Different versions of a gene are called alleles. Alleles are described as either dominant or recessive depending on their associated traits.
Illustration to show the inheritance of dominant and recessive alleles for eye colour. Image credit: Genome Research Limited
What are sex-linked genes?
These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl). Females have two Xchromosomes (XX), while males have one X chromosomeand one Y chromosome (XY). This means females have two alleles for X-linked genes while males only have one. The allele for haemophilia is recessive so two copies are needed for a female to have the disease However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease. This means haemophilia is much more common in males than in females.
Functioning allele = H
Haemophilia allele = h
XHXH = healthy female
XHXh = carrier female
XhXh = haemophilia female
XHY = healthy male
XhY = haemophilia male
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What is a gene?
Genes are small sections of DNA within the genome that code for proteins. They contain the instructions for our individual characteristics – like eye and hair colour.
What is inheritance?
Inheritance is the process by which genetic information is passed on from parent to child. This is why members of the same family tend to have similar characteristics.
What is genetic variation?
Genetic variation is a term used to describe the variation in the DNA sequence in each of our genomes. Genetic variation is what makes us all unique, whether in terms of hair colour, skin colour or even the shape of our faces.
What are haemophilia A & B?
Haemophilia A and B are two disorders characterised by slow and inefficient formation of blood clots leading to prolonged bleeding and spontaneous internal bleeding.
What are single gene disorders?
Single gene disorders are caused by DNA changes in one particular gene, and often have predictable inheritance patterns.
What is a genetic disorder?
A genetic disorder is a disease that is caused by a change, or mutation, in an individual’s DNA sequence.
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