chapter 16

Question 1

Explain the terms ‘haploid’ and ‘diploid’.

Answer 1

● Haploid (n) - having half of a set of
homologous chromosomes e.g. gametes

● Diploid (2n) - having the full set of homologous
chromosomes e.g. somatic cells

Question 2

Explain the term ‘homologous chromosomes’.

Answer 2

In diploid cells, the chromosomes are in pairs - they are homologous chromosomes. Each chromosome of a pair carries the same genes, but may have different alleles. Therefore, there are two loci for each gene.
Humans have 23 pairs of chromosomes, or 46 chromosomes in total.
One chromosome of each pair is from the mother (maternal) and the other is from the father (paternal).

Question 3

Why must the number of chromosomes be halved before fertilisation?

Answer 3

Each of the gametes contains half the number of
chromosomes (23 in humans) so that when they fuse
during fertilisation the diploid number is restored (46 in humans). If this did not occur, the number of
chromosomes would double with each generation.

Question 4

Name the process of cell division that gives rise to gametes.

Answer 4

Meiosis

Question 5

Describe the events of meiosis I.

Answer 5

● Prophase I - centrosomes move to opposite poles of the cell; homologous chromosomes pair up. Crossing over occur here.
● Metaphase I - homologous chromosomes line up along the equator of the cell
● Anaphase I - homologous chromosomes are separated and pulled to opposite poles of the cell by spindle fibres
● Telophase I - the nuclear envelope forms around the genetic material at each pole and the cell divides (cytokinesis)

Question 6

Describe the events of meiosis II.

Answer 6

● Prophase II - centrosomes move to opposite poles of the cell
● Metaphase II - the chromosomes align along the equator of the cell
● Anaphase II - spindles pull the sister chromatids apart to the opposite poles of the cell
● Telophase II - the nuclear envelopes reform. Four genetically different daughter cells are produced, each with the haploid number of chromosomes.

Question 7

Explain how crossing over in meiosis increases genetic variation.

Answer 7

During prophase I, the chromatids within a homologous pair of chromosomes twist around each other. Parts of the chromatid break off and may be rejoined with the chromatid of the other homologous chromosome. The same loci are exchanged. This creates new combinations of alleles and therefore increases genetic variation.

Question 8

Explain how random assortment in meiosis increases genetic variation

Answer 8

During metaphase I, the homologous chromosomes align randomly along the equator. This means that in the daughter cells of meiosis I, there will be a mix of
maternal and paternal chromosomes, increasing genetic variation.

Question 9

Why is the random fusion of gametes important?

Answer 9

It increases genetic variation due to the random combination of chromosomes generated.

Question 10

Define the term ‘gene’

Answer 10

A gene is a sequence of DNA that codes for a polypeptide (protein) or RNA.

Question 11

What is meant by the term ‘locus’?

Answer 11

A locus is the specific position of a gene on the chromosomes.

Question 12

What is an allele?

Answer 12

An allele is one of the different versions of a gene. With homologous chromosomes, there are two loci for each gene; these may carry different alleles, or the same.

Question 13

What is meant by homozygous and heterozygous?

Answer 13

Homozygous - the alleles on each of the homologous
chromosomes in a pair are the same.
Heterozygous - the alleles in the homologous pair are different

Question 14

State what is meant by dominant and recessive alleles.

Answer 14

A dominant allele is a form of a gene that will be expressed even if only one is present (heterozygous) - it can override recessive alleles.
Recessive alleles will only be expressed if they are homozygous.

Question 15

Define the term ‘codominant’.

Answer 15

Both alleles (which are different) are fully expressed and contribute to the phenotype.

Question 16

Explain what is meant by ‘linkage’.

Answer 16

Two genes are said to be linked if they are close to each other on the same chromosome. They are unlikely to be separated during crossing over of meiosis, and so are inherited together.

Question 17

What is a test cross?

Answer 17

A genetic cross to determine the genotypes of the parents and offspring. An organism with a recessive genotype is crossed with an organism whose genotype is unknown, but their phenotype shows a dominant allele is present. The phenotypes of the offspring will determine whether the parent organism is heterozygous or homozygous.

Question 18

What is meant by F1 and F2?

Answer 18

F1 is the first generation, produced from one homozygous dominant parent and one homozygous
recessive parent. The F1 generation is therefore heterozygous.
F2 is the second generation, produced from inbreeding of the F1 generation.

Question 19

Define ‘phenotype’.

Answer 19

The observable features of an organism.
The phenotype is a result of the
interaction between the genotype and
environment.

Question 20

Define ‘genotype’.

Answer 20

The alleles an organism possesses.

Question 21

What does a monohybrid cross show?

Answer 21

The possible genotypes of the offspring
for one gene only.

Question 22

What is autosomal linkage?

Answer 22

When two genes are positioned close
together on the same autosome (any
chromosome except the sex chromosomes)
they are likely to be inherited together and so
are autosomally linked.

Question 23

Explain what is meant by ‘sex linkage’.

Answer 23

Sex-linked genes are any genes found
on the X or Y chromosomes.

Question 24

Why are males at greater risk of sex-linked disorders caused by recessive alleles?

Answer 24

Females = XX

Males = XY

The X chromosome is much larger than the Y chromosome, so for many of the genes on the X chromosome there is no homologue on the Y
chromosome. Therefore, recessive alleles on the X chromosome will appear more frequently in the phenotype in males, because they only have one copy
of that gene.

Question 25

Why is the chi-squared test used?

Answer 25

To test whether the difference between
the observed and expected frequencies
is significant, or whether the difference is
due to chance.

Question 26

Describe the types of gene mutations that can occur.

Answer 26

● Substitution - one nucleotide base is swapped for
another
● Deletion - one or more bases are removed from the DNA sequence
● Insertion - one or more bases are added into the DNA
sequence

Question 27

Explain how a gene mutation can affect the gene product.

Answer 27

● Silent mutation - a change in one base does not affect the amino acid the codon codes for. This is
due to the degenerate nature of the genetic code
● Nonsense mutation - the new base creates a stop codon, which means the protein is not fully
produced
● Missense mutation - the new codon codes for a different amino acid. It may change the shape of
the protein produced
● Frameshift - deletions or insertions shift the sequence, so every codon downstream is read
differently, resulting in different amino acids and potentially a completely different protein.

Question 28

Describe how albinism can arise as a result of a mutation.

Answer 28

One form of albinism is caused by mutations of the
tyrosinase (TRY) gene. Such mutations alter the
tyrosinase enzyme which is responsible for the
production of melanin; an inactive or absent
enzyme results in albinism.

Question 29

Outline how a mutation can cause sickle cell anaemia.

Answer 29

There is a glutamine-to-valine substitution in the
HBB gene for β-globin. This changes how the
haemoglobin molecules interact; they form
strands and produce sickle-shaped red blood
cells.
Sickle-shaped cells are less efficient at
transporting oxygen, and may get stuck in the
capillaries.

Question 30

Describe how a mutation can lead to haemophilia.

Answer 30

The F8 gene, which codes for factor VIII
(involved in clotting) is mutated in
haemophilia. This gene is found on the X
chromosome, so this is a sex-linked disorder
caused by a recessive allele.

Question 31

Explain how Huntington’s disease can arise from a mutation.

Answer 31

In people with Huntington’s disease, the HTT
gene, which codes for the protein huntingtin,
contains a large number of CAG repeats
within the DNA sequence - a ‘stutter’.

Question 32

Explain the effects of the dominant allele Le and the recessive allele le on gibberellin synthesis.

Answer 32

The dominant allele (Le) of the gene codes for a functional enzyme in the pathway which synthesises gibberellin. This leads to stem elongation and tall plants.
The recessive allele (le) of the gene codes for a non-functional enzyme in this pathway, so gibberellin is not produced. This results in a short plant.

Question 33

Describe the difference between a
regulatory gene and a structural gene.

Answer 33

A regulatory gene controls the expression of other
genes.
A structural gene is one that codes for a protein or RNA
that does not regulate the expression of other genes.

Question 34

What is the difference between a
repressible enzyme and an inducible enzyme?

Answer 34

The synthesis of a repressible enzyme is only
stopped when a repressor protein is activated.

The synthesis of an inducible enzyme is only
started when a substrate is added.

Question 35

What type of operon is the lac operon?

Answer 35

An inducible operon.

Question 36

How is the genetic control of protein production achieved using the lac operon?

Answer 36

● The expression of the genes in the lac operon depends on whether lactose is
present. The lac repressor senses lactose.
● When lactose is not present, the repressor binds to the operator and prevents transcription of the genes.
● When lactose is present, it binds to the repressor and changes its shape. It can no longer block the transcription of the lacZ, lacY and lacA genes, which
code for enzymes involved in lactose metabolism.

Question 37

Name the molecules that regulate gene expression in eukaryotes.

Answer 37

Transcription factors

Question 38

Explain how transcription factors control gene expression.

Answer 38

Transcription factors bind to the promoter regions
of genes. They may activate or repress the
expression of a gene by changing how easy it is
for RNA polymerase to access the DNA for
transcription.

Question 39

Explain how gibberellin can activate the expression of genes.

Answer 39

Gibberellin causes the DELLA protein repressors,
which normally inhibit transcription factors, to
break down. This means the gene can be
transcribed (such as the gene for amylase).