Question Key
We’ve covered biological sex determination in a content video,
including the idea that we distinguish between somatic and sex
chromosomes. Note that somatic chromosomes are also called autosomes.
Some ways somatic and sex chromosomes differ are that the sexes have
differing sets of sex chromosomes and the two sex chromosomes do not
contain all of the same genetic information as each other (some genes
are shared but most are found only on one of the two sex chromosomes).
Consider an organism with ZZ/ZW sex determination, where ZZ are male and
ZW are female – this is what birds and some other animals do.
We can see that there appears to be a difference in copy number (aka
dosage) for ZW individuals:
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ZW and ZZ have 2 copies of all genes on the autosomes.
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ZW have only 1 copy of genes on Z or W.
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ZZ have 2 copies of Z genes and 0 copies of W genes.
That is, for genes found only on Z or only on W, the ZW organism only
has one version but for all of the autosomes (like Chromosome 1), the
same organism has two copies of every gene. This suggests that when
there are sex chromosomes, an organism must have some way to deal with
this difference in dosage, some mechanism of dosage compensation.
Remember from thinking about the BarH1 gene that the number of copies of
a gene can impact the phenotype, it can change gene expression such as
by increasing the amount of transcript produced (and thus possibly the
amount of protein).
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Considering what you have learned about transcription and translation,
suggest 2 different mechanisms by which gene expression can be altered
to achieve similar levels of protein production in ZZ and ZW
individuals, for a gene found on the Z chromosome. (Note that telling me
“chromatin state could be altered” is not sufficient.) Remember the
alternative splicing case study where we discussed things like this!
Multiple possiblities exist,
here are a few.
In order to equalize gene
expression, a ZZ individual could highly methylate the histones of one
Z, which would greatly reduce or prevent gene expression from that
chromosome (this would be like the Barr body in human
females).
Alternatively, the ZW
individual could enhance expression of the genes on their single Z,
perhaps producing extra transcription factors to bind enhancers and in-
crease expression of individual genes.
Another possibility is that the
ZZ individual could reduce transcription on both of its Zs so that it
matches the amount of transcription from just one Z.
In humans, it turns out that having an extra or a missing sex
chromosome often has few effects, compared to having an extra or missing
autosome. In fact, for 1 in every 1000 live births, the newborn is
either XXX (“female”) or XYY (“male”). Such individuals experience
typical physical and mental development expected for XX or XY; they may
never know of the extra chromosome unless they are karyotyped for some
other reason. These individuals are also fertile, producing typical
haploid gametes possessing only a single sex chromosome (their children
do not inherit their condition).
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The question before us is: what happens with gene expression that
results in XXX and XYY individuals being just like XX or XY individuals?
Propose an explanation for the lack of phenotypic effects of each
karyotype, XXX and XYY (the reasons are likely to be different…).
For the XXX individual,
remember that XX individuals already shut down one X by forming a Barr
body (X-inactivation). In the case of triple-X, two of the X’s form Barr
bodies. This would result in the usual phenotypic expression of just one
X.
For the XYY individual, it
could be that genes on the extra Y are expressed but there aren’t that
many genes on the Y-chromosome (only 55) and overexpression of them may
just not have that much of an effect. In fact, the National Human Genome
Research Institute says that as they age, many men lose the Y chromosome
from their cells, without apparent problems.
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Furthermore, given that the parents of XYY or XXX were XX and XY, what
happened during meiosis to yield a YY or XX gamete (thus giving XXX or
XYY zygotes)? (Or you can consider: what happens in meiosis to result in
the triploid individual producing typical haploid gametes, X or Y?)
The problem in cell division
is most likely to have happened during gamete formation in one of the
parents. Alternatively, it could occur during very early embryonic
development (and thus be present in most of that person’s
cells).
Assuming the event occurred
during meiosis, for an individual to have XYY, the gamete from their
father must have been YY (since females generally have no Y chromosome).
For an individual to have XXX, the gamete from one parent must have been
XX (possibly either parent since both carry X). This can happen when
there is non-disjunction during meiosis I or meiosis II.
If during meiosis I, the
homologous pairs would fail to separate and both end up in the same
daughter cell, yielding 2 daughter cells with 2 copies of the chromosome
and 2 with no copies at the end of the process.
If during meiosis II, sister
chromatids fail to separate and you may have 2 regular hap- loid
daughter cells and 2 that are aneuploid (one cell with 2 copies of the
chromosome and one cell with no copies).
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Consider Mendel’s Law of Segregation. What observations of his led him
to the idea of segregation? What actually happens during meiosis that
explains the patterns Mendel observed in pea plants?
Mendel observed that when
parents with different phenotypes had offspring, sometimes one of the
parental phenotypes seemed to disapear in the immediate offspring (F1
generation) but then might reappear in the F2 generation,
like white flower color. This led him to suggest that the factor causing
white flower color and the factor causing purple flower color remained
distinct from each other. They didn’t meld together to make something
new, rather each version maintained its own integrity and could thus
reappear intact in a later generation. Now we know that segregation of
factor, or chromosomes, occurs during anaphase I of meiosis, when
homologous pairs of chromosomes are separated from each other. This also
happens to be the point at which the daughter cells become haploid. As a
result of segregation, each gamete (egg or sperm) carries only 1 copy of
the flower color gene, either the purple allele or the white
allele.
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From Epigenetics: Provide one genetic and one epigenetic explanation for
an individual having increased risk of obesity. If we are doing an
experiment, for example with rats or mice, how can we distinguish
between an individual that is obese due primarily to genetic factors and
one that is obese due primarily to epigenetic factors? Consider what
variables you need to control to tell whether a difference is due to
genetics compared to due to the environment influencing epigenetics.
Many potential reasonable
responses. It’s important to figure out how you can tell the difference
between organisms being different due to genetic differences versus due
to epigenetic (environment-induced) differences. So, it might be good to
have multiple genotypes and multiple environmental conditions (such as a
control diet versus one that is thought to lead to obesity). If
organisms with the same genotype differ in their propensity to obesity
depending on diet, that strongly implicates a role for epigenetic
effects on obesity. If genotypes differ in obesity risk even when in the
same environment (same diet), that suggests a role for genetic variation
in the risk of obesity.