Topic 3: Genetics

Question 1

Define gene

Answer 1

A section of DNA that codes for a specific trait/characteristic via the production of proteins.

Question 2

Define allele

Answer 2

An alternate form of a gene.

Question 3

Define locus

Answer 3

The specific location of a gene on a chromosome.

Question 4

Define genome

Answer 4

The totality of genes in a cell, organism, or organelle.

Question 5

Identify 3 types of mutagens and mention an example for each

Answer 5

1. Physical (e.g. radiation)
2. Chemical (e.g. tobacco and other carcinogens)
3. Biological (e.g. viruses)

Question 6

Distinguish between germline and somatic mutations

Answer 6

Somatic mutations affect body cells and cannot be inherited by offspring while germline mutations affect sex cells and will be passed on to offspring.

Question 7

Explain the consequences of a base substitution in the development of sickle cell anaemia

Answer 7

The base substitution mutation on the 6th codon changes GAG to GUG on the mRNA transcript. The amino acid is changed from glutamic acid to valine which alters the structure of haemoglobin causing it to have a sickle shape. This makes the haemoglobin unable to carry oxygen effectively and cause clots in the capillaries.

Question 8

Compare the structure of prokaryotic and eukaryotic chromosomes

Answer 8

In prokaryotic chromosomes, DNA is circular, DNA is naked (not bound to proteins), they only have a single chromosome (genophore) and it is found in the nucleoid region of the cytosol. While in eukaryotic chromosomes, DNA is linear, DNA is bound to histone proteins, chromosomes may exist in pairs (diploid), and it is found in the nucleus.

Question 9

Outline the role of plasmids in bacterial cells

Answer 9

Plasmids are circular DNA molecules capable of autonomous replication and transcription. They can be transferred between bacteria via bacterial conjugation. They are also used as a vector for gene transfer in scientific experiments.

Question 10

Outline how John Cairns elucidated the length of chromosomes via autoradiography

Answer 10

Cells were grown in a solution including radioactive thymidine. The radioactive thymidine was incorporated into the DNA of the cell. The chromosomes were then isolated and fixed to a photographic surface. The surface was immersed in a solution of silver bromide which turns into metal grain if exposed to radiation. The silver grains appear only where the DNA was present, allowing chromosome length to be determined.

Question 11

Define homologous chromosome

Answer 11

Chromosome pairs that share the same structural features and have the same genes at the same loci positions. They represent the maternal and paternal copies of a chromosome.

Question 12

Differentiate between autosomes and sex chromosomes (heterosomes)

Answer 12

Sex chromosomes (X and Y) determine sex and autosomes represent all other chromosomes.

Question 13

Distinguish between diploid and haploid

Answer 13

Diploid nuclei possess pairs of homologous chromosomes and haploid nuclei possess only one copy of each chromosome.

Question 14

Outline the importance of chromosome number to the reproduction of species

Answer 14

Chromosome number is a characteristic feature of a species. Organism with different haploid numbers will generally be unable to produce viable diploid zygotes.

Question 15

Define karyogram and identify the purpose of karyograms

Answer 15

A karyogram is a chromosome profile of an individual organism. Karyograms are generally used to identify sex or to identify chromosomal abnormalities.

Question 16

Define meiosis

Answer 16

The reduction division of a diploid cell to produce four haploid daughter cells.

Question 17

Identify the main differences between meiosis I and meiosis II

Answer 17

Meiosis I involved the separation of homologous chromosomes, is a reduction division, and promotes genetic variation.

Meiosis II involved the separation of sister chromatids, is a mitotic division, and does not promote genetic variation.

Question 18

Differentiate between homologous chromosomes and sister chromatids

Answer 18

Homologous chromosomes are the maternal and paternal copies of given chromosomes. Homologous chromosomes have the same structure and the same genes at the same loci positions. Sister chromatids are the duplicated copies of the chromosomes’s DNA.

Question 19

Compare the processes of meiosis and mitosis

Answer 19

Type of cell produced:
Meiosis - sex cells (gametes)
Mitosis - body (somatic) cells

Number of cells produced:
Meiosis - four
Mitosis - two

Number of divisions:
Meiosis - two
Mitosis - one

Ploidy of daughter cells:
Meiosis - haploid
Mitosis - diploid

Genetics of daughter cells:
Meiosis - shows genetic variation
Mitosis - are genetically identical

Question 20

Outline how crossing over give rise to infinite genetic variety

Answer 20

Crossing over involved the exchange of genetic material between non-sister chromatids of a bivalent. Bivalents are connected at points called chiasma during the process of synapsis. It is at these chiasma that recombination occurs.

Question 21

Outline how independent assortment give rise to infinite genetic variety

Answer 21

Bivalents will line up at the cell’s equator in a random orientation during metaphase I. Meaning there is a equal probability of a gamete containing the maternal OR paternal copy for any chromosome pair. Because human cells have 23 chromosome pairs, there are 2^23 possible combinations.

Question 22

Explain how random gamete fusion promotes variation within a species

Answer 22

When two haploid gametes fuse, they form a diploid zygote which can grow into a new organism. Because gamete fusion is random, each successive offspring will be composed of a distinct combination of maternal and paternal chromosomes. This means every member of a species is unique.

Question 23

Explain how non-disjunction can give rise to aneuploidy

Answer 23

Non-disjunction describes the failure of chromosomes to separate during cell division. If non-disjunction occurs in anaphase I, all four gametes will be affected. If non-disjunction occurs in anaphase II, only two of the gametes are affected. If a gamete with one extra chromosome fuses with a normal gamete, the offspring will have trisomy.

Question 24

List the genetic condition that causes down syndrome and identify a contributing factor

Answer 24

Down syndrome is caused by trisomy 21. Increased maternal age increases the risk of non-disjunction.

Question 25

Describe the method by which cells are obtained for karyotyping

Answer 25

Cells are isolated and treated will drugs to promote cell division (makes chromosomes visible to microscope). Cells are arrested during mitosis and then chromosomes are isolated and visualised.

Question 26

Describe the methodology and conclusions drawn from Mendel’s pea plant experiment

Answer 26

Mendel crossed pure breeding pea plants (P generation), then crossed large numbers of the offspring (F1). He did this for a variety of different characteristics (e.g. flower colour, plant height, etc.). Mendel concluded that organisms have discrete factors that control inheritance of traits (i.e. genes). There are different versions of these factors (i.e. alleles) and each parent passes on one copy to offspring. Only one version gets expressed (i.e. dominance / recessiveness)

Question 27

Explain how sexual reproduction results in diploid zygotes with two alleles for each characteristic

Answer 27

Parents each pass on one copy of every chromosome to offspring via haploid gametes. The resulting zygote is diploid and therefore possesses two copies of every gene. These alternative copies of a gene are called alleles

Question 28

Differentiate between homozygous, heterozygous, and hemizygous

Answer 28

Homozygous – Alleles are the same for a given characteristic
Heterozygous – Alleles are different for a given characteristic
Hemizygous - There is only one allele (occurs in males for sex chromosome traits)

Question 29

Distinguish between complete dominance and co-dominance

Answer 29

Complete Dominance – One allele (dominant) masks expression of other allele (recessive) in heterozygotes
Codominance – Both alleles are expressed within a heterozygous phenotype

Question 30

Identify which blood group is the universal donor and which is the universal acceptor

Answer 30

O is the universal donor
AB is the universal acceptor

Question 31

Define sex linkage and identify two examples of sex-linked conditions

Answer 31

Sex linkage refers to any gene/trait that is located on the sex chromosome. Two examples of X-linked recessive traits are red-green colour blindness and haemophilia.

Question 32

Outline why X-linked recessive disorders are more common in males

Answer 32

Males only have one chromosome and so cannot be carriers for recessive conditions.

Question 33

Describe the inheritance and cause of the following genetic diseases: cystic fibrosis, Huntington’s disease

Answer 33

Cystic fibrosis is an autosomal recessive disease caused by a mutation to the CFTR gene on chromosome 7 which causes the production of sticky mucus leading to obstructed airways and digestive issues.

Huntington’s disease is an autosomal dominant disease caused by a mutation to the HTT gene on chromosome 4 which causes abnormal production of the Huntington protein leading to neurodegeneration and dementia.

Question 34

Describe the purpose and process of the polymerase chain reaction (PCR)

Answer 34

PCR is used to rapidly amplify minute quantities of DNA. It involves a thermal cycler and three repeating steps. First, denaturation where DNA is heated to separate strands. Second, annealing where primers are introduced to designate copying points. Third, elongation where taq polymerase synthesises new strand. They double the amount of DNA.

Question 35

Describe the purpose and process of gel electrophoresis

Answer 35

Gel electrophoresis separates fragments of DNA (broken by restriction enzymes) or protein based on size. DNA is run in an agarose gel that moves towards the positive terminus as phosphate is negatively charged.

Question 36

Identify two types of vectors for gene transfer

Answer 36

Plasmids and viruses

Question 37

Distinguish between blunt and sticky end restriction endonucleases

Answer 37

Sticky ends are overhanging sequences which allow for specificity in ligation. Blunt ends have no overhag.

Question 38

Outline the process of gene transfer

Answer 38

Gene of interest and plasmid vector are isolated from cells and amplified with PCR. Gene and plasmid are cut with restriction enzymes. Gene and plasmid are fused together by DNA ligase to form a recombinant construct. The construct is then inserted into a hose cell. Transgenic cells are then selected by growing on antibiotic media. The transgenic cells are then grown in culture and the newly expressed protein can then be extracted. An example of this process is the mass production of human insulin by transgenic bacteria.

Question 39

Explain the role of short tandem repeats (STRs) play in DNA profiling

Answer 39

STRs are short tandem repeating sequences in non-coding DNA. Different individuals have different numbers of repeats, so by comparing STR loci, unique profiles emerge.

Question 40

Outline two potential applications for DNA profiling

Answer 40

Forensic investigations and paternity testing

Question 41

Outline the benefits and risks associated with the use of genetically modified crops

Answer 41

Benefits: improve yields, potentially longer shelf life, reduce need for chemical pesticides, allows growth in wider areas

Risks: risks of cross-contamination, threatens native biodiversity, difficult to police internationally, patenting could lead to monopolies.

Question 42

Define clone

Answer 42

Groups of genetically identical cells or organisms