Transcript Sex Linked Traits and Pedigrees

Sex Linked Traits
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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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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.
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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
X
X
XX
XX
Y
XY
XY
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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.
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X Chromosome & Y Chromosome
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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.
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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
– 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.
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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 sexlinked 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_Thomas
HuntMorgan.jpg
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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 sexlinked trait in humans.
• Are any of you colorblind?
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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
colorblind?
Mom
• No, but she is a carrier.
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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.
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