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Biology 110 - Basic Concepts and Biodiversity

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Chromosome Behavior and Sex Chromosomes

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As with autosomal traits, you can utilize the Punnett square to study the transmission of sex-linked traits. When completing these crosses, the gametes identify the sex chromosomes only. The genes located on the X chromosome are indicated with superscript letters. (We will not examine any cases where we track a Y-linked gene.) As with Mendelian traits, a lowercase letter indicates a recessive allele, while an uppercase letter indicates a dominant allele. When discussing sex-linked traits, carriers (females who are heterozygous for a recessive trait) need to be considered. Figure 5 illustrates several examples of genetic crosses involving sex-linked traits. White boxes represent unaffected individuals, light-colored boxes represent carriers, and dark-colored boxes represent affected individuals. 

 

Transcript for Sex Chromosomes - Part I

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Figure 5. The transmission of sex-linked recessive traits for different genetic crosses. (Click image to enlarge)

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The gene for the inactivation of the X chromosome, called the XIST gene, is located on the X chromosome. This gene does not appear to code for a protein. It is one of the few genes expressed on the inactivated X chromosome. The gene is transcribed, producing many copies of an RNA molecule. These RNAs attach to the DNA on the same X chromosome, inactivating it. Scientists are still researching the phenomenon. For example, it is uncertain how cells determine if an X chromosome should be inactivated or not. There is still a lot to learn about X-inactivation in mammals.

 Transcript for Sex Chromosomes - Part II

 

Transcript for Sex Chromosomes - Part III

Transcript for Sex Chromosomes - Part IV

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Sex-linked Gene Example:  Retinitis pigmentosa

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Questions?  Either send Send your instructor a message through ANGEL or attend an office hour (the times are posted on ANGEL).Canvas!

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