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The Inheritance of Thalassemia
Recall that hemoglobin is the protein that is affected
in thalassemia. Hemoglobin is made of heme, alpha globins, and beta globins.
At least 9 different genes direct the production of heme. Changes in these
genes may lead to disorders of heme production, a group of conditions
separate from the thalassemias. Alpha thalassemia occurs when a mutation
in the gene that codes for alpha globin results in reduced or absent production
of alpha globins. Beta thalassemia occurs with a corresponding change
in the beta globin gene. Therefore, the thalassemias are a result of quantitative
mutations in the globin genes.
Below is a karyotype, a picture of all of an individual’s genetic
information as seen through a microscope. Notice there are 22 pairs of
numbered chromosomes (autosomes) and one pair of sex-determining chromosomes.
Also notice that a male carries both an X- and a Y-chromosome, whereas
a female has two X-chromosomes. Because a man can pass down either an
X- or a Y-chromosome, he is the one who randomly determines the sex of
the baby.
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Female Karyotype (46-XX)
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Male Karyotype (46-XY)
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[images courtesy of Phil Cotter,
PhD FACMG]
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