16. Inherited Change

Question 1

Homologous pairs of chromosomes

Answer 1

Pairs of chromosomes that are similar in structure and composition

• same length
• same shape
• same order of gene
• same banding pattern

Question 2

Haploid

Answer 2

Containing a single set of unpaired chromosomes

Question 3

Diploid

Answer 3

Containing two sets of chromosomes

Question 4

COMPARE the role of meiosis in gametogenesis to produce sperm cells in humans with the role of meiosis in gametogenesis in producing pollen grains in flower plants. [4m]

Answer 4

Similarities

1. both produce 4 cells
2. both halve the chromosome number
3. both produce genetically different cells

Differences

1. sperm is a gamete; pollen grain is not
2. sperm cell has 1 haploid nucleus; pollen grains contain 2 haploid nuclei
3. sperm formed by mitosis then meiosis; pollen grain cells formed by meiosis then mitosis

Question 5

Gene

Answer 5

A sequence of nucleotides that forms part of a DNA molecule and codes for a polypeptide

Question 6

Locus

Answer 6

The position of a gene on a chromosome

Question 7

Allele

Answer 7

• Different forms of a gene
• Due to different nucleotide sequence in DNA
• Resulting in different polypeptide produced

Question 8

Dominant

Answer 8

An allele that is expressed if it is present

Question 9

Recessive

Answer 9

An allele that is only expressed when there is no dominant allele of the gene present

Question 10

Codominant

Answer 10

Alleles that both have an effect on the phenotype of a heterozygous organism

Question 11

Test cross

Answer 11

A genetic cross in which an organism with a dominant phenotype is crossed with an organism that is homozygous recessive, to determine whether the organism is heterozygous or homozygous

Question 12

F1 generation

Answer 12

The offspring of two different pure-breeding, homozygous organisms

Question 13

F2 generation

Answer 13

The offspring of two F1 (heterozygous) organisms

Question 14

Phenotype

Answer 14

The observable features of an organism

Question 15

Genotype

Answer 15

The genetic make-up of an organism in terms of the alleles present

Question 16

Homozygous

Answer 16

Having two identical alleles of a particular gene

Question 17

Heterozygous

Answer 17

Having two different alleles of a particular gene

Question 18

Sex linkage

Answer 18

Gene carried on the X chromosome

Question 19

Autosomal linkage

Answer 19

2 or more genes on the same chromosome (not a sex chromosome) will be inherited together, without independent assortment

Question 20

Gene mutation

Answer 20

Random change in nucleotide sequence in DNA, during DNA replication

Question 21

Explain how the presence of galactosaemia in a foetus may be determined. [2m]

Answer 21

• genetic screening
• obtain fetal cells/DNA
• by amniocentesis / chorionic villus sampling
• electrophoresis + probe

Question 22

A female has a family history of ocular albinism (non-progressive disorder and clarity of vision remains stable throughout life) but she does not have any symptoms.
A test to find out if she has the mutant allele is available.

2 reasons for taking this test and 3 reasons against taking this test.

Answer 22

FOR

1. decide whether to have children
2. earlier treatment if child born with ocular albinism

AGAINST

1. test may be expensive
2. condition is not life-threatening
3. cannot be treated

Question 23

Explain why some genes show constitutive expression. [1m]

Answer 23

The gene products / proteins are needed all the time.

Question 24

Discuss the link between the frequency of sickle cell anaemia and the number of cases of malaria. [9m]

• Plasmodium (malaria parasite) cannot complete life cycle in sickle cells
• heterozygotes have resistance to malaria
• sickle cell anaemia ppl die due to SCA
• normal ppl die due to malaria

2 selection pressures are involved!

Answer 24

1. frequency of sickle cell anaemia is highest in areas where malaria is common
2. sickle cell anaemia RBCs cannot carry O2 very well
3. homozygous H^S have sickle cell anaemia and may die
4. homozygous H^A have normal Hb / red blood cells
5. heterozygotes have sickle cell trait
6. Plasmodium affects red blood cells
7. malaria is lethal
8. heterozygotes less likely to suffer from (severe effects of) malaria
9. have selective advantage
10. pass on both H^A and H^S alleles
11. malaria selects against homozygous H^A
12. sickle cell anaemia selects against homozygous H^S
13. idea that sickle cell ALLELE is maintained within population because of sickle cell trait individuals

Question 25

Explain why there are usually more than 100 copies of mtDNA in a cell, but only two copies of nuclear DNA. [2m]

Answer 25

• many mitochondria per cell but only 1 nucleus

- cell is diploid / has 2 copies of each chromosome in nucleus

Question 26

❖ Explain the function of transcription factors in gene expression in eukaryotes. [6m]

Answer 26

• bind to DNA
• so RNA polymerase binds to promoter
• so transcription begins / gene expressed
  1. can activate genes in correct order; for correct pattern of development
  2. determine sex
  3. allow responses to environmental stimuli
  eg. correct genes expressed in response to very high temp. / light exposure
  4. regulate cell cycle - role in cell cycle checkpoints & apoptosis
  5. in cell signalling - response to hormones

Question 27

Gene R/r, which codes for a protein involved in pigment production, has 2 alleles.
Suggest ways in which the expression of allele R allows pigment production. [3m]

Answer 27

• example of gene interaction / epistasis
• (may code for) an enzyme in pigment production pathway
• (may code for) transcription factor
• which binds to promoter
• & allows binding of RNA polymerase
• (may code for) co-factor to activate enzyme