4.3 Questions part II (includes review)
1. Outline how the process of meiosis can lead to Down’s Syndrome.
Accept any of the points below if clearly drawn and correctly
labelled in a diagram.
in metaphase homologues in centre of cell / spindles attached;
homologues are separating;
one pair doesn’t separate / non-disjunction;
in telophase cells divide into two;
cells have either one more / one less chromosome;
can occur in second division of meiosis;
sister chromatids fail to separate;
fertilization with one gamete / sperm / egg carrying extra chromosome;
Down’s syndrome is trisomy of chromosome 21;
4 max
2. Discuss the advantages and disadvantages of genetic screening
for chromosomal and genetic disorders.
genetic screening is testing for the presence or absence of gene /chromosome;
screening for chromosomes can involve karyotyping;
genetic screening is controversial;
advantages: [4 max]
parents can choose to avoid having children with disorder;
parents can prepare for a child with a disorder;
parents can use IVF to select embryos that are normal;
can use gene therapy to correct the problem;
treatment can start to prevent symptoms;
fewer children with the disorder are born;
disadvantages: [4 max]
frequency of abortion can increase;
parents can select embryos for sex of the child;
can have harmful side effects such as depression if you know you will
develop a disorder later;
can create a genetic underclass;
health insurance / treatment can be denied if there is genetic predisposition;
8 max
3. Outline the differences between the behaviour of the
chromosomes in mitosis and meiosis.
two divisions in meiosis, only one in mitosis;
meiosis results in haploid cells, mitosis in diploid cells;
crossing over only occurs in meiosis;
no S phase precedes meiosis II;
chromosome behaviour in meiosis II and mitosis is similar / chromosome
behaviour in meiosis I and mitosis is different;
chiasmata only form during meiosis;
homologous chromosomes move to the equator in pairs only in meiosis;
Do not accept number of cells produced - it is a result not a behaviour.
[5]
4. Why is meiosis referred to as reduction division?
i. “reduction” for producing cells containing fewer chromosomes
ii. “division” for dividing cells to produce new cells
5. Explain why meiosis rather than mitosis is necessary for gamete
production.
i. Meiosis halves the number of chromosomes in preparation for
two of these half (haploid) cells combining to make a whole cell
(diploid) which contains pairs of all the chromosomes.
6. State the name of two different cells in your body which are
diploid.
i. [any somatic cell: ex. muscle, fat, liver, nerve, etc.]
7. State the name of a cell in your body which is haploid.
i. [any germ cell: ex. “gamete” like ova or sperm.]
8. Explain the reason more women age 35-40 have karyotypes of
their unborn babies prepared than do pregnant women who are
aged 25-30.
i. [see tiger book question & graph]
9. Make a list of all variations and disorders and traits described
through 4.3 in textbook. See the partial example below:
i. trait - blood types - codominance - A (I^AI^A or I^Ai), B ( .....
ii. trait - sickle cell - codominance - Hb^A for disc shaped cell; Hb^S for
sickle cell shape
iii. Down's - disorder - nondisjunction of chromosome 21 (trisomy)
Pedigrees
Use the information provided below to create a pedigree. Then answer the
question at the end of each description.
1. Diagram and annotate a key for the symbols for pedigrees.
2. Make a pedigree showing a mother who carries the hemophilia gene
and a father who is normal, having children. In your pedigree make as
many children as possible who each have a unique combination of
alleles for this condition.
3. A man and woman marry. They have five children, 2 girls and 3 boys.
The mother is a carrier of hemophilia, an X-linked disorder. She passes
the gene on to two of the boys who died in childhood and one of the
daughters is also a carrier. Both daughters marry men without
hemophilia and have 3 children (2 boys and a girl). The carrier
daughter has one son with hemophilia. One of the non-carrier
daughter’s sons marries a woman who is a carrier and they have twin
daughters. What is the percent chance that each daughter will also be
a carrier?
4. The great-great maternal grandmother of a boy was a carrier for
color-blindness, an X-linked disorder. His great uncle on his mother’s
side was colorblind but this great uncle’s father was unaffected. The
boy’s mother has 2 brothers (1 colorblind, 1 unaffected) and 1 sister
(unaffected). The boy’s grandmother on his mother’s side had 1
brother who was colorblind and 3 sisters. Two of these sisters were
unaffected and one was a carrier. The boy’s great grandmother on his
mother’s side had 4 sisters. The boy has one unaffected sister and he
is colorblind. What is the probability of the boy’s sons being colorblind
if he marries a non-carrier?
5. An unaffected man marries a woman who is a carrier for Duchenne
Muscular Dystrophy, which is attributed to an X-linked gene. They
have four children, one with Duchenne, one carrier daughter and a
daughter and son who are unaffected. The child with Duchenne
Muscular Dystrophy dies in childhood. The carrier daughter marries
and has three children of her own, two of which are carriers and one
of which is unaffected. What is the most likely sex of these two carrier
children given the fact that they are unaffected by the X-linked gene?