6.1.2 Patterns Of Inheritance

Question 1

Discontinuous variation

Answer 1

Discontinuous variation is genetic variation producing discrete (discontinuous)
phenotypes, so two or more non-overlapping categories. These traits tend to be
monogenic, so different alleles have very different effects on the phenotype. Can be
represented using a bar chart. Is unaffected by the environment.

Question 2

Continuous variation

Answer 2

Continuous variation is genetic variation that produces phenotypic variation where
quantitative traits vary by very small amounts between one group and the next. Such
characteristics tend to be polygenic (so each of the alleles involved has an additive
effect).The greater the number of gene loci involved, the more continuous the variation
(the greater the range). Such data can be represented on a histogram. It is influenced by
the environment.

Question 3

How does meiosis create genetic variation ?

Answer 3

Crossing over of chromatids where pairs of homologous chromosomes line up and
exchange some of their genetic material
Independent assortment of chromosomes – there are various combinations of
chromosome arrangement

Question 4

Allele

Answer 4

A version of a gene

Question 5

Locus

Answer 5

the specific position of a gene on a chromosome, the two alleles of a gene are
found at the same loci on the chromosome pairs

Question 6

Phenotype

Answer 6

visible characteristics of an organism

Question 7

Genotype

Answer 7

genetic makeup of an organism

Question 8

Codominance

Answer 8

both alleles contribute to the phenotype

Question 9

Dominant

Answer 9

only a single allele is required for the characteristic to be expressed, that
allele is always expressed in the phenotype

Question 10

Reccesive

Answer 10

the characteristic is only expressed if there is no dominant allele present

Question 11

Homozygous

Answer 11

two identical alleles

Question 12

Heterozygous

Answer 12

two different alleles

Question 13

Discuss mendel’s work

Answer 13

Mendel studied pea plants and found through his crossings that the tall allele is dominant
and the dwarf allele is recessive, the purple flower is dominant and the white flower is
recessive, the axial flower position is dominant and the terminal position is recessive, the
full pod shape is dominant and then constricted pod shape is recessive, the yellow seed
colour is dominant and the green seed colour is recessive, and the round seed shape is
dominant and the wrinkled seed shape is recessive. All of these characteristics that he
studied were monogenic. He was able to determine this by getting true breeding strains
for each trait (where the trait was shown to be unchanged in the strain over
generations), and by keeping accurate and quantitative records of the data he obtained,
which he then analysed.

Question 14

What is monogenic inheritance, give an example

Answer 14

when a phenotype or trait is controlled by a single gene. For instance, cystic fibrosis where the individuals with doubly recessive genotype are affected. When parental (P1) organisms, one homozygous recessive and one homozygous dominant, are crossed, all offspring are heterozygous for the trait. When these offspring the F1 (F for filial) generation interbreed, ¾ of the offspring (F2 generation) will show the dominant phenotype,and ¼ will show the recessive phenotype. Huntington’s is an example of a monogenic condition in humans. It is dominant, but a double dose of the allele is fatal, so if both parents have it then 50% of the offspring will have the condition, 25% will be unaffected, and 25% will not survive to be born.

Question 15

Describe what a dihybrid cross is

Answer 15

inheritance of two genes simultaneously. The two
genes are inherited independently of each other and so each gamete has
one allele for each of the two gene loci. This means that during fertilisation
any one of an allele pair can combine with any one of another allele pair.
The probability of any two traits being inherited together if they are not
linked is the product of the individual probabilities

Question 16

Define linkage

Answer 16

is the phenomenon where genes for different characteristics are located at
different loci on the same chromosome and so are inherited together.

Question 17

Describe sex linkage and give example

Answer 17

Sex linkage – expression of an allele dependent on the gender of the individual as the
gene is located on a sex chromosome. Sex Chromosomes (XY in men, XX in women)
are not fully homologous, but a small part on one matches up with a small part on the other
so that they can pair up before meiosis. Many genes on the X chromosome are not to do
with sex determination (i.e: involved in metabolic functions) and have no partner allele on
the Y chromosome. So if a female inherits an X chromosome with a faulty allele she will
likely have a functioning allele for the same gene on the other X chromosome, whereas if
a man has a faulty allele on his X chromosome then he will suffer from a genetic disease
as he will not have a functioning allele for that gene. So women can be hetero or
homozygous for X-linked genes and men can’t be as they only have one, so they are
functionally haploid, or hemizygous, for X-linked genes. Women are homogametic as
they have two identical sex chromosomes, but in birds, butterflies and moths males are
homogametic and females are heterogametic. Examples of recessive sex linked
disorders are hemophilia (X^H is dominant for normal blood clotting, X^h is recessive and
leads to abnormal blood clotting)and colourblindness.

Question 18

Describe autosomal linkage

Answer 18

genes which are located on the same chromosome (which is not
a sex chromosome) and tend to be inherited together in the offspring. This occurs as the
chromosome, not the gene, is the unit of transmission during meiosis, so linked genes are very unlikely to be separated by independent assortment. If linked genes are not affected
by crossing over of non-sister chromatids during prophase 1 then they are always inherited
as one unit.

Question 19

Describe epistasis

Answer 19

The interaction between non-linked gene loci where one masks expression
of the other. The genes can be located at different loci on the same (or different) genes or
chromosomes. They interact to form one phenotypic characteristic, and can work
antagonistically or in a complementary fashion. As the loci are not linked , they assort
independently during gamete formation. Epistasis reduces the number of phenotypes
produced in the F2 generation of dihybrid crosses and therefore reduces phenotypic
variation.

Question 20

What is the chi-squared test

Answer 20

The chi squared test is a statistical test which can be used to establish whether the
difference between observed and expected results is small enough to occur purely due to
chance.

Question 21

When can we use use chi-squared test

Answer 21

● It can be used if the sample size is sufficiently large, that is over 20. It can only be
used for discontinuous variation (categoric) data in the form of raw counts, and if
there are no zero scores in the raw data. We should also have a strong biological theory
to use to predict expected values.
● The chi squared test can be used to determine whether the null hypothesis is correct or
not. The null hypothesis is the assumption that there is no significant difference
between observed and expected results.
● The value obtained is compared to the critical value, and in a case where the value
obtained is less than the critical value, the null hypothesis is accepted as the difference
due to chance is not significance
● Whereas in a case where the x2 value is greater than critical value, the null hypothesis is
rejected meaning that the difference between observed and expected results is not due
to chance, as it is significant.

Question 22

How to calculate chi-squared

Answer 22

To calculate Chi squared:
1. Minus the expected from the observed of each value, squaring the result and then
dividing it by the expected.
2. Add all the values you gained from step 1 together to get x squared
3. Determine the degrees of freedom (this is the number of categories -1)
4. Determine the value of p from a distribution table
5. Decide whether the difference is significant at the p=0.05 level

Question 23

What is the Hardy weinburg equation

Answer 23

The Hardy-Weinberg Equation can be used to estimate the frequency of alleles in a 
population and to see whether a change in allele frequency is occurring in a population 
over time.
● p = the frequency of the dominant allele (represented by A) q = the frequency of the 
recessive allele (represented by a) For a population in genetic equilibrium:
● p + q = 1.0 (The sum of the frequencies of both alleles is 100%.)
● (p + q)2 = 1 so p2+ 2pq + q2 = 1
● The three terms of this binomial expansion indicate the frequencies of the three 
genotypes:
● p2 = frequency of AA (homozygous dominant)
● 2pq = frequency of Aa (heterozygous)
● q2 = frequency of aa (homozygous recessive)

Question 24

What are the hardy-weinburg principles assume

Answer 24

★ The population is large enough to make sampling error negligible
★ Mating within the population occurs at random
★ There is no selective advantage for any genotype, so no selection
★ There is no mutation, migration, or genetic drift

Question 25

Define the niche of a species

Answer 25

its role within the environment.

Question 26

Describe the 3 types of adaptation to the environment

Answer 26

● Anatomical adaptations are physical adaptations, either external or internal e.g.
presence of loops of Henlé which allow desert mammals to produce concentrated urine
and minimise water loss
● Behavioural adaptations are changes in behaviour which improve the organism’s
chance of survival e.g. mating calls
● Physiological adaptations are processes inside an organism’s body that increase
its chance of survival e.g. regulation of blood flow through the skin

Question 27

Describe natural selection

Answer 27

is the process in which fitter individuals who are better adapted to the
environment survive and pass on the advantageous alleles to future generations .

Question 28

Describe evolution

Answer 28

Evolution is the process by which the frequency of alleles in a gene pool changes over time
as a result of natural selection.

Question 29

What is directional selection

Answer 29

Directional selection is a type of natural selection that occurs when an environmental
change favours a new phenotype, so results in a change in the population’s mean.

Question 30

What is stabilising selection

Answer 30

Stabilising selection is natural selection that favours constancy within a population.
Intermediate phenotypes are favoured and extreme phenotypes are selected against.
Results in reduced genetic variation within a population.

Question 31

What is genetic drift

Answer 31

Genetic drift is a phenomenon where there is a small change in allele frequency which
occurs as a result of the fact that not all the individuals in a population reproduce.
This effect is amplified in very small groups, isolated from the rest of the population
(i.e; in the aftermath of a Genetic bottleneck or the Founder Effect).

Question 32

What is genetic bottleneck

Answer 32

rapid reduction in population size which has an effect on the
population size and genetic variation in future generations such as that caused by a
natural disaster. If those who are too closely related survive then this could affect the
fertility of the population, causing it to go extinct. If individuals with deleterious alleles
survive then this may also put the population’s survival at risk. However if those best
adapted survive, then whilst genetic diversity may be less the gene pool may be better.

Question 33

What is the founder effect

Answer 33

Founder effect – decrease in genetic diversity which occurs when the population
descends from a small number of ancestors

Question 34

What is speciation

Answer 34

the process by which new species arise after a population becomes
separated and cannot interbreed

Question 35

Define and describe the two types of speciation

Answer 35

allopatric speciation is caused by a
physical barrier. As the two groups become separated and reproductively isolated as a
result, the gene flow is reduced. Each group experiences a different selection pressure
as the environment they live in is different. Over time, the frequency of alleles changes
through natural selection and the two parts of the population can no longer
interbreed and become separate species.

sympatric speciation where new species evolve from a
single ancestral species when inhabiting the same geographic region, for example
as a result of a chromosomal error during cell division which leads to reproductive
isolation. Change in chromosome number may prevent gamete fusion, make zygotes
less viable so that they fail to develop, and/or lead to infertile hybrid offspring with an
odd number of chromosomes such that chromosome pairing during meiosis cannot
occur. Animals may become reproductively isolated for other reasons such as courtship
behaviour changing (i.e: fertile at different time of year or require different behaviours to
mate) or change in flower or animal genitalia structure.

Question 36

Describe artificial selection and give an example

Answer 36

Artificial selection is the process where selection pressures are artificially created
by humans thus allowing the breeding of the desired characteristics.
An example of artificial selection is the dairy cow. The milk yield of each cow is
measured and recorded to identify the cows with the highest milk yields. This
enables the identification of the best quality bulls. The cows with the highest yields are
given hormones to increase their egg production. The eggs are fertilised in vitro
and subsequently implanted into surrogate mothers.

Question 37

What are gene banks and give examples of

Answer 37

Gene banks store genomes still within their organisms. Examples of gene banks include:
● Rare breed farms
● Crops in cultivation
● Botanic gardens
● Zoos
● Seed banks
● Sperm banks
● Cells in tissue culture
● Frozen embryos

Question 38

Ethical considerations of artificial selection

Answer 38

● Livestock have been selected to have leaner meat and less fat might succumb to low
environmental temperatures during Winter if not housed
● The traits in dogs considered desirable by humans might put a dog at selective
disadvantage in the wild.
● Some dogs have a susceptibility to disease due to the inbreeding used to produce a
breed
● However domesticated animals provide humans with economically useful work such as
companionship, protection, and herding. They also have aesthetic qualities such as
looking pretty