Inheritance (A-level only)

Question 1

Gene

Answer 1

A gene is a section of DNA located at a particular site on a DNA molecule, called its locus.

The base sequence of each gene carries the genetic information that determines the sequence of amino acids in a protein.

Question 2

Allele

Answer 2

Alleles are the different variations of a gene.

Diploid organisms have two alleles (one on each chromosome).

Question 3

Genotype

Answer 3

A genotype is an organism’s underlying genetic makeup.

The genotype consists of both physically visible and non-expressed alleles.

Question 4

Phenotype

Answer 4

A phenotype is the observable traits expressed by an organism.

The phenotype is determined by the interaction between its genetic constitution and the environment.

Question 5

Dominant alleles

Answer 5

Dominant alleles are always expressed in the phenotype of an organism.

Question 6

Recessive alleles

Answer 6

Recessive alleles are only expressed in the phenotype if there are two copies of the allele.

Question 7

Co-dominant alleles

Answer 7

Co-dominant alleles are both expressed in the heterozygote.

Question 8

Homozygote

Answer 8

Homozygotes are organisms with two copies of the same allele.

Homozygotes can be homozygous dominant (both alleles are dominant) or homozygous recessive (both alleles are recessive).

Question 9

Heterozygote

Answer 9

Heterozygote

Question 10

Monohybrid crosses

Answer 10

When two parents that differ in only one characteristic breed, the process is called a monohybrid cross.

Monohybrid crosses allow the genotype of offspring to be predicted.

Question 11

Stages of making a monohybrid cross:

Answer 11

Parental genotype
Gamete alleles
F1 offspring
Gamete alleles
F2 offspring
Predicting genotypic ratios

Question 12

Parental genotype

Answer 12

The first step in constructing a monohybrid cross involves identifying the parental genotypes.

E.g. Two true-breeding pea plants have yellow or green peas.

The dominant seed colour is yellow so the parental genotype is YY for yellow pea plants and yy for green pea plants.

Question 13

Gamete alleles

Answer 13

Gametes are haploid, so only one allele from each parent is found in the gametes.

All possible combinations of the parental alleles should be identified.

This represents the meiotic segregation into haploid gametes. In our true-breeding pea plant example:
100% of the gametes of green pea plants will have y alleles.
100% of the gametes of yellow pea plants will have Y alleles.

Question 14

F1 offspring

Answer 14

F1 offspring are the first generation of offspring.

A monohybrid cross produces four different combinations of possible offspring.

For the pea plants, both parents are heterozygous.

This means 50% of the offspring are homozygous (25% yy and 25% YY), and that 50% of the F1 offspring produced have a Yy genotype.

Question 15

Gamete alleles

Answer 15

The F1 pea plants have two different alleles. They are heterozygous.

The gametes for an individual F1 offspring may contain either the Y allele or the y allele.

50% of an organism’s gametes will contain the Y allele.

50% of an organism’s gametes will contain the y allele.

Question 16

F2 offspring

Answer 16

F2 offspring are the second generation of offspring.

When the F1 pea plants breed, there are three possible genotypic combinations:
YY
Yy
yy

Question 17

Predicting genotypic ratios

Answer 17

Monohybrid crosses allow predictions to be made about the genotypic and phenotypic ratios of offspring.

In the pea plant example, the ratio of yellow peas to green peas is 3:1. A monohybrid cross between two heterozygotes will always produce this ratio.

Monohybrid crosses can be drawn in two ways:
Genetic diagrams.
Punnett squares.

Question 18

Dihybrid crosses

Answer 18

When two parents that differ in two characteristics breed, the process is called a dihybrid cross.

Question 19

Stages of making a dihybrid cross:

Answer 19

Independent assortment
Dihybrid gamete alleles
F1 offspring
F1 gamete alleles
F2 offspring
Predicting phenotypic ratios

Question 20

Independent assortment

Answer 20

Mendel proved that genes do not influence each other with regard to the sorting of alleles into gametes.

This is called the law of independent assortment.

The law of independent assortment means that genes separate independently of each other when gametes are made.

The combination of alleles can be shown in a dihybrid cross.

Question 21

Dihybrid gamete alleles

Answer 21

In a dihybrid cross between two homozygotes, there is one possible gamete allele combination for each homozygote.

E.g. two pea plants differ in two characteristics: seed colour and seed texture.

One plant has green, wrinkled seeds (yyrr) and one plant has yellow, round seeds (YYRR).

100% of the gametes of the green/wrinkled plant are yr.

100% of the gametes of the yellow/round plant are YR.

Question 22

F1 offspring

Answer 22

When two homozygotes breed, all the F1 offspring have the same genotype.

E.g. The offspring of the pea plants all have a YyRr genotype.

Question 23

F1 gamete alleles

Answer 23

The law of segregation predicts that each gamete in F1 generation has an equal probability of receiving any allele (e.g. R, r, Y or y).

This means there are four possible combinations of gametes produced by the F1 offspring. For example:
YR.
Yr.
yR.
yr.

Question 24

F2 offspring

Answer 24

When the F1 offspring breed, the four possible gametes from one individual can combine with any of the four possible gametes from the other individual.

The total possible combinations in the F2 generation is 16.

Question 25

Predicting phenotypic ratios

Answer 25

Dihybrid crosses can be used in this way to predict genotypic ratios of the F2 offspring.

In the pea plant example, the ratio of offspring is:
Nine round/yellow.
Three round/green.
Three wrinkled/yellow.
One wrinkled/green.

When two dihybrid heterozygotes breed, the ratio is always expected to be 9:3:3:1.

Question 26

Codominance

Answer 26

Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote.

This can influence the outcome of monohybrid and dihybrid crosses.

Question 27

Example of codominance
Sickle-cell anaemia

Answer 27

An example of codominance is sickle-cell anaemia.

There are two alleles for sickle-cell anaemia:
HN - normal haemoglobin.
HS - sickle haemoglobin.

Question 28

Sickle-cell phenotypes

Answer 28

People who have two copies of the HN allele (homozygotes) do not have sickle-cell anaemia.

People who have two copies of the HS alleles (homozygotes) do have sickle-cell anaemia.

People who have one HN allele and HS allele (heterozygotes) have both normal haemoglobin and sickled haemoglobin.

HN and HS are codominant.

Question 29

Phenotypic ratios in codominance

Answer 29

Codominance affects the phenotypic ratios of monohybrid and dihybrid crosses.

E.g:
If two heterozygous (HNHS) breed, the ratio becomes 1:2:1 instead of the normal 3:1 that is expected in a monohybrid cross.

Question 30

Autosomal linkage

Answer 30

Linkage is when genes that are close to one another on a chromosome are likely to be inherited together.

Linkage shows that some allele combinations are not inherited independently of each other.

Question 31

Mendel’s law

Answer 31

Mendel’s law of independent assortment states that genes do not influence the sorting of alleles into gametes.

This is not always the case.

Some allele combinations are not inherited independently of each other.

Genes that are located close to each other on the same chromosome are more likely to be inherited as a pair.

This is called linkage.

Question 32

Autosomes

Answer 32

Autosomes are all the chromosomes except the sex chromosomes (X and Y).

Autosomes are arranged in pairs called homologous chromosomes (one from the father and one from the mother).

Homologous chromosomes consist of the same genes in the same order along the chromosome.

There is some variation if the chromosomes have different alleles but the genes are the same.

Question 33

Recombination

Answer 33

When gametes are produced by meiosis, multiple sections of homologous chromosomes are exchanged in a process called recombination.

If two genes are located in close proximity on the same chromosome, they are more likely to be exchanged together and not separated in recombination.

The genes are more likely to be transmitted to a gamete together.

Question 34

Linkage

Answer 34

Genes that are close together on the same autosome are more likely to be transmitted to a gamete together.

The genes are linked.

This is autosomal linkage.

Question 35

Sex linkage

Answer 35

Linkage is where genes that are close to one another on a chromosome are likely to be inherited together.

Sex linkage is different from autosomal linkage because it takes place in the sex chromosomes.

Question 36

Sex chromosomes

Answer 36

In many organisms, the sex chromosomes (X and Y) determine the sex of an individual.

Sex chromosomes differ from autosomes because they are non-homologous.

Non-homologous chromosomes do not consist of the same genes in the same order along the chromosome.

Question 37

Y-chromosome

Answer 37

Human females have a homologous pair of X chromosomes (XX) and human males are heterozygous (XY).

X and Y chromosomes contain a small region of similarity.

The Y chromosome is considerably shorter than the X chromosome and contains fewer genes.

If a gene is only found on the X chromosome, it is X-linked.

Question 38

X-linked chromosome

Answer 38

Males only have one copy of an allele for X-linked genes.

There is no allele for the same gene on the Y chromosome.

This is called hemizygosity.

Hemizygosity means that there is no role of dominance and recessiveness.

If a recessive gene is inherited on the X chromosome, it will always be present in the phenotype as it is the only allele present.

Question 39

Haemophillia

Answer 39

Haemophilia is a blood disorder that is X-linked.

Humans males only need to inherit one recessive mutant X allele to be affected by the disorder but females must inherit two copies of the recessive allele.

This means haemophilia is more common in males.

Females can be carriers for the disorder when they are heterozygous.

Question 40

Multiple alleles

Answer 40

Although only two alleles per gene are found in an individual diploid organism, there may be multiple alleles at the population level.

This means many possible combinations of alleles can exist.

Question 41

Homologous chromosomes

Answer 41

Chromosomes in diploid organisms exist in homologous pairs.

Homologous chromosomes consist of the same genes in the same order.

The only variation between homologous chromosomes is in the alleles.

Two alleles can be present for one gene.

Question 42

Multiple alleles definition

Answer 42

Although only two alleles can be present in an organism, there could be multiple different alleles for a single gene.

Multiple alleles provide many different genotype combinations.

This creates large variation at the population level.

Question 43

Function of epistasis

Answer 43

A gene at one location can mask the expression of another gene.

This is called epistasis.

The alleles that are being masked are hypostatic to the epistatic alleles.

Epistasis often involves a pathway where expression of one gene is dependent on the function of another gene.

Question 44

Recessive epistasis

Answer 44

Recessive epistasis is where the epistatic allele (the allele that masks another gene) is recessive.

This means two copies of the epistatic allele must be present for expression of the hypostatic allele to be affected.

Question 45

Dominant epistasis

Answer 45

Dominant epistasis is where the epistatic allele is dominant.

This means only one copy of the epistatic allele must be present for expression of the hypostatic allele to be affected.

Question 46

Chi-squared

Answer 46

The chi-squared test is used in genetics to compare the goodness of fit of observed data with expected data.

It tests if the difference between observed and expected values is due to chance.

Question 47

Chi-squared: Inheritance

Answer 47

Genetic diagrams are used to predict the expected phenotypic ratio of offspring.

Predictions are rarely 100% accurate because of the random nature of gametes fusing during fertilisation.

Chi-squared is used to compare observed phenotypic ratios with expected ratios.

Chi-squared tells us if the difference between the observed and expected ratios are due to chance.

Question 48

Requirements for Chi-squared

Answer 48

The Chi-squared test is used when:
Variation is discrete not continuous. This means the data are in categories (e.g. Aa and aa).
Data show absolute numbers (whole numbers), normally frequencies.

Question 49

Null hypothesis

Answer 49

Before using chi-squared, a null hypothesis is stated.

The null hypothesis is:
‘There is no significant difference between observed and expected data, the difference is due to chance’.

The chi-squared test is used to reject or accept the null hypothesis.

Question 50

Chi-squared equation

Answer 50

x = sum of (O-E)^2/E

O = observed values.
E = expected values.