Sex Linked traits
Chi-square tests, cont.
Try this Experiment
You and your neighbor each choose a number, 1 or 2. Repeat 20x
and record the number of times you said the same
number.
This is analogous to Mendelian inheritance of a simple cross
Aa x Aa, and asking how many offspring out of 20 are homozygotes.
In other words, do the two alleles match (either AA or
aa).
Compute the X2 value for your
experiment
Sum of (observed - expected)2 /
(expected)
What should you use for the expected value in this
experiment?
If all of you are doing the experiment the same way,
why aren't all of your Chi-squre values the
same?
Collect class data
Here is an example of what the results might look like, with
X2 values on the y axis and frequency of occurrence on
the x axis.
>10
9
8 x
7
6
5 x
4 xx
3 xxxxxxxxx
2 xxxxxxxxxxxxxxxx
1 xxxxxxxxxxxxxxxxxxxxxxxx
0 xxxxxxxxxxxxxxxx
Frequency
Most Chi-square values are near zero, because everyone did the
experiment the same way and there was no reason to expect much
deviation from 10 matches out of 20 tries.
Occasionally we got a big value by chance, but 95% of the time
the Chisquare values were less than about 4.
\
Therefore, if you collect data from a similar type of experiment
and you end up with a large Chi-square value (say, 8), you could
say that those results are significantly unusual, and the data
probably don't fit the model.
In practice, we don't have to do the experiments because the
probabilities of getting Chi-square values of a certain size have
been collected in a table- see Table 10.5 in your
book.
**************************
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?
Fig. 11.3a X-linked inheritance of white eyes in
Drosophila: Red-eyed female ´
white-eyed male
Fig. 11.3b X-linked inheritance of white eyes in
Drosophila:The F1 flies are interbred
to produce the F2s
We went over that cross in some detail: make sure you
understand how to keep track of the x and y chromosomes and why
white eyes are sometimes found in one sex but not the
other.
For sex-linked traits, the reciprocal cross shows
different results
See and understand Fig. 11.4
Summary of patterns for sex linked
traits:
Males are "hemizygous" for genes on the
X
You often see "criss-cross inheritance
The reciprocal crosses can give different results
Why doesn't that happen with
autosomes?
Non-disjunction of X
Bridges discovery of rare non-disjunction events confirmed
patterns of sex-linked inheritance.
Can you work out the inheritance patterns in crosses
with attached x chromosomes?
Attached X still a useful genetic tool in flies
(because Y0 and XXX flies die)
Example from current literature-- screening for x-linked
mutations. By crossing males to attached X females, they could see
the phenotypes of recessive mutanions in F1 males.
Diagram that cross for yourself to see how it
works.
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!
(actually, we know a lot about sex determination in plants, but
it happens in a lot of different ways).
Y Chromosome
Determines maleness in mammals
Only a couple of genes are important- most is
junk
SRY region at tip of Y does everything- transform a female mouse
with SRY and it becomes male.
Here is a cartoon view of the
Y
X dosage compensation in mammals
Problem: how can males have only one copy of X and still
have normal cellular function?
Answer: every mammal has only one functional
X
In females, one copy is inactivated to become a Barr
Body
Molecular basis of X inactivation
Insert Xic onto a male autosome -> results in Barr
bodies
Calico Cats
X linked coat color genes Oo and B-
Random inactivation of one copy early in development gives
patches of orange and black fur. Calico cats also have a dominant
gene for white, which determines the white patches.
Dosage compensation in other organisms
In flies transcription rate of single X in males is 2x
higher
No Barr bodies are produced.
Pedigrees with sex-linked traits
Don't see father to son transmission of trait.
(why?)
Usually expressed in males (why?)
Sex-biased expression 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 of a sex-linked trait:
hemophilia
Fig. 11.13b Pedigree of Queen Victoria (III-2) and her
descendants, showing the X-
linked recessive inheritance of
hemophilia