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CHROMOSOMES What Is a Chromosome? A long, continuous strand of DNA, plus several types of associated proteins, and RNA. In X-Y system, male determines sex of offspring. Inheritance of Genes on Sex Chromosomes • X-Linked traits are carried on the X chromosome and most are recessive Examples: color blindness, hemophilia • More common in females; affected males are hemizygous, cannot be carriers • Male inherits X-linked condition from carrier or affected mother • Y-linked traits are very rare because Y has few genes Hemophilia: recessive X-linked trait Genotype Phenotype XHXH non-carrier female XHXh carrier female XhXh female with hemophilia XHY normal male XhY male with hemophilia What is the probability that a carrier female and a normal male will have a son with hemophilia? ¼ or 25% What is the probability that a non-carrier female and a hemophiliac male will have a son with hemophilia? X Inactivation Female mammals have 2 alleles for every gene on the X chromosome, while males have only 1. This inequality is balanced by “turning off” one X chromosome in each cell of a 3 week old female embryo. – some cells turn off paternal X – some cells turn off maternal X Inactivated X appears as a dark- staining structure called a Barr body. How many Barr bodies would cells of a male possess? X inactivation is responsible for the appearance of calico cats. The earlier X inactivation occurs, the larger the patches. Chromosome Abnormalities 1. Polyploidy - extra full sets of chromosomes. – animal polyploids spontaneously abort or die shortly after birth – plant polyploids are relatively common (wheat, lilies) Aneuploidy - an extra (trisomy) or missing (monosomy) chromosome. Aneuploidy is usually due to a meiotic error called nondisjunction. • Autosomal aneuploids – trisomy 13 – trisomy 18 – trisomy 21 (Down syndrome) • Sex chromosome aneuploids – Turner syndrome XO female – Triplo-X XXX female – Klinefelter syndrome XXY male – Jacobs syndrome XYY male Deletion - part of a chromosome is missing. Duplication - part of a chromosome is present twice. Inversion - part of a chromosome is reversed. Translocation - nonhomologous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation).
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