Topic 3: Genetics

Question 1

Distinguish between autosomes and sex chromosomes in humans. [4]

Answer 1

• X and Y chromosomes determine sex
• females XX and males XY
• X chromosome is larger than the Y chromosome
• 22 pairs of autosomes
• males and females have the same types of autosomes

Question 2

Describe the inheritance of hemophilia including an example using a Punnett grid. [6]

Answer 2

• sex-linked / due to gene on the X chromosome
• more common in males (only receive one X chromosome)
• female is hemophilic if homozygous recessive
• X^H for dominant allele, X^h for recessive allele

Question 3

Explain how meiosis results in an effectively infinite genetic variety of gametes. [8]

Answer 3

• one (homologous) chromosome is from the mother and one from the father;

prophase I

• homologous chromosomes pair up
• crossing over
• alleles/genes on non-sister chromatids swapped
• many possible points of crossing over
• occurs at random positions

metaphase I
* chromosomes are not identical due to crossing over
* random orientation of bivalents

anaphase I

• chromosomes move to opposite poles;
• independent assortment of genes
• 2^n combinations (without considering crossing over);
• four genetically different nuclei/gametes from each meiosis;

Question 4

Describe karyotyping and one application of its use. [4]

Answer 4

• cells undergoing mitosis are used for karyotyping;
• process of mitosis is stopped at (mitotic) metaphase;
• chromosomes (cut from photographs) are arranged in pairs of similar structure/ homologous chromosomes;
• allows abnormalities in the chromosome number/appearance to be seen;
• any valid example (e.g. in Down syndrome / gender of fetus);
• detected by identifying unique feature (e.g. trisomy 21 / one extra chromosome / 47 chromosomes);

Question 5

Describe a technique used for gene transfer. [5]

Answer 5

• restriction enzymes/endonucleases cut a small fragment of DNA from an organism;
• same restriction enzymes used to cut DNA of plasmid
• sticky ends are the same in both cases;
• fragment of DNA is inserted into the plasmid;
• spliced together by ligase;
• to make recombinant DNA;
• recombinants can be inserted into host cell and cloned;

Question 6

Using a named example, discuss the benefits and harmful effects of genetic modification. [9]

Answer 6

• genetic modification is when the DNA of an organism is artificially changed;
• genetic modification alters some characteristics of the organism;
• named example with modification (e.g. salt tolerance in tomato plants);

benefits: [5 max]

• allows crops to be grown where they would not grow naturally;
• provides more food;
• economic benefits;
• expands world’s productive farmland;
• lowers COP;
• less pesticides/fertilizers needed so better for the environment;

harmful effects: [5 max]

• may be released into the natural environment;
• may affect food chains / unintended effects on other organisms;
• may affect consumers e.g. allergies/health risks;
• long-term effects are unknown;
• risk of cross-pollination;
• risk of long-term contamination of soil;

Question 7

Horses (Equus ferus) diploid chromosome number: 64
Donkeys (Equus africanus) diploid chromosome number: 62
Mated male donkey and a female horse: mule with 63 chromosomes

a. State the haploid number for horses.

b. Explain reasons that mules cannot reproduce.

c. Discuss whether or not horses and donkeys should be placed in the same species.

d. A mule was born at the University of Idaho in the USA with 64 chromosomes. Suggest a mechanism by which this could happen.

Answer 7

a. 32

b. chromosome number not even;
cannot divide by two during meiosis;
one chromosome has no homologue;
cannot produce viable gametes;

c. no, because
organisms in the same species must have the same chromosome number;
members of the same species produce fertile offspring;

d. non-disjunction

Question 8

Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species. [7]

Answer 8

Genes

• mutations cause genetic differences
• genes have multiple alleles
• variation in alleles between individuals
• alleles are dominant or recessive
• both alleles influence characteristics with co-dominance
• all members in a species are genetically similar
• genes inherited from parents
• not all genes expressed (epigenetics/methylation/acetylation)

Chromosomes

• same locus
• same chromosome number within a species
• some individuals have extra chromosome (trisomy)
• sex chromosomes X and Y
• meiosis gives new chromosome combinations

Question 9

State one type of environmental factor that may increase the mutation rate of a gene. [1]

Answer 9

exposure to radiation/chemical mutagens/carcinogens

Question 10

Identify one type of gene mutation. [1]

Answer 10

base substitution/insertion/deletion

Question 11

Meiosis in humans produces cells that participate in fertilization. Outline the processes involved in meiosis. [5]

Answer 11

• meiosis produces four haploid cells from one diploid cell

prophase I

• homologous chromosomes pair up
• chromatids break and rejoin, crossing over

metaphase I

• line up at spindle equator (random assortment)

anaphase I

• chromosomes separate and move to opposite poles

telophase I

• reach poles and unwind

prophase II

• chromosomes condense, new spindles form

metaphase II

• line up at equator

anaphase II

• sister chromatids separate and move to opposite poles

telophase II

• reach poles and unwind

Question 12

Define linked genes. [1]

Answer 12

Genes located on the same chromosome

Question 13

Explain the inheritance of colour blindness. [8]

Answer 13

• caused by recessive allele
• sex-linked, located on X chromosome
• Xb is allele for colour blindness and XB is allele for normal colour vision
• male has one X and one Y chromosome;
• male has only one copy of gene(s) located on X chromosome;
• X chromosome in males comes from female parent;
• any male receiving allele from mother will express the trait;
• XbY is genotype for colour blind male;
• more males have colour blindness than females;
• female only express colour blindness if homozygous recessive
• female carrier if heterozygous
• colour blind female could be born to colour blind father and carrier mother;

Question 14

Outline outcomes of the human genome project. [4]

Answer 14

• complete human DNA sequenced
• identification of all human genes
• discover protein structures
• evidence for evolutionary relationships
• find mutations
• screening for diseases
• develop new gene theories

Question 15

Define codominant allele, recessive allele, locus and sex linkage. [4]

Answer 15

Co-dominant allele: both alleles are expressed and affects the phenotype when present in a heterozygote

Recessive allele: allele that expressed only when dominant allele is not present

Locus: position of a gene on the chromosome

Sex-linkage: gene positioned on a sex chromosome (X and Y chromosomes)

Question 16

State the technique used to collect cells for prenatal testing. [1]

Answer 16

amniocentesis (sampling amniotic fluid)/chorionic villus (sampling placenta tissue)

Question 17

State the method used to arrange the chromosomes in a karyotype. [1]

Answer 17

chromosomes grouped by pairs according to size and structure/band pattern/location of centromeres

Question 18

State at what stage in the cell cycle the cells would be when the karyotype photograph was taken. [1]

Answer 18

metaphase or late prophase of mitosis

Question 19

Using an example, describe polygenic inheritance. [3]

Answer 19

• more than one gene contributes to characteristics
• as the number of genes increases so do phenotypes
• leads to continuous variation
• e.g. human skin color (differing amounts of melanin)

Question 20

Explain why carriers of sex-linked (X-linked) genes must be heterozygous. [2]

Answer 20

• carrier has one copy of the recessive allele
• must also have dominant to prevent carrying the condition

Question 21

Define the term allele as used in genetics. [1]

Answer 21

a specific form of a gene

Question 22

Following fertilization, cells in the developing embryo differentiate. Outline a technique for cloning using differentiated animal cells. [5]

Answer 22

• diploid cell taken from sheep udder
• diploid nucleus removed
• egg cells taken from female sheep
• haploid nucleus removed
• diploid nucleus fused with egg to form zygote
• embryo implanted in uterus of surrogate
• offspring is a genetic copy of diploid nucleus

Question 23

Discuss ethical issues of therapeutic cloning in humans. [8]

Answer 23

• involves producing embryos from which embryonic stem cells can be harvested for medical use;

argument in favour:

• stem cells used to replace tissues that are lost/damaged in a patient;
• pain and suffering can be reduced/lives can be saved/life quality improved;
• cells can be removed from embryos that have stopped developing and would have died anyway;
• cells are removed at a stage when no pain can be felt by the embryo;
• use embryos from IVF that would otherwise be destroyed;

argument against:

• danger of embryonic stem cells developing into tumour cells/other harmful effects are not yet known;
• every human embryo is a potential human with the right to development;
• more embryos may be produced than can be used and so some would be killed;

Question 24

Describe the application of DNA profiling to determine paternity. [5]

Answer 24

• DNA for child, mother and possible fathers used
• done for legal reasons/divorce/inheritance
• or for personal reasons/self-esteem issues
• DNA amplified using PCR
• DNA cut using restriction enzymes
• Gel electrophoresis used to separate DNA fragments
• pattern of bands produced in gel
• analyzed for matches between child and mother and possible father
• half of the child’s bands match with the father

Question 25

Outline a technique used for gene transfer. [5]

Answer 25

• plasmid used for gene transfer removed from bacteria;
• plasmid is a small circle of DNA;
• restriction enzyme cleaves the DNA strand;
• each restriction enzyme creates sticky ends;
• same restriction enzyme used to cut DNA with (desired) gene;
• DNA can be added to the open plasmid;
• ligase used to join together;
• recombinant DNA inserted into host cell;

Question 26

State two general types of enzymes used in gene transfer. [1]

Answer 26

restriction enzyme, DNA ligase, reverse transcriptase

Question 27

Discuss the potential benefit and possible harm of one named example of gene transfer between species. [3]

Answer 27

• Bt gene transferred from bacterium (Bacillus) to maize
• insect pest killed by Bt toxin to increase crop production
• less pesticides needed, better for the environment
• non-target insects may be killed as well
• risk of cross-pollination, introducing gene to unintended species

Question 28

Outline, with examples, different types of inheritance that produce non-Mendelian ratios. [4]

Answer 28

• polygenic inheritance that involves multiple genes, e.g. skin color, height
• inheritance of linked genes where alleles of different genes are located on the same chromosome
• recombinant phenotypes due to crossing over of linked genes
• co-dominance of specific alleles that form intermediate characteristics, e.g. blood groups
• sex-linked effects e.g. color blindness
• environmental influence, e.g. methylation

Question 29

Explain how changes to the cell cycle results in tumor formation. [4]

Answer 29

• cell cycle includes interphase, mitosis and cytokinesis
• cyclins regulate the cycle
• extra cells produced when needed
• tumor formation is the result of uncontrolled cell division
• repeated mitosis
• due to mutations in oncogenes, contribute to development of cancer cells
• carcinogens causes tumors