember, take a guess and find out.


Oops, males are the gender with a Y chromosome.


What's the difference between dominant and recessive alleles?


Nope, one chromosome can contain thousands of genes.


Females have two X chromosomes, and as haemophilia is linked to an X chromosome, females can get this disorder.


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Yes, there are. Remember haemophilia is a recessive disease on the X chromosome. There is one male offspring who has an **h** on the X chromosome. This means he will be affected by the disease.


As males don't have a second X chromosome, a recessive allele on their only X chromosome will show in the phenotype, as they don't have another paired allele which could mask their effect. So the affected offspring is the one in the bottom right box.


This person is female. She has two X chromosomes. This means the dominant normal allele **H** has overridden the recessive **h** allele. This makes her a **carrier** for the disease. She can pass it on, but she does not herself suffer from it.


There are many important genes on the X chromosome.


It's more dangerous in a **male** embryo, because they only have 1 copy of the X chromosome. This means that they lose all the important genes on the X chromosome that are vital for survival. Therefore, if a male embryo has this syndrome, the embryo will not develop.


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Chromosomal abnormalities occur when chromosomes are made to be put in an egg or sperm  cell.


No, it isn't inherited. It's a chromosomal abnormality that occurs when chromosomes are made to be put in an egg or sperm.


Remember what happens to male embryos with Turner syndrome.


Turner syndrome isn't inherited, so it's neither recessive nor dominant. Turner syndrome is the lack of all or part of the X chromosome. Male embryos without an X chromosome don't develop.


Sex-linked disorders are related to a sex chromosome - either faulty genes on the sex chromosome, or the absence of one or part of one. Examples include haemophilia and Turner syndrome.

Sex-Linked Disorders

Mendel

Variation

Dominant and Recessive Alleles

Monohybrid Inheritance

Polygenic Variation

Constructing Punnett Squares

Blood Groups

Inherited Disorders

Sex Determination

The Human Genome Project

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