6.1.2: Patterns of inheritance

Question 1

Define phenotype.

Answer 1

The expression of an organism’s genetic
constitution e.g. low metabolism,
combined with its interaction with the
environment e.g. diet.

Question 2

How can meiosis bring about genetic variation?

Answer 2

● Random arrangement of
chromosomes during lining up.
● Crossing over of chromatids before the
first division.

Question 3

How does random fertilisation bring about genetic

variation?

Answer 3

Gametes are haploid cells, meaning they only
contain half of a person’s DNA. As this is
determined by meiosis, every gamete contains
different DNA. Therefore the same two individuals
can produce genetically different offspring.

Question 4

Define monogenic inheritance.

Answer 4

Where one phenotypic characteristic is

controlled by a single gene.

Question 5

Define dihybrid inheritance.

Answer 5

Where two phenotypic characteristics
are determined by two different genes
present on two different chromosomes at
the same time.

Question 6

What is meant by sex-linkage?

Answer 6

Where an allele is located on one of the
sex chromosomes, meaning its
expression depends on the sex of the
individual.

Question 7

What is meant by multiple alleles?

Answer 7

A gene with more than two alleles.

Question 8

What is meant by codominant alleles?

Answer 8

Two dominant alleles that both contribute
to the phenotype, either by showing a
blend of both characteristics, or the
characteristics appearing together

Question 9

What is meant by autosomal linkage?

Answer 9

Where two or more genes are located on the
same (non-sex) chromosome. In this case, only
one homologous pair is needed for all four
alleles to be present. For genes that aren’t
linked, two homologous pairs are needed.

Question 10

What is meant by epistasis?

Answer 10

Where two non-linked genes interact, with
one gene either masking or suppressing
the other gene.

Question 11

What is the chi-squared test?

Answer 11

A statistical test to find out whether the
difference between observed and expected
data is due to chance or a real effect. Can
be used to compare expected phenotypic
ratios with observed ratios

Question 12

How is a chi-squared test performed?

Answer 12

The formula results in a number, which is then
compared to a critical value (for the
corresponding degrees of freedom). If the number
is greater than or equal to the critical value, we
conclude there is no significant difference and the
results occured due to chance.

Question 13

How can the number of genes coding for a

characteristic influence variation?

Answer 13

● Discontinuous variation= characteristic
determined by one gene (monogenic inheritance)
● Continuous variation= characteristic determined
by more than one gene (polygenic inheritance)

Question 14

What is stabilising selection?

Answer 14

Occurs when environmental conditions
stay the same. Individuals closest to the
mean are favoured, and any new
characteristics are selected against.
Results in low diversity.

Question 15

What is directional selection?

Answer 15

Occurs when environmental conditions
change. Individuals with phenotypes suited to
the new conditions will survive and pass on
their genes. Over time the mean of the
population will move towards these
characteristics

Question 16

What is genetic drift?

Answer 16

A change in a population’s allele
frequencies that occurs due to chance
rather than selective pressures. In other
words, it is caused by ‘sampling error’
during reproduction.

Question 17

What is meant by a genetic bottleneck?

Answer 17

Where a catastrophic event dramatically
reduces the size of a population, thereby
decreasing the variety of alleles in the
gene pool and causing large changes in
allele frequencies

Question 18

What is meant by the founder effect?

Answer 18

When a small number of individuals
become isolated, forming a new
population with a limited gene pool, with
allele frequencies not reflective of the
original population

Question 19

What is the Hardy-Weinberg principle?

Answer 19

Allows us to estimate the frequency of
alleles in a population, as well as if allele
frequency is changing over time.

Question 20

Explain the Hardy-Weinberg equation for calculating

allele frequency.

Answer 20

The frequencies of each allele for a
characteristic must add up to 1.0. The
equation is therefore; p + q = 1
Where p= frequency of the dominant allele,
and q= frequency of the recessive allele.

Question 21

Explain the Hardy-Weinberg equation for calculating

genotype frequency.

Answer 21

The frequencies of each genotype for a
characteristics must add up to 1.0. The equation is
therefore; p2 + 2pq + q2 = 1
Where p2= frequency of homozygous dominant,
2pq= frequency of heterozygous, and q2=
frequency of homozygous recessive.

Question 22

Define speciation.

Answer 22

Where a population is split and isolated, there
are different selective pressures on the two
groups. If the genetic makeup changes to the
extent the two groups can not longer
interbreed, they have become separate
species.

Question 23

What is meant by allopatric speciation?

Answer 23

Speciation resulting from a physical
barrier e.g. river, mountain range. The
environments occupied by the two
groups are different, and therefore
different alleles are favoured.

Question 24

What is meant by sympatric speciation?

Answer 24

Speciation resulting from a non-physical
barrier e.g. a mutation that no longer allows
two organisms to produce fertile offspring.
Any changes in anatomy or behaviour may
also prevent breeding.

Question 25

What is artificial selection?

Answer 25

Humans choose particular organisms to
breed together in order to produce a
desired characteristic in the offspring

Question 26

Give examples of artificial selection in plants and

animals.

Answer 26

● Plants= seeds used from plants that
produce larger fruit and vegetables.
● Animals= cows with higher milk yield
are chosen and selectively bred.

Question 27

Why is it important to keep a resource of genetic

material when selective breeding?

Answer 27

Allows any traits that were accidentally
bred out to be reintroduced, or to revert
back to a point before any negative traits
were introduced.

Question 28

Give some ethical issues around the use of artificial

selection.

Answer 28

● Anatomical changes in animals e.g.
respiratory issues in pugs.
● Higher susceptibility to disease in both
plants and animals.