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Unformatted text preview: hemizygous
***Sex-Determining Region Y (480000): Although the distinctive pattern of inheritance of Y-linked genes (only
found in males) should make them easy to identify, only a handful of genes have yet been mapped to the Y
chromosome. The SRY gene is one of the most interesting and important of those, as it codes for the "testes
determining factor," the transcription factor that initiates the pathway of gonad differentiation into the testes, the first
step in the development of the male phenotype. This single gene can cause XX zygotes to develop into males
(although sterile) as has been found in experimentally manipulated mice and naturally occurring human XX males
resulting from rare translocations of the SRY gene from the Y to the X during meiosis in the father. XY females
have also been identified - in these sterile individuals the SRY gene has been lost from the Y chromosome.
***Pattern Baldness (109200): Loss of scalp hair in a pattern which itself seems to be inherited. The loss of hair
is due to the presence of a dominant allele (B > b) and the presence of high levels of testosterone and
dihydrotestosterone (DHT). Because its expression is a function of hormones that are more prevalent in males than
in females, the mutation behaves like a simple dominant in males but a recessive in females. Hairy Pinna (139500): Hair on the external pinna (ears). Long thought to be a Y-linked trait, it has been found in
females and its inheritance is most consistent with sex-influenced expression of an autosomal dominant trait.
POLYGENIC (QUANTITATIVE) INHERITANCE
***Skin color and eye color are two among many examples of quantitative inheritance that are not so readily
AUTOSOMAL CHROMOSOME ABNORMALITIES
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