As we have seen, there are two types of cloning – reproductive and therapeutic. Reproductive cloning involves an embryo that has been created and implanted into a surrogate mother, where it is allowed to develop into a new organism. This has now been done for many diff erent species of animals but not, as yet, for humans.
In the therapeutic cloning process, no sperm fertilisation is needed nor is it necessary to implant an embryo into the uterus to create a child.
In the future, human therapeutic cloning could begin with the extraction of a nucleus from a donated egg. This nucleus holds the genetic material from the donor. Scientists might then take a body cell from their patient and extract the nucleus from it. The patient might need a stem cell transplant to treat a health condition or disease.
The patient’s nucleus would be substituted into the egg cell so that the egg then contains the patient’s genetic material. The egg could be stimulated to divide and soon form a cluster of cells rich in stem cells.
Stem cells could be isolated and infused into the patient where they are needed to restore structure or function.
One of the major benefi ts of therapeutic cloning is that the stem cells are pluripotent, which means they can give rise to almost any cell type in the body. Another advantage is that there is no risk of immunological rejection because the patient’s own genetic material is used. Therapeutic cloning has the potential to dramatically reduce waiting times for organ transplants as well as eliminating the immunological concerns associated with organ transplants.
Questions to consider
1 Therapeutic cloning is still at the research stage, using unused IVF embryos. Is it appropriate to take it to the next stage and create patient-specifi c embryos for therapeutic use?
2 The creation of an embryo requires a human egg cell. Some women are willing to donate eggs but is it ethical to buy them from willing donors?
3 The embryonic stem cells are genetically identical to the patient’s cells and therefore there is no need to use immunosuppressant drugs and no danger of tissue rejection. Is this a useful argument supporting therapeutic cloning?
4 Since embryonic stem cells can develop into any cell type, how can a surgeon be certain that the implanted cells will develop into the desired diff erentiated cells and not some other cell type such as cancer cells? Are these risks worth taking?
(1)
3 The karyogram below shows:
A Down’s syndrome
B a sex chromosome trisomy C a normal human female D a normal human male
End-of-chapter questions
1 Which of the following statements is correct?
A Both men and women can be carriers of a sex-linked allele.
B Mitosis reduces the number of chromosomes in a cell from the diploid to the haploid number.
C All of the genetic information in an organism is called a genome.
D The polymerase chain reaction separates fragments of DNA in a gel. (1)
2 The fruit fl y, Drosophila, has sex chromosomes X and Y, the same as humans. ‘White eye’ is caused by a sex-linked allele, which is recessive.
In the pedigree shown opposite, which two individuals must be carriers of the white allele?
A I4 and II3 B I4 and II2 C II1 and II2 D II2 and III2
III II I
this fly has white eye 1
1
1 2 3
2 3
2 3 4
1
6
13
21 22
14 15 16 17 18 19 20
7 8 9 10 11 12
2 3 4 5
(1)
4 GENETICS 1 97 4 Which of the following describes the behaviour of chromosomes during prophase I and metaphase II of meiosis?
Prophase I Metaphase II
(1) A Chromosomes undergo supercoiling. Sister chromatids are separated.
B Homologous chromosomes pair up together. Homologous pairs of chromosomes line up on the equator.
C Homologous chromosomes pair up together. Chromosomes line up on the equator.
D The nuclear envelope reforms. Chromosomes line up on the equator.
5 A test cross is carried out on a plant of unknown genotype; 50% of the off spring have the same phenotype as the test-cross parent. The conclusion is:
A the unknown parent is homozygous recessive B the unknown parent is homozygous dominant C the gene is sex linked
D the unknown parent is heterozygous (1)
6 The diagram shows a DNA electrophoresis gel. The results are from a single probe showing a DNA profi le for a man, a woman and their four children.
Which child is least likely to be the biological off spring of the father?
A Child 1
B Child 2
C Child 3
D Child 4
(1) 7 Ludovica is blood group AB and is expecting a baby with her husband Mikhail who is blood group A.
Mikhail’s mother was group O. Deduce the possible genotypes and phenotypes of their baby using a
Punnett grid. (4)
8 Discuss the ethical arguments for and against the therapeutic cloning of humans. (4)
9 Genetic modifi cation involves the transfer of DNA from one species to another. Discuss the potential
benefi ts and possible harmful eff ects of genetic modifi cation. Include a named example. (8) 10 Outline how a DNA profi le is obtained, including one way in which it has been used. (5)
11 Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application
of this process, including the way in which the chromosomes are obtained. (5)
1 2 3 4
child 1
child 2 father mother
child 4 child 3
origin
5 6
III IV I II
12 Outline two positive outcomes of the Human Genome Project. Do not include international cooperation. (2)
13 The diagram opposite shows the pedigree of a family with red–green colour-blindness, a sex-linked condition.
4 3
2 1
5 4
3 2
2nd generation
3rd generation
normal
male normal
female male with
condition female with condition Key
1st generation
1
2 1
a Defi ne the term ‘sex-linkage’. (1)
b Deduce, with a reason, whether the allele producing the condition is dominant or recessive. (2) c i Determine all the possible genotypes of the individual (2nd generation 1) using appropriate symbols. (1) ii Determine all the possible genotypes of the individual (3rd generation 4) using appropriate symbols. (1) (total 5 marks)
© IB Organization 2009
14 Potatoes with more starch have a lower percentage water content. This has an advantage in the transport, cooking and processing of potatoes.
In a strain of Escherichia coli, scientists found an enzyme that increases the production of starch. Using biotechnology, the gene for this enzyme was transferred to potatoes, increasing their starch content (transgenic potatoes). The gene was transferred to three potato varieties to create three transgenic lines.
The table shows the mean amount of starch and sugar contained in three lines of transgenic potatoes and normal potatoes (control), after storage for four months at 4 °C.
Carbohydrate/% of fresh weight
Potato Line Sugar Starch
transgenic
I 0.60 11.07
II 1.56 11.61
III 1.46 12.74
mean 1.21 11.81
control
I 5.14 5.88
II 5.61 3.70
III 4.32 6.35
mean 5.02 5.31
source: Stark et al, (1999), Annals of the New York Academy of Sciences, 792, pages 26–36
4 GENETICS 1 99 a State which line of transgenic potato has the greatest amount of starch. (1) b i Compare the levels of carbohydrate between the transgenic lines and the control potatoes. (2)
ii Suggest reasons for these diff erences. (2)
Potato tubers were harvested from the fi eld and stored in high humidity at 4 °C for three months. After this period, the tubers were stored at 16 °C, and samples were removed after 0, 3, 6 or 10 days, cut into strips, and fried. The colour of the fried potatoes was then measured and values reported using a 0to 4 rating (light to dark), where a score of 2 or lower indicates acceptable colour. The results are shown in the table.
c Evaluate the eff ect of transferring the E. coli gene on the suitability of the potatoes for frying. (2) (total 7 marks)
© IB Organization 2009
Transgenic 3
Number of days stored at 16°C
source: Stark et al, (1999), Annals of the New York Academy of Sciences, 792, pages 26–36
Key
Transgenic 2 Transgenic 1 Control 2 Control 1
Colour of fried potatoes 0
0 3 6 10
1 2 3 4
Control 3
Assessment statements