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More complex genetics: 5. The genetics of inheriting the genetic disorder of Huntington's disease

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INDEX of biology notes on more complex genetics - inherited sex/non-sex linked examples

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(6) Huntington's disease (genetic disorder) (an inherited genetic disorder, but not sex-linked)

Huntington's disease is a rare inherited disease and the condition is caused by the presence of one dominant gene, so only one allele is necessary to give the disease.

Huntington's disease affects nerve cells in the brain, leading to brain damage and usually becomes apparent in middle age, affecting around 1 in 10 000 people.

Associated with the Huntington's disease condition, with varying degrees, is the loss of intellectual ability, slurring of speech, hallucinations, mood and personality swings and memory loss.

There is no cure for Huntington's disease, and the deterioration of the brain cells and gradual loss of control of voluntary muscles by motor nerves is progressive and the condition eventually proves fatal.

Examples of a Punnett square analysis of Huntington's disease for the offspring's genotypes and phenotypes

H denotes the dominant allele and h denotes the recessive allele for Huntington's Disease.

Example 1. Parents A (heterozygous) and B (homozygous)

Punnett square analysis of offspring - the resulting allele pairings - phenotypes Parent A's gametes genotypes - alleles
H h
Parent B's gametes genotypes - alleles h Hh hh
h Hh hh

If one of the parents suffers from Huntington's disease, there is a 50% (1 in 2) of the child suffering from Huntington's disease, the other 50% chance would be normal and non-carries of the defective gene.

 

Example 2. Both parents A and B (heterozygous)

Punnett square analysis of offspring - the resulting allele pairings - phenotypes Parent A's gametes genotypes - alleles
H h
Parent B's gametes genotypes - alleles H HH Hh
h Hh hh

If my some great misfortune, both parents were sufferers of Huntington's disease, there would be a 75% (3 in 4) chance of any children suffering from Huntington's disease too. There is only a 1 in 4 chance of a child being normal and hence not a carrier of Huntington's disease.


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