Download X linked traits

Lecture Outline 9/6/05
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More about pedigree analysis
Sex linked traits
Sex determination
Pedigrees for X linked traits
First, at look at the news this
week:
• Comparison of chimp genome with humans
• 35 million bp different (out of 3 billion total)
Check the course web site for homework problems
(due Sept 12)
Y chromosome has fewer genes than human Y
Phenylalanine-tyrosine metabolic pathways
The story of baby Pierre
• Born March 7, 1964, but “failed to thrive”; died
November 30
– He gained only 1/2 pound in 6 months
– His urine “smelled like rotten cabbage”,
• Similar children were also born in that same small village
– Chicoutimi area of northern Quebec.
• Occurred in both boys and girls.
– In families with one affected child, about 1/4 of children showed
the condition.
• Is this a genetic disorder? Why?
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Baby Pierre
• Here is the pedigree:
– Is the condition dominant or recessive?
– What is the genotype of L? of C?
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Pedigree for an autosomal recessive trait (albinism)
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Pedigree for an autosomal dominant trait (woolly hair)
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
What is the mode of inheritance for this trait?
Pedigree Analysis
• Recessive mutant alleles
– normal, heterozygous parents
– often "skips" generations
– matings between normal carriers should produce 25%
affected offspring
• Dominant mutant alleles
– every affected person has at least one affected parent
– trait does not usually "skip" generations
– on average, an affected person has 50% affected
offspring
I
II
C
D
III
A
B
A slightly harder problem:
A slightly harder problem:
• If K marries her cousin R, what is the
probability that they will have a normal
child?
• If K marries her cousin R, what is the
probability that they will have a normal
child?
– Practice using the rules of probability!
1. What genotypes are possible for K?
What is the probability of each?
2. What genotypes are possible for R?
3. Given those genotypes what is the
probability of having a normal
offspring?
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X-linked inheritance of white eyes in Drosophila: Red-eyed female ×
white-eyed male
Sex linkage
www.amphilsoc.org
www.amphilsoc.org
• 1910, TH Morgan found a white-eyed fly
• Cross of white male to red female gave:
– F1: All red eyes
– F2:
– 3,470 red-eyed flies.
– 782 white-eyed flies.
– All of the white-eyed flies were male.
• Morgan inferred that the w gene must be on the X
• WHY?
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
X-linked inheritance of white eyes in Drosophila:
The F1 flies are interbred to produce the F2 s
Sex linkage
• Males are “hemizygous” for genes on the X
• Cris-cross inheritance
• The reciprocal cross (ww female x WY
male gives different results.
– Try that yourself to see . . .
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Reciprocal cross shows different
results
• Red-eyed female x
white-eyed male
– XX x X wY
– F1:
• all red
– F2:
• 1/4 white, 3/4 red
• All white-eyed flies are
male
Reciprocal cross:
Homozygous white-eyed female × red-eyed ( wild-type) male
• White-eyed female x
red-eyed male
– XwXw x XY
– F1:
• Females: all red
• Males: all white
– F2:
• 1/2 white, 1/2 red
• Same in M and
F
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
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Reciprocal cross:
The F1 flies are interbred to produce the F2 s
Sex determination
• Lots of different mechanisms:
– In mammals, sex is determined by the Y
– In flies and C. elegans, sex is determined by #
copies of the X
– In birds and reptiles, females are heterogametic
(WZ females, ZZ males)
– In microorganisms (eg.yeast) sex is determined
by a few mating type genes
– In plants, who knows!
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Y Chromosome
Determines maleness
Pedigrees with sex-linked traits
• Never see father to son transmission of trait.
(why?)
• Usually expressed in males (why?)
• but that does not necessarily mean that the trait is sex linked.
Actually, the Y contains very few
genes: testes genes and SRY.
SRY alone is necessary and
sufficient for maleness.
– E.g. male pattern baldness: rare in females but not sex linked.
» Allele is dominant in males, recessive in females.
» Also horns in sheep, coat color in Ayrshire cattle etc.
• Famous example: hemophilia
– Queen Victoria’s gene . . .
Pedigree of Queen Victoria (III-2) and her descendants, showing the
X-linked recessive inheritance of hemophilia
For next time:
• Read Chapter 3
• Check website for second homework
assignment (due 9/12)
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
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