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,likeHuntington’sdisease. IfoneparenthasHuntington’sdisease, whatisthechance
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 (Figure6.10).
Figure 6.10: 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
inflicted with the trait. ( 13 )
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,