lecture outline 9/6/05 first, at look at the news this weekdstratto/bcor101/0906.pdf · x-linked...

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Lecture Outline 9/6/05

• More about pedigree analysis• Sex linked traits• Sex determination• Pedigrees for X linked traits

Check the course web site for homework problems(due Sept 12)

First, at look at the news thisweek:

• Comparison of chimp genome with humans• 35 million bp different (out of 3 billion total)

Y chromosome has fewer genes than human Y

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.

Phenylalanine-tyrosine metabolic pathways

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)


Pedigree for an autosomal dominant trait (woolly hair)

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

III

A B

C D

What is the mode of inheritance for this trait?

A slightly harder problem:• If K marries her cousin R, what is the

probability that they will have a normalchild?

A slightly harder problem:• If K marries her cousin R, what is the

probability that they will have a normalchild?– 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 theprobability of having a normaloffspring?

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Sex linkage

• 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?

www.amphilsoc.orgwww.amphilsoc.org


X-linked inheritance of white eyes in Drosophila: Red-eyed female ×white-eyed male


X-linked inheritance of white eyes in Drosophila:The F1 flies are interbred to produce the F2s

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 . . .

Reciprocal cross shows differentresults

• Red-eyed female xwhite-eyed male– XX x XwY– F1:

• all red

– F2:• 1/4 white, 3/4 red• All white-eyed flies are

male

• White-eyed female xred-eyed male– XwXw x XY– F1:

• Females: all red• Males: all white

– F2:• 1/2 white, 1/2 red• Same in M and

F


Reciprocal cross:Homozygous white-eyed female × red-eyed ( wild-type) male

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Reciprocal cross: The F1 flies are interbred to produce the F2s

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!

Y ChromosomeDetermines maleness

Actually, the Y contains very fewgenes: testes genes and SRY.

SRY alone is necessary andsufficient for 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.– 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 theX-linked recessive inheritance of hemophilia

For next time:

• Read Chapter 3

• Check website for second homeworkassignment (due 9/12)

lecture outline 9/6/05 first, at look at the news this weekdstratto/bcor101/0906.pdf · x-linked...

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