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Sex Linked Traits

Sex Linked Traits. J. R. Jones Spring 2008. Human Chromosomes. Chromosomes are only visible during cell division (mitosis/meiosis). To analyze chromosomes, biologist photograph cells during mitosis and align the chromosomes in homologous pairs. This picture is known as a karyotype.

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Sex Linked Traits

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  1. Sex Linked Traits J. R. Jones Spring 2008

  2. Human Chromosomes • Chromosomes are only visible during cell division (mitosis/meiosis). • To analyze chromosomes, biologist photograph cells during mitosis and align the chromosomes in homologous pairs. • This picture is known as a karyotype. http://www.miscarriage.com.au/images/pages/karyotype_normal.jpg http://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg

  3. Types of Chromosomes • On a human karyotype: • Chromosome pairs 1-22 are known as autosomes. • They code for traits that are not related to sex. • Both males and females have the same types of autosomes. • Chromosome pair 23 includes the sex chromosomes. • They determine the biological sex traits of an individual. • Males have XY and females have XX.

  4. Punnett Squares with Sex Chromosomes • Remember: Sex is determined by the father of a child. • Every time a couple has a baby, there is a 50% chance of having a boy or a girl. X Y X XY XX XX XY X

  5. Sex-linked Traits • Sex chromosomes can also determine traits that do not effect your biological sex. • Only the X chromosome carries alleles for sex-linked traits. • These alleles are written as superscripts. • Remember Dragon Meiosis – XH for horns, Xh for no horns. • The Y chromosome does not carry any alleles for sex-linked traits.

  6. X Chromosome

  7. Y Chromosome

  8. Thomas Morgan’s Experiment • One of the first scientists to investigate sex-linked traits was Thomas Morgan (1866-1945). • Morgan was an embryologist by trade, and wanted to determine what factors influenced human inheritance. • He conducted experiments to observe the inheritance patterns of eye color in Drosophila (fruit flies). • There are three possible eye colors in fruit flies – red, sepia and white. • Red is dominant to sepia both are dominant to white. http://www.cas.vanderbilt.edu/bsci111b/drosophila/flies-eyes-phenotypes.jpg

  9. Morgan’s Experiment • Consider the following crosses that Morgan conducted: R – red eyes r – white eyes • Red eyed male x heterozygous red eyed female • What are the genotypes of the parents? • XRY, XRXr • Show the cross between these two parents, giving the phenotype and genotype ratios. • White eyed male x homozygous red eyed female • What are the genotypes of the parent? • XrY, XRXR • Show the cross between these two parents, giving the phenotype and genotype ratios. • What sex are the white eyed offspring? • All would be males. • Could a male be a carrier for white eye color? • No, his one allele for recessive eye color will be expressed. • Is it possible for a female fly to have white eyes? • Yes, but both of her parents would have to be white-eyed.

  10. Morgan’s Conclusions • The inheritance of certain traits is linked to biological sex. • Morgan noticed that a large majority of the white-eye flies were male. • Males are more likely to show a sex-linked trait because they have only one X chromosome. • If they inherit a recessive allele X chromosome, there would be no dominant allele present to mask it. • A carrier has the recessive allele in it’s genotype, but this is not expressed in their phenotype. • Males cannot be carriers of sex-linked traits. http://www7.nationalacademies.org/archives/4x6_ThomasHuntMorgan.jpg

  11. Sex-linked Traits in Humans • Morgan’s conclusions explained the high incidence of certain traits/diseases within predominantly male populations. • Color blindness is a commonly known sex-linked trait in humans. • Are any of you colorblind?

  12. When Harry Met Sally… • Harry and Sally are two normal vision adults who have a color blind son. • B – normal vision b – color blind • Show the cross between Harry and Sally. • What are the genotypes of Harry and Sally? • XBY, XBXb • Give all of the possible genotypes and phenotypes of their offspring. • XBXB – normal vision daughter • XBXb – normal vision daughter (carrier) • XBY – normal vision son • XbY – color blind son • Sally’s dad has normal vision. Is Sally’s Mom colorblind? • No, but she is a carrier. http://www1.istockphoto.com/file_thumbview_approve/2341727/2/istockphoto_2341727_cute_cartoon_couple_drinking_vector.jpg

  13. Other sex-linked traits: • Baldness (some types) • Muscular Dystrophy • Hemophilia • All of these traits happen to be sex-linked recessive.

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