Human Genetics

5. Human Genetic Disorders

Human Genetic Disorders

Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes are known as autosomes or autosomal chromosomes.

Some genetic disorders are caused by recessive or dominant alleles of a single gene on an autosome. These disorders would then have the same inheritance pattern as any other dominant or recessive trait. An example of an autosomal recessive genetic disorder is cystic fibrosis. Children with cystic fibrosis have excessively thick mucus in their lungs which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will have a child with cystic fibrosis.

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Another recessive trait that we mentioned previously was sickle cell anemia. A person with two recessive alleles for the sickle cell trait (aa) will have sickle cell disease. In this disease the hemoglobin protein is formed incorrectly and the person’s red blood cells are misshapen. A person who does not carry the sickle trait has a homozygous dominant genotype (AA). Remember the trait showed incomplete dominance, so a person who is heterozygous for the trait (Aa) would have some sickle-shaped cells and some normal red blood cells.

You can also use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntington’s disease. If one parent has Huntington’s disease, what is the chance of passing it on to their children? If you draw the Punnett square, you will see that there is a 50 percent chance of the disorder being passed on to the children. Huntington’s disease causes the brain’s cells to break down, leading to muscle spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become apparent until middle age.

Genetic diseases can also be carried on the sex-chromosomes. An example of a recessive sex-linked genetic disorder is hemophilia. A hemophiliac’s blood does not clot, or clots very slowly, so he or she can easily bleed to death. As with colorblindness, males are much more likely to be hemophiliacs since the gene is on the X chromosome. Because Queen Victoria of England was a carrier of hemophilia, this disorder was once common in European royal families. Several of her grandsons were afflicted with hemophilia, but none of her granddaughters were affected by the disease, although they were often carriers. Because at the time medical care was very primitive, often hemophiliacs bled to death, and usually at a young age. Queen Victoria’s grandson Frederick died at age 3, and her grandson Waldemar died at age 11 (Figure below).

A pedigree chart shows all the phenotypes for a particular trait in the family. This pedigree chart traces back the occurrence of hemophilia in the British royal family. Those individuals with boxes around them are either female carriers of the trait or males afflicted with the trait.

Many genetic disorders are recessive, meaning that an individual must be homozygous for the recessive allele to be affected. Sometimes these disorders are lethal (deadly), however, heterozygous individuals (unaffected individuals with one dominant allele and one recessive allele) survive. This allows the allele that causes the genetic disorder to be maintained in a population's gene pool. A gene pool is the complete set of unique alleles in a species or population. A mutation is a change in the DNA sequence. New mutations are constantly being generated in a gene pool.