Sex Linked Traits

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

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

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

Sex-linked Traits Sex chromosomes can also determine traits that do not effect your biological sex. Sex chromosomes can also determine traits that do not effect your biological sex. Only the X chromosome carries alleles for sex-linked traits. Only the X chromosome carries alleles for sex-linked traits. These alleles are written as superscripts. These alleles are written as superscripts. Remember Dragon Meiosis – X H for horns, X h for no horns. Remember Dragon Meiosis – X H for horns, X h for no horns. The Y chromosome does not carry any alleles for sex-linked traits. 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 ( ). One of the first scientists to investigate sex-linked traits was Thomas Morgan ( ). Morgan was an embryologist by trade, and wanted to determine what factors influenced human inheritance. 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). 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. There are three possible eye colors in fruit flies – red, sepia and white. Red is dominant to sepia both are dominant to white. Red is dominant to sepia both are dominant to white.

Morgan’s Experiment Consider the following crosses that Morgan conducted: R – red eyes r – white eyes Consider the following crosses that Morgan conducted: R – red eyes r – white eyes Red eyed male x heterozygous red eyed female Red eyed male x heterozygous red eyed female What are the genotypes of the parents? What are the genotypes of the parents? X R Y, X R X r X R Y, X R X r Show the cross between these two parents, giving the phenotype and genotype ratios. Show the cross between these two parents, giving the phenotype and genotype ratios. White eyed male x homozygous red eyed female White eyed male x homozygous red eyed female What are the genotypes of the parent? What are the genotypes of the parent? X r Y, X R X R X r Y, X R X R Show the cross between these two parents, giving the phenotype and genotype ratios. Show the cross between these two parents, giving the phenotype and genotype ratios. What sex are the white eyed offspring? What sex are the white eyed offspring? All would be males. All would be males. Could a male be a carrier for white eye color? Could a male be a carrier for white eye color? No, his one allele for recessive eye color will be expressed. No, his one allele for recessive eye color will be expressed. Is it possible for a female fly to have white eyes? Is it possible for a female fly to have white eyes? Yes, but both of her parents would have to be white-eyed. 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. The inheritance of certain traits is linked to biological sex. Morgan noticed that a large majority of the white-eye flies were male. 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. 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. 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. 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. Males cannot be carriers of sex-linked traits. HuntMorgan.jpg

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

When Harry Met Sally… Harry and Sally are two normal vision adults who have a color blind son. Harry and Sally are two normal vision adults who have a color blind son. B – normal vision b – color blind B – normal vision b – color blind Show the cross between Harry and Sally. Show the cross between Harry and Sally. What are the genotypes of Harry and Sally? What are the genotypes of Harry and Sally? X B Y, X B X b X B Y, X B X b Give all of the possible genotypes and phenotypes of their offspring. Give all of the possible genotypes and phenotypes of their offspring. X B X B – normal vision daughter X B X B – normal vision daughter X B X b – normal vision daughter (carrier) X B X b – normal vision daughter (carrier) X B Y – normal vision son X B Y – normal vision son X b Y – color blind son X b Y – color blind son Sally’s dad has normal vision. Is Sally’s Mom colorblind? Sally’s dad has normal vision. Is Sally’s Mom colorblind? No, but she is a carrier. No, but she is a carrier /2/istockphoto_ _cute_cartoon_couple _drinking_vector.jpg

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