Copyright Pearson Prentice Hall Sex-Linked Genes Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes(X & Y) are called sex-linked genes. HOWEVER; MOST WE WILL DISCUSS ARE ON THE X CHROMOSOME! More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Almost all are recessive. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes X Chromosome Duchenne muscular dystrophy The Y chromosome is much smaller than the X chromosome and appears to contain fewer genes. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Why are sex-linked disorders more common in males than in females? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. What does this mean…males only have to have one recessive allele and they will have the recessive phenotype. Females with one allele for these conditions are called carriers. They are “normal”. Ex: Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Colorblindness Three human genes associated with color vision are located on the X chromosome. In males, a defective version of any one of these genes produces colorblindness. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Possible Inheritance of Colorblindness Allele X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles. Copyright Pearson Prentice Hall

Hemophilia; “bleeder’s disease” Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Practice problem… IF an affected male (with hemophilia) marries a woman who is a normal, what is the probability that their children will have hemophilia? Copyright Pearson Prentice Hall

X-Chromosome Inactivation British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly switched off. This chromosome forms a dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males because their single X chromosome is still active. This explains why XXX females don’t show symptoms. Copyright Pearson Prentice Hall

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Copyright Pearson Prentice Hall Tallulah Copyright Pearson Prentice Hall

Ectodermal dysplasias. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–2 Which of the following genotypes indicates an individual who is a carrier for colorblindness? XCX XCXc XcY XCY Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–2 Colorblindness is much more common in males than in females because the recessive gene on the male’s single X chromosome is expressed. genes on the Y chromosome make genes on the X chromosome more active. females cannot be colorblind. colorblindness is dominant in males and recessive in females. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–2 The presence of a dense region in the nucleus of a cell can be used to determine the sex of an individual. blood type of an individual. chromosome number of an individual. genotype of an individual. Copyright Pearson Prentice Hall