Genetics

Question 1

What are Eukaryote chromosomes made of?

Answer 1

Eukaryote chromosomes are made of DNA and proteins.

Question 2

define gene

Answer 2

a heritable factor that controls a specific characteristic.

Question 3

define allele

Answer 3

one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.

Question 4

define genome

Answer 4

the whole of the genetic information of an organism.

Question 5

define gene mutation

Answer 5

a change to the base sequence of a gene

Question 6

Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia.

Answer 6

Sickle cell anaemia is a genetic disease that affects red blood cells in the body. It is due to a mutation on the Hb gene which codes for a polypeptide of 146 amino acids which is part of haemoglobin (haemoglobin is an important protein component in red blood cells). In sickle cell anaemia the codon GAG found in the normal Hb gene is mutated to GTG. This is called a base substitution mutation as adenine (A) is replaced by thymine (T). This means that when the mutated gene is transcribed, a codon in the messenger RNA will be different. Instead of the normal codon GAG, the messenger RNA will contain the codon GUG. This in turn will result in a mistake during translation. In a healthy individual the codon GAG on the messenger RNA matches with the anticodon CUC on the transfer RNA carrying the amino acid glutamic acid. However, if the mutated gene is present then GUG on the messenger RNA matches with the anticodon CAC on the transfer RNA which carries the amino acid valine. So the base substitution mutation has caused glutamic acid to be replaced by valine on the sixth position on the polypeptide. This results in haemoglobin S being present in red blood cells instead of the normal haemoglobin A. This has an effect on the phenotype as instead of normal donut shaped red blood cells being produced some of the red blood cells will be sickle shaped. As a result these sickle shaped red blood cells cannot carry oxygen as efficiently as normal red blood cells would. However, there is an advantage to sickle cell anemia. The sickle cell red blood cells give resistance to malaria and so the allele Hbs on the Hb gene which causes sickle cell anemia is quite common in parts of the world where malaria is found as it provides an advantage over the disease.

Question 7

What is meiosis?

Answer 7

Meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

Question 8

Define homologous chromosomes.

Answer 8

Homologous chromosomes: chromosomes with the same genes as each other, in the same sequence but do not necessarily have the same allele of those genes.

Question 9

Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

Answer 9

Meiosis involves two divisions. Meiotic cells have an interphase stage before the start of meiosis I which is similar to mitosis. It includes G1, S and G2 phases. (See notes on mitosis) After meiosis I there is another brief interphase stage which is followed by meiosis II.

Question 10

summarize Meiosis I

Answer 10

The first stage of meiosis I is prophase I. In prophase I the chromosomes pair up so that the chromosomes in each pair are homologous. Once the homologous chromosomes are paired up, crossing over occurs. Crossing over is the exchange of genetic material between non-sister chromatids. The nuclear membrane also starts to break down and the spindle microtubules stretch out from each pole to the equator.

The second stage is metaphase I. Here the paired up homologous chromosomes line up at the equator and the spindle fibbers attach to the chromosomes in a way that ensures that for each homologous pair, one chromosome moved to one pole and the other moves to the opposite pole.

The third stage is anaphase I. This is the stage where the homologous chromosomes are separated and pulled to opposite poles. This halves the chromosome number however each chromosome is still composed of two sister chromatids. The cell membrane starts to prepare for its separation at the equator to form two cells.

The fourth stage is telophase I. Here each chromosome from the homologous pair are found at opposite poles and the nuclear membrane reforms around each daughter nucleus. The membrane then divides through citokinesis.

There is a brief interphase stage before the start of meiosis II. This stage does not include the S phase.

Question 11

summarize meiosis II

Answer 11

The first stage of meiosis II is prophase II. Here the cell has divided into two daughter haploid cells however the process does not end here as these two cells immediately start to divide again. The spindle microtubules stretch out from each pole again and the nuclear membrane breaks down as in prophase I.

The second stage is metaphase II. Here the chromosomes in each cell line up at the equator and the spindle microtubules attach to the centromere of each chromosome.

The third stage is anaphase II. Here the centromere devised as a result of the spindle microtubules pulling each sister chromatid to opposite poles in both cells. Each sister chromatid then becomes a chromosome.

The fourth stage is telophase II. Here the nuclear membrane reforms around the four sets of daughter chromosomes. Cytokinesis then follows to divide the cytoplasm of the two cells and so the result is four daughter cells each with a haploid set of chromosomes.

Question 12

Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome

Answer 12

A number of problems can arise during meiosis. A common problem is non-disjunction. This is when the chromosomes do not separate properly during meiosis, either in meiosis I (in anaphase I) or meiosis II (in anaphase II). This leads the production of gametes that either have a chromosome too many or too few. Gametes with a missing chromosome usually die quite fast however gametes with an extra chromosome can survive. When a zygote is formed from the fertilization of these gametes with an extra chromosome, three chromosomes of one type are present instead of two. An example of this is Down syndrome. Down syndrome is a disease in which the chromosomes failed to separate properly during meiosis leading to three chromosomes of type 21 instead of two. A person with the condition therefore has a total of 47 chromosomes instead of 46. The non-disjunction can take place either in the formation of the egg or the sperm. Down syndrome leads to many complications and also the risk of having a child with the condition increases with age.

Question 13

How are chromosomes arranged in kareotyping?

Answer 13

In karyotyping, chromosomes are arranged in pairs according to their size and structure.

Question 14

Where are the cells used in kareotyping collected from?

Answer 14

Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Question 15

How can you use a kareotype to ascertain whether non-disjunction has been performed or gender?

Answer 15

Karyotyping can be used to determine gender of a fetus and look for chromosome abnormalities such as non-disjunction. The gender can be deduced by looking at the sex chromosomes. Females will have two X chromosomes while males have one X and one Y. We can distinguish this on with karyotyping as the Y chromosome is smaller than the X. As for non-disjunctions we can see if a chromosome is missing or if their is an extra one by looking at the number of chromosomes. If There should only be two of each chromosome. Each 23 chromosomes should have a pair resulting in 46 chromosomes in total. For example, if we notice that there are three chromosomes 21 then we can conclude that a non-disjunction occurred. In this case, the non-disjunction results in Down’s syndrome. (trisomy 21)

Below are two images of a karyotype. The first one is of a normal healthy male patient as on the karyotype there are two chromosomes for each chromosome number and a Y chromosome is present. The second image shows the karyotype a person with Down’s syndrome would get.

Question 16

define Genotype

Answer 16

the alleles of an organism.

Question 17

define phenotype

Answer 17

the characteristics of an organism.

Question 18

define dominant allele

Answer 18

an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.

Question 19

define recessive allele

Answer 19

an allele that only has an effect on the phenotype when present in the homozygous state.

Question 20

define Codominant alleles

Answer 20

pairs of alleles that both affect the phenotype when present in a heterozygote.

Question 21

define locus

Answer 21

the particular position on homologous chromosomes of a gene.

Question 22

define homozygous

Answer 22

having two identical alleles of a gene.

Question 23

define heterozygous

Answer 23

having two different alleles of a gene.

Question 24

define carrier

Answer 24

an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

Question 25

define test cross

Answer 25

testing a suspected heterozygote by crossing it with a known homozygous recessive.

Question 26

how many alleles do genes have?

Answer 26

Some genes have more than two alleles (multiple alleles).

Question 27

Describe ABO blood groups as an example of codominance and multiple alleles.

Answer 27

The ABO blood group is a good example of codominance and multiple alleles. There are three allele that control the ABO blood groups. If there are more than two allele of a gene then they are called multiple allele. The allele IA corresponds to blood group A (genotype IAIA) and the allele IB corresponds to blood group B (genotype IBIB). Both of these are dominant and so if IA and IB are present together they form blood group AB (genotype IAIB). Both allele affect the phenotype since they are both codominant. Codominant allele are pairs of allele that both affect the phenotype when present together in a heterozygote. The allele i is recessive to both IA and IB so if you have the genotype IA i you will have blood group A and if you have the genotype IB i you will have blood group B. However if you have the genotype ii then you are homozygous for i and will be of blood group O. Below is a table to summaries which genotypes give which phenotypes.

Question 28

Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

Answer 28

There are two chromosomes which determine gender. These are called the sex chromosomes and there are two types, the X and the Y chromosome. Females have two X chromosomes whereas males have one X and one Y chromosome. The X chromosome is relatively large compared to the Y (which is much smaller) and contains many genes. The Y chromosome on the other hand only contains a few genes. The female always passes on to her offspring the X chromosome from the egg (female gamete). The male can pass on either the Y or the X chromosome from the sperm (male gamete). If the male passes on the X chromosome then the growing embryo will develop into a girls. If the male passes on the Y chromosome then the growing embryo will develop into a boy. Therefore gender depends on whether the sperm which fertilizes the egg is carrying an X or a Y chromosome.

Question 29

Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

Answer 29

There are two chromosomes which determine gender. These are called the sex chromosomes and there are two types, the X and the Y chromosome. Females have two X chromosomes whereas males have one X and one Y chromosome. The X chromosome is relatively large compared to the Y (which is much smaller) and contains many genes. The Y chromosome on the other hand only contains a few genes. The female always passes on to her offspring the X chromosome from the egg (female gamete). The male can pass on either the Y or the X chromosome from the sperm (male gamete). If the male passes on the X chromosome then the growing embryo will develop into a girls. If the male passes on the Y chromosome then the growing embryo will develop into a boy. Therefore gender depends on whether the sperm which fertilizes the egg is carrying an X or a Y chromosome.

Question 30

Are all genes present on both the X and Y chromosome?

Answer 30

Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

Question 31

Define sex linkage.

Answer 31

Sex linkage: when the gene controlling the characteristic is located on the sex chromosome and so we associate the characteristic with gender.

Question 32

Describe the inheritance of colour blindness and hemophilia as examples of sex linkage.

Answer 32

Most of the time sex-linked genes are carried on the X chromosome. Since females have two X chromosomes they have two copies of the sex-linked gene whereas males only have one since they only have one X chromosome. Hemophilia and colour blindness are both examples of sex linkage.

Hemophilia
XH is the allele for normal blood clotting and is dominate over Xh which is recessive and causes hemophilia. If a mother is heterozygous she is a carrier of the disease but does not have hemophilia as the dominate allele is present. She can however pass the disease on to her offspring. Below is a punnett showing how a carrier mother and an unaffected father can pass the disease on to their offspring.
From our four possible outcomes we can see that a female child cannot get hemophilia but can be a carrier. This is because the father will always pass on the dominate allele (XH) on the X chromosome in females. Depending on whether the mother passes on the dominant or recessive allele will determine if the female child is a carrier or is unaffected by the hemophilia. If the child is a boy then the father has passed on the Y chromosome which does not contain the allele of the gene. So whether the child has the disease or is unaffected depends on which allele the mother had passed on. If she has passed on the recessive allele (Xh) then the male child will have hemophilia, however if she has passed on the dominate allele (XH) then the child will be unaffected.

So there is a 50% chance of the child having hemophilia if it is male as half of the eggs produced by the mother will carry the recessive allele. The chance of a female offspring having hemophilia is 0% since the father always passes on the dominant allele on the X chromosome. Finally there is a 25% chance overall that the offspring will be affected.

Question 33

Is a female homozygous or heterozygous with respect to sex linked genes?

Answer 33

A human female can be homozygous or heterozygous with respect to sex-linked genes.

Question 34

Are female carriers homozygous or heterozygous for X linked recessive alleles?

Answer 34

Female carriers for X-linked recessive alleles are always heterozygous since they require a dominant allele and a recessive allele to be carriers. They inherit the recessive allele from one parent and the dominate allele from the other. For example hemophilia is a sex-linked disease. If a carrier mother and an unaffected father have offspring then the unaffected father will always pass on his dominate allele to his female offspring. The carrier mother can either pass on the dominate or recessive allele. If she passes on the recessive allele to her female offspring than the female offspring will be a carrier as well.

Question 35

Explain that female carriers are heterozygous for X-linked recessive alleles.

Answer 35

Female carriers for X-linked recessive alleles are always heterozygous since they require a dominant allele and a recessive allele to be carriers. They inherit the recessive allele from one parent and the dominate allele from the other. For example hemophilia is a sex-linked disease. If a carrier mother and an unaffected father have offspring then the unaffected father will always pass on his dominate allele to his female offspring. The carrier mother can either pass on the dominate or recessive allele. If she passes on the recessive allele to her female offspring than the female offspring will be a carrier as well.

Question 36

What do symbols in pedigree charts mean?

Answer 36

Squares represent males
Circles represent females
Shaded symbols represent affected individuals
Unshaded symbols represent unaffected individuals.

Question 37

Identify trends in pedigree charts

Answer 37

If most of the males in the pedigree are affected the disorder is X-linked.
If it is a 50/50 ratio between men and women the disorder is autosomal.
If the disorder is dominant, one of the parents must have the disorder.
If the disorder is recessive than neither of the parents has to have the disorder as they can be heterozygous.

Question 38

Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA.

Answer 38

Polymerase chain reaction is used to copy and amplify minute quantities of DNA. It can be useful when only a small amount of DNA is available but a large amount is required to undergo testing. We can use DNA from blood, semen, tissues and so on from crime scenes for example. The PCR requires high temperature and a DNA polymerase enzyme from Thermus aquaticus (a bacterium that lives in hot springs).

Question 39

What are pieces of DNA separated by in electrophoresis?

Answer 39

In gel electrophoresis, fragments of DNA move in an electrical field and are separated according to their size.

Question 40

When is gel electrophoresis used?

Answer 40

Gel electrophoresis of DNA is used in DNA profiling.

Question 41

Describe the application of DNA profiling to determine paternity and also in forensic investigations.

Answer 41

Organisms have short sequences of bases which are repeated many times. These are called satellite DNA. These repeated sequences vary in length from person to person. The DNA is copied using PCRand then cut up into small fragments using restriction enzymes. Gel electrophoresis separates fragmented pieces of DNA according to their size and charge. This gives a pattern of bands on a gel which is unlikely to be the same for two individuals. This is called DNA profiling. DNA profiling can be used to determine paternity and also in forensic investigations to get evidence to be used in a court case for example.

Question 42

Analyse DNA profiles to draw conclusions about paternity or forensic investigations.

Answer 42

For a suspect look for similarities between the DNA found at the crime scene and the suspect. For a paternity test, look for similarities between the child and the possible father.

Question 43

Outline three outcomes of the sequencing of the complete human genome.

Answer 43

It is now easier to study how genes influence human development.
It helps identify genetic diseases.
It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded for by these genes.
It will give us more information on the origins, evolution and migration of humans.

Question 44

What happens when genes are transferred between species?

Answer 44

When genes are transferred between species, the amino acid sequence of polypeptides translated from them is unchanged because the genetic code is universal.

Question 45

Outline a basic technique used for gene transfer involving plasmids, a host cell (bacterium, yeast or other cell), restriction enzymes (endonucleases) and DNA ligase.

Answer 45

The human gene that codes for insulin can be inserted into a plasmid and then this plasmid can be inserted into a host cell such as a bacterium. The bacterium can then synthesis insulin which can be collected and used by diabetics. This is done as follows. The messenger RNA which codes for insulin is extracted from a human pancreatic cell which produces insulin. DNA copies are then made from this messenger RNA by using the enzyme reverse transcriptase and these DNA copies are then given extra guanine nucleotides to the end of the gene to create sticky ends. At the same time, a selected plasmid is cut using restriction enzymes which cut the DNA at specific base sequences. Then extra cytosine nucleotides are added to create sticky ends. Once we have both the plasmid and the gene ready, these are mixed together. The two will link by complementary base pairing (between cytosine and guanine) and then DNA ligase is used to make the sugar phosphate bonds. The plasmids with the human insulin gene (called recombinant plasmids) can then be mixed with host cells such as bacterium. The bacterium will take in the plasmid and start producing insulin which can then be collected and purified.

Question 46

State two examples of the current uses of genetically modified crops or animals.State two examples of the current uses of genetically modified crops or animals.

Answer 46

The transfer of a gene for factor IX which is a blood clotting factor, from humans to sheep so that this factor is produced in the sheep’s milk.
The transfer of a gene that gives resistance to the herbicide glyphosate from bacterium to crops so that the crop plants can be sprayed with the herbicide and not be affected by it.

Question 47

Discuss the potential benefits and possible harmful effects of one example of genetic modification.

Answer 47

It is quite common to see genetic modifications in crop plants. An example of this is the transfer of a gene that codes for a protein called Bt toxin from the bacterium Bacillus thuringiensis to maize crops. This is done because maize crops are often destroyed by insects that eat the corn and so by adding the Bt toxin gene this is prevented as the toxin kills the insects. However this is very controversial as even though it has many positive advantages, it can also have some harmful consequences. The table below summarizes the benefits and possible harmful effects of genetically modifying the maize crops.

Question 48

Discuss the potential benefits of one example of genetic modification.

Answer 48

Since there is less damage to the maize crops, there is a higher crop yield which can lessen food shortages.

Since there is a higher crop yield, less land is needed to grow more crops. Instead the land can become an area for wild life conservation.

There is a reduction in the use of pesticides which are expensive and may be harmful to the environment, wild life and farm workers.

Question 49

Discuss possible harmful effects of one example of genetic modification.

Answer 49

We are not sure of the consequences of humans and animals eating the modified crops. The bacterial DNA or the Bt toxin itself could be harmful to human as well as animal health.

Other insects which are not harmful to the crops could be killed. The maize pollen will contain the toxin and so if it is blown onto near by plants it can kill the insects feeding on these plants.

Cross pollination can occur which results in some wild plants being genetically modified as they will contain the Bt gene. These plants will have an advantage over others as they will be resistant to certain insects and so some plants may become endangered. This will have significant consequences on the population of wild plants.

Question 50

define clone

Answer 50

Clone: a group of genetically identical organisms or a group of genetically identical cells derived from a single parent cell.

Question 51

Outline a technique for cloning using differentiated animal cells.

Answer 51

Dolly the sheep was cloned by taking udder cells from a donor sheep. These cells were than cultured in a low nutrient medium to make the genes switch off and become dormant. Then an unfertilized egg was taken from another sheep. The nucleus of this egg cell was removed by using a micropipette and then the egg cells were fused with the udder cells using a pulse of electricity. The fused cells developed like normal zygotes and became embryos. These embryos were then implanted into another sheep who’s role was to be the surrogate mother. One lamb was born successfully and called Dolly. Dolly was genetically identical to the sheep from which the udder cells were taken.

Question 52

Outline a technique for cloning using differentiated animal cells.

Answer 52

Dolly the sheep was cloned by taking udder cells from a donor sheep. These cells were than cultured in a low nutrient medium to make the genes switch off and become dormant. Then an unfertilized egg was taken from another sheep. The nucleus of this egg cell was removed by using a micropipette and then the egg cells were fused with the udder cells using a pulse of electricity. The fused cells developed like normal zygotes and became embryos. These embryos were then implanted into another sheep who’s role was to be the surrogate mother. One lamb was born successfully and called Dolly. Dolly was genetically identical to the sheep from which the udder cells were taken.

Question 53

Discuss the ethical issues of therapeutic cloning in humans. (arguments for)

Answer 53

Embryonic stem cells can be used for therapies that save lives and reduce pain for patients. Since a stem cell can divide and differentiate into any cell type, they can be used to replace tissues or organs required by patients.

Cells can be taken from embryos that have stopped developing and so these cells would have died anyway.

Cells are taken at a stage when the embryos have no nerve cells and so they cannot feel pain.

Question 54

Discuss the ethical issues of therapeutic cloning in humans. (arguments against)

Answer 54

Every human embryo is a potential human being and should be given the chance of developing.

More embryos are generally produced than are needed and so many are killed.

There is a risk of embryonic stem cells developing into tumour cells.

Question 55

Discuss the ethical issues of therapeutic cloning in humans. (arguments against)

Answer 55

Every human embryo is a potential human being and should be given the chance of developing.

More embryos are generally produced than are needed and so many are killed.

There is a risk of embryonic stem cells developing into tumour cells.