Download MCDB 1041 Activity 4 Complex Patterns and Errors in Meiosis

MCDB 1041 Activity 4 Complex Patterns and Errors in Meiosis
Recognizing complex patterns.
1. The disease shown in the pedigree is most likely inherited
in what kind of manner (s) ? (Autosomal dominant,
Autosomal recessive, X-linked dominant, X-linked recessive,
Mitochondrial)
Explain!
2. The pedigree at right could be an example of:
a. Normal autosomal dominant inheritance
b. Mitochondrial inheritance
c. Incomplete dominance
d. Co-dominance
e. Incomplete penetrance
Explain!
3. Niemann Pick Type C disease is a recessive disorder that causes accumulation of cholesterol and other lipids
in lysosomes, ultimately affecting both the liver and the nervous system. Below are the genotypes and
phenotypes of offspring of a family with a history of Niemann Pick.
7 NN ( all normal phenotype)
3 Nn (all normal phenotype)
4 nn (1 early onset dementia, 1 mid-life onset dementia, 2 late-onset dementia).
From this information, Niemann-Pick disease is an example of:
a. variable expressivity
b. incomplete dominance
c. incomplete penetrance
d. variable expressivity and incomplete penetrance
e. multiple alleles
Explain!
4. Barth syndrome, severe X-linked cardiomyopathy, and X-linked endocardial fibroelastosis are all diseases in
which there is damage to heart muscle. Recently, gene sequencing showed that these diseases result from
different mutations in the same gene. How can you explain this?
5. Genes that cause deafness are on chromosomes 6, 11, 17 and 19; these forms of deafness are all inherited in
an autosomal recessive fashion. An individual who has one deafness allele on chromosome 6 and one on 11 is
not deaf. Why?
Chromosomal separation errors (non-disjunction) during meiosis are a common cause of miscarriages
(approximately 10-20% of all human fertilized eggs contain chromosome abnormalities!); however, this is still
a relatively rare event. Usually, during meiosis, chromosomes separate to produce gametes with 23
chromosomes (one of each kind of chromosome). Nondisjunction can happen to any chromosome; inheriting
the incorrect number (more or less than 2) of a chromosome is usually fatal (although not in the case of the
smallest chromosome, #21, and in the case of the sex chromosomes). The dish with paper chromosomes can be
used to work on this problem if you like.
Let's practice visualizing non-disjunction and its effects
1. Kari has the genotype: Aa on chromosomes 1 and Ff on chromosomes 3. Draw Kari’s chromosomes as they
are entering into meiosis I in the cell below. Make sure you label the chromosomes with the correct genotypic
letters (A, a, F, f)
2. Kari has a chromosome separation error in meiosis I. Let’s say this error occurs in the separation of the
chromosome 1’s in meiosis I (these are the chromosomes with the A/a alleles). Show here with a drawing what
the resulting cells of meiosis I will look like. Then, draw the outcome of the gametes after meiosis II is
complete.
3. Pete (from the handout for Wednesday’s class) has a daughter with genotype XXX. A person with this
genotype can be the result of nondisjunction in either the production of sperm or egg. We’re going to follow
nondisjunction in the formation of sperm in this example. Draw Pete’s sex chromosomes, and another
chromosome (say, chromosome 3) as they would look in metaphase I of meiosis.
Let’s say that meiosis I is normal, but in meiosis II, the sister chromosome pairs of the X chromosome do not
properly separate in anaphase II (immediately after metaphase II of meiosis II) in one of the cells produced
in meiosis I.
Draw the outcome of meiosis I (remember it’s normal!) and line the chromosomes up as they would be in
metaphase of meiosis II:
3. Now draw the chromosomal contents of Pete’s sperm after meiosis is complete. Star * the gametes with
abnormal chromosomal number.
4. If Pete produces 2 sperm that each have an X and a Y, and 2 sperm that have no sex chromosomes, when did
the nondisjunction event occur: meiosis I, meiosis II, or it cannot be determined? Explain your answer (it may
help to draw out the possibilities!).
5. Let’s say one of Pete’ abnormal sperm that is genotype XY fertilizes a normal egg. What sex chromosomes
will the embryo have?
6. Claire is a carrier for the X-linked recessive disorder muscular dystrophy. Mutations in the DMD gene cause
muscular dystrophy. Draw the alignment of the X chromosomes (using D for the normal version of the gene
and d for the mutant version) and chromosome 21 in metaphase of meiosis I for an oocyte:
7. Assuming meiosis I is normal, draw the alignment of the chromosomes in metaphase of meiosis II:
8. Claire experiences a nondisjunction event for the X chromosome in meiosis II (say the event occurs in the
cell with the X chromosome that has the D allele). Draw the possible gametes that will be produced (with
respect to the X chromosome and chromosome 21)?
9. If one of Pete’ sperm (from question 3 or 5) that has no sex chromosomes fertilizes one of these eggs above,
what sex chromosomes will the embryo have? An individual can survive without a Y chromosome, but not
without an X chromosome (everyone needs at least one X). Knowing this, mark off the embryos that will die.
10. Summarize: what do you notice about the gametes produced from a non-disjunction event in meiosis I vs.
meiosis II? Compare the outcomes and write a summary statement that may help you interpret such problems
in the future. Please turn this in as a group on a separate piece of paper.
Practice Problems--Non-disjunction
1. You are presented with a patient who has Klienfelter’s syndrome (XXY). You determine that the
nondisjunction event occurred during meiosis II in the mother. Would the two X chromosomes from the
mother have the same or different alleles? (assume no recombination occurs)
a. 50% of the alleles would be the same and 50% of the alleles would be different
b. Different alleles
c. Same alleles
2. You are presented with a patient who has Down syndrome (3 copies of chromosome 21). The mother was
Bb for a gene of interest on chromosome 21; the father was BB. You know the nondisjunction event
occurred in the mother. If the resulting genotype of the patient is BBb, when during meiosis did the
nondisjunction event occur (assuming no recombination)?
a. During meiosis I
b. During meiosis II
c. It could have happened during meiosis I or II
3. Jill is a carrier for the autosomal recessive disease cystic fibrosis (Ff). She is also a carrier for the X-linked
trait muscular dystrophy (XD Xd). Jill produces an egg that has the F allele of the cystic fibrosis gene and two d
alleles of the muscular dystrophy gene on the X chromosome. What can you conclude?
a. The egg is normal
b. A meiosis I nondisjunction event occurred
c. A meiosis II nondisjunction event occurred
d. A nondisjunction event occurred, but you cannot determine if it occurred during meiosis I or meiosis II
4. If Dave is genotype AaBb with respect to two genes you are studying, and he produces a gamete that has
genotype b (no A alleles), in which phase of meiosis did a nondisjunction event occur?
a) During meiosis I
b) During meiosis II
c) It could have happened during meiosis I or II.