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. http://www.miscarriage.com.au/images/pages/karyotype_normal.jpg http://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg

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.

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

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.

X Chromosome

Y Chromosome

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

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 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.

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

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?

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

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