Download Sex Linked Traits and Pedigrees

Sex Linked Traits
J. R. Jones
Spring 2008
Human Chromosomes
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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
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On a human karyotype:
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Chromosome pairs 1-22 are known as autosomes.
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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.
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They determine the biological sex traits of an individual.
Males have XY and females have XX.
Punnett Squares with Sex
Chromosomes
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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
Sex-linked Traits
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Sex chromosomes can also determine
traits that do not effect your biological
sex.
Only the X chromosome carries alleles for
sex-linked traits.
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These alleles are written as superscripts.
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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
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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.
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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
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Consider the following crosses that Morgan conducted: R – red eyes r –
white eyes
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Red eyed male x heterozygous red eyed female
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What are the genotypes of the parents?
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XRY, XRXr
Show the cross between these two parents, giving the phenotype and genotype ratios.
White eyed male x homozygous red eyed female
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What are the genotypes of the parent?
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What sex are the white eyed offspring?
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All would be males.
Could a male be a carrier for white eye color?
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XrY, XRXR
Show the cross between these two parents, giving the phenotype and genotype ratios.
No, his one allele for recessive eye color will be expressed.
Is it possible for a female fly to have white eyes?
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Yes, but both of her parents would have to be white-eyed.
Morgan’s Conclusions
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The inheritance of certain traits is
linked to biological sex.
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Males are more likely to show a sexlinked trait because they have only one
X chromosome.
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Morgan noticed that a large majority of the
white-eye flies were male.
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.
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Males cannot be carriers of sex-linked
traits.
http://www7.nationalacademies.org/archives/4x6_Thomas
HuntMorgan.jpg
Sex-linked Traits in Humans
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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?
When Harry Met Sally…
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Harry and Sally are two normal vision adults who have a
color blind son.
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B – normal vision
b – color blind
Show the cross between Harry and Sally.
What are the genotypes of Harry and Sally?
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Give all of the possible genotypes and phenotypes of their offspring.
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XBY, XBXb
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?
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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:
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Baldness (some types)
Muscular Dystrophy
Hemophilia
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All of these traits happen to be sex-linked
recessive.