Inheritnce and selection

Question 1

Define ‘genotype’

Answer 1

Genotype is the genetic make-up of an organism. It describes all the alleles that an organism contain and sets the limits within which the characteristics of an individual may vary

Question 2

Define ‘mutation’

Answer 2

Any change to the genotype as a result of a change in the DNA - can be inherited if it occurs in the formation of gametes

Question 3

Define ‘phenotype’

Answer 3

The observable characteristics of an organism - the result of interaction between the expression of the genotype and the environment.

Question 4

Define a ‘modification’

Answer 4

An alter in an organism’s appearance due to the environment

Question 5

Define a ‘gene’

Answer 5

A section of DNA that is a sequence of nucleotide bases which usually determines a single characteristic of an organism (eg, eye colour) by coding for particular polypeptides. These make up the enzymes that are needed in the biochemical pathway that leads to the production of the characteristic

Question 6

Define ‘locus’

Answer 6

The position of a gene on a chromosome

Question 7

Define an ‘allele’

Answer 7

One of the different forms of a gene

Question 8

Define ‘homologous chromosome’

Answer 8

A pair of chromosomes that have the same gene loci and therefore determine the same features

Question 9

Define ‘homozygous’

Answer 9

If the allele on each of the chromosomes is the same (eg, both alleles for blue eyes) then the organism is said to be homozygous for this characteristic

Question 10

Define ‘heterozygous’

Answer 10

If the two alleles are different (eg, one chromosome has an allele for blue eyes and the other for green) then the organism is said to be heterozygous for the characteristic

Question 11

Define ‘homozygous dominant’

Answer 11

A homologous organism with two dominant alleles

Question 12

Define ‘homozygous recessive’

Answer 12

A homologous organism with two recessive dominant alleles

Question 13

Define ‘co-dominant’

Answer 13

When two alleles both contribute to the phenotype, they are referred to as co-dominant. The phenotype is either a blend of both features or both features are represented

Question 14

Define ‘monohybrid inheritance’

Answer 14

The inheritance of a single gene

Question 15

Define ‘pure breeding’

Answer 15

A kind of breeding in which the parents with a particular phenotype produce offspring only with the same phenotype eg, plants with green pods consistently give rise to plants with green pods
This means that the organisms are homozygous

Question 16

Define ‘first filial’

Answer 16

Offspring resulting from a cross between strains of distinct genotypes - written as F1

Question 17

The law of segregation

Answer 17

In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be presented by a single gamete

Question 18

Define a ‘test cross’

Answer 18

A genetic cross carried out to test whether an unknown genotype is homozygous (two dominant alleles) or heterozygous (one dominant allele, one recessive allele)

Question 19

What chromosomes do male and female humans have?

Answer 19

Unlike other features of an organism, sex is determined by chromosomes rather than genes
Female - XX - all gametes are the same as they contain a single X chromosome
Male - XY - half have an X chromosome and half have a Y chromosome

Question 20

Define ‘sex-linked’

Answer 20

A characteristic is said to be sex-linked when the alleles that codes for it is located on a sex chromosome (X or Y)

Question 21

Why will characteristics controlled by recessive alleles on this non-homologous portion of the X chromosome appear more frequently in males?

Answer 21

For most of the length of the X chromosome there is no equivalent homologous portion of the Y chromosome as it is much shorter. ‘There is no homologous portion of the Y chromosome that might have the dominant allele’, in the presence of which the recessive allele does not express itself.

Question 22

What is haemophilia?

Answer 22

A condition in humans where the blood clots only slowly and there may be slow/persistent internal bleeding especially in the joints. It is lethal if not treated causing some selective removal of the gene from the population - mostly occurs in males.
One cause of haemophilia is a recessive allele with altered DNA nucleotides that do not code for the required protein and therefore the individual is unable to produce a protein required in the clotting process. This protein can be extracted from donated blood and given to haemophiliacs.
Males inherit the Y chromosome from their father which does not carry the allele that does not code for the clotting protein - it is linked to the X chromosome. Females can inherit it via their mothers or fathers in the X chromosome and males can inherit it from their mother.

Question 23

What is a pedigree chart?

Answer 23

Can be used to trace the inheritance of sex-linked characteristics such as haemophilia

• Male = represented by a square
• Female = represented by a circle
• Shaded shape = indicates presence of a character in phenotype
• Dot within a circle = represents a woman with normal phenotype but carries the defective allele

Question 24

Define ‘co-dominance’

Answer 24

Both alleles are equally dominant

Question 25

Define ‘multiple alleles’

Answer 25

Where there are more than two alleles of which only two may be present at the loci of an individual’s homologous chromosomes

Question 26

Writing co-dominance

Answer 26

In snapdragon plants one allele codes for an enzyme that catalyses the formation of a red pigment in flowers and another allele codes for an altered enzyme that lacks this catalytic activity and so does not produce the pigment. When writing this we CANNOT use upper and lower case letters as this would suggest that one allele was dominant to the other, therefore, we use different letters eg, R for red and W for no pigment (white)
SEE TEXTBOOK PAGE 122

Question 27

Define ‘multiple alleles’

Answer 27

Sometimes a gene has more than two alleles eg, the inheritance of the human ABO blood group; there are three alleles associated with the gene I (immunoglobulin gene) which leads to the production of different antigens on the surface membrane of red blood cells

Question 28

Multiple alleles and dominance hierarchy

Answer 28

Sometimes there may be more than three alleles, each of which are arranged into a hierarchy with each allele being dominant to those below it and recessive to those above eg, coat colour in rabbits
Agouti coat
Chinchilla coat
Himalayan coat
Albino coat

Question 29

Define ‘gene pool’

Answer 29

All of the alleles of all the genes of all the individuals in a population at any one time
If there are 10,000 people in a population then there will be 20,000 alleles (twice as many) in the gene pool of a particular gene

Question 30

Define ‘allelic frequency’

Answer 30

The number of times an allele occurs within the gene pool

Question 31

Total number of alleles

Answer 31

The total number of alleles in a population is 1.0
If everyone had the homozygous dominant gene (FF) then the dominant allele (F) would be 1.0 and the recessive allele (f) would be 0.0
If everyone had the heterozygous allele then the dominant allele (F) would be 0.5 and the recessive allele (f) would be 0.5 however the population is not made up of one genotype but a mixture

Question 32

How can we work out the allele frequency of mixed populations?

Answer 32

Using the Hardy-Weinberg principle (can be used to calculate the frequencies of the alleles of a particular gene in a population)

Question 33

The Hardy-Weinberg principle predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next provided that these 5 conditions are met:

Answer 33

• No mutations arise
• The population isolated (no flow of alleles into/out of the population)
• No selection (all alleles are equally likely to be passed to the next generation)
• Large population
• Mating within the population is random

Question 34

Hardy-Weinberg principle

Answer 34

Frequency of allele A = p
Frequency of allele a = q
p + q = 1.0 
As there are only 4 possible arrangements of the two alleles, it follows that the frequency of all four added together must equal 1.0
AA + Aa + aA = aa = 1.0
p² +2pq + q² = 1.0

Question 35

Differences between the reproductive success of individuals affects allele frequency of populations because …

Answer 35

• All organisms produce more offspring than can be supported by the supply of food, light, space etc
• Despite overproduction of offspring, most populations remain relatively constant in size - less competition between members of a species to survive
• Gene pool with a wide variety of alleles
• Some individuals possess combinations of alleles making them fitter to survive in competition with others as they are able to obtain available resources thus grow more rapidly and live longer so have a better chance of successfully reproducing and pass on their alleles
• As new individuals have advantageous alleles, they are more likely to survive and reproduce successfully
• Over many generations, the number of individuals with advantageous alleles will increase and those with less advantageous alleles will decrease

Question 36

Define ‘selection’

Answer 36

A process that results in the best-adapted individuals in a population surviving and so pass on their favourable alleles to the next generation

Question 37

Give 2 types of selection

Answer 37

• Directional selection - selection may favour individuals that vary in one direction from the mean of the population
• Stabilising selection - preserves the characteristics of a population and may favour average individuals

Question 38

Directional selection

Answer 38

If the environmental conditions change, so will the phenotypes needed for survival and some individuals which fall to the left/right of the mean will possess a phenotype more suited to the new conditions - these will be more likely to survive and breed therefore contributing more offspring and alleles to the next generation
Over time the mean will move in the direction of these individuals
‘Directional selection therefore results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against’

Question 39

Stabilising population

Answer 39

If environmental conditions remain stable then the individuals with phenotypes closest to the mean are favoured and are more likely to pass their alleles on to the next generation
Individuals with phenotypes at the extreme ends are less likely to pass on their alleles so stabilising selection tends to eliminate the phenotypes at the extremes
‘Stabilising selection therefore results in the phenotypes around the mean of the population being selected for and those at both extremes being selected against’

Question 40

Define ‘speciation’

Answer 40

The evolution of new species from existing species
It depends upon groups within a population becoming isolated in some way so the flow of alleles between them may cease and environmental factors will affect the two groups differently. Selection will cause the alleles in each group to differ and each population will evolve so that when reunited the two groups (originally of the same species) may be incapable of breeding with one another as they have become separate species - each with its own gene pool

Question 41

Geographical isolation

Answer 41

Occurs when a physical barrier prevents two populations from breeding with one another eg, rivers, mountain ranges and deserts
What proves a barrier to one species may not be a problem for another eg, a river may isolate hedgehogs but not fish
A small stream could isolate woodlice yet the whole ocean may not separate birds
Geographical separation of populations can lead to the formation of a new species