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A human karyotype - female A human karyotype - male Sex determination in the fruit fly Sex determination depends on the ratio between number of X chromosomes and sets of autosomes Results when there is nondisjunction of chromosomes during meiosis Sex chromosome non-disjunction in humans XO – Turner syndrome Sex chromosome non-disjunction in humans XXY – Kleinfelter syndrome Achieving balance of genetic traits that are sex-linked in mammals - Barr bodies A calico kitten Pattern of inheritance of X and Y chromosomes determines sex Pattern of inheritance of traits that are Sex (X) linked - recess Pattern of inheritance of traits that are Sex (X) linked - recessive Non-disjunction can affect pattern of inheritance of sex-inked traits Pedigree analysis – sex linked traits what to look for: Recessive trait: 1. More males that females affected 2. Affected sons are usually born to unaffected mothers, zig-zag pattern – from grandfather to grandson through an unaffected female. 3. Approximately 50% of a carrier female are affected 4. It is never passed from father to son 5. All daughters of affected fathers are carriers Pedigree analysis – sex linked traits what to look for: Dominant trait: 1. Both males and females are affected; often more females than males 2. Does not skip a generation. Affected sons must have an affected daughter; affected daughters must have either an affected mother or an affected father. 3. Affected fathers will pass the trait on to all their daughters. 4. Affected mothers ( if heterozygous) will pass on the trait to approximately 50% of their daughters and 50% of their sons. X-linked dominant inheritance Faulty enamel Reminder If you are working with pedigrees and probability, and the trait is autosomal (dominant or recessive) the probability of inheriting the trait is independent from the sex of the individual. If the trait is sex linked the probability of inheriting the trait includes the sex of the individual.