A2 Biology

unit F215 – exam 1hour 45 mins50% of marks for A2

Population genetics

includes

Hardy Weinberg equations,

natural selection and artificial selection

Call for European Cystic Fibrosis healthcare gap to be closed

Press release 19 March 2010 by Bristol University• A healthcare gap amounting to a ‘death sentence’ for

Cystic Fibrosis (CF) children born in Eastern Europe must be closed say researchers from the EuroCareCF Coordination Action for Cystic Fibrosis.

• “We know that this disease occurs randomly in about 1 in 4,000 children born to healthy parents across the EU,” he said.  “Despite this, the team encountered many fewer people with CF in poorer countries.  CF patients there die far younger than in long standing EU countries.”

• Understanding the inheritance of characteristics and of population genetics can help governments to plan healthcare.

2 inherited characters in humans – both involve autosomal recessive alleles

Cystic fibrosis – excessive mucus in bronchioles reduces gas exchange + few digestive enzymes from pancreas reach gut

PKU = phenylketonuria – enzyme to convert phenylalanine into tyrosine not present; child has light skin and hair + brain damage (now blood test at birth + special diet)

Key definitions page 2

1) A population of a species is:-

a group of organisms which have similar characteristics e.g. anatomy, physiology, biochemistry and behaviour and which can interbreed to produce fertile offspring

2) The gene pool is:-

all the genes in the gametes of a population from which the genotypes or genomes of the next generation are formed

Hardy was a British

Mathematician;

Weinberg was a

German doctor

The Hardy Weinberg principle predicts that:-the frequency of an allele in the gene pool will not

change from one generation to the next (and so the frequency of genotypes / genomes also remains constant)

this is only true if

• the population is very large• there is no immigration or emigration from the

population • mating is random• all genotypes are equally fertile • there are no random mutations

Which could be met by a natural population?

Hardy Weinberg equations

• Allele frequency • frequency of dominant allele = p• frequency of recessive allele = q• total frequency of alleles for a character in the

population = 1

p + q = 1

So if r = 0.6 then frequency of R = 0.4

Hardy Weinberg equations

p2 = frequency of homozygous dominant genotype2pq = frequency of heterozygous genotypeq2 = frequency of homozygous recessive genotype

total frequency of all genotypes in population = 1

p2 + 2pq + q2 = 1

So if frequency rr is 0.39 and RR is 0.34 frequency of Rr = 0.27

Tables for Hardy-Weinberg calculations

decimal %

allele

p

allele

q

decimal%

genotype

p2

genotype

2pq

genotype

q2

Hardy Weinberg - answers

1) p = 0.3 and q = 0.7;

Genotype frequencies = 9% 49% 42%

2)a) q = 0.3 or 30% b) 0.49 or 49%

c) 0.42 or 42%

3) q = 0.008 or 0.8%

Hardy Weinberg - answers

4)a) q = 0.42 or 42% b) p = 0.58 or 58%

c) 33% and 49%

5)a) q = 0.07 so p = 0.93

b) 0.49% c) 0.13 or 13%

6)a) q = 0.01 b) 0.0198 or 1.98%

Hardy Weinberg - answers7) a) q = 0.4 homozygotes = 0.36 or 36%

heterozygotes = 0.48 or 48%

b) Allele for sickle cell anaemia gives resistance to malaria = an environmental selection pressure

People with allele for normal haemoglobin and for sickle cell haemoglobin have selective advantage where malaria is present so more likely to survive and pass on alleles to offspring

c) If malaria eradicated no advantage to have sickle cell allele

Also need large population, no immigration or emigration, random mating, equal fertility and no mutations

Hardy Weinberg - answers8)a) q = 0.6

Homozygotes = 0.16 or 16%

Heterozygotes = 0.48 or 48%

b) No selective advantage to be tongue roller – remains same if all the conditions of Hardy-Weinberg equation are fulfilled

9)a) no – until very recently people with cystic fibrosis have not reproduced

b) Termination of pregnancies may reduce allele frequencies – but gene may still mutate

Genetic drift in small populations p7

Genetic drift = effect of chance factors on allele frequency in gene pool

Genetic drift in frog-hoppers on the Scilly Islands

Genetic drift and frog hoppers

1) St Agnes + St Mary’s – all striped;Other islands – striped and melanic but proportions vary;On all but one island more striped than melanic

2) Small population on island; may only have genes for striped = founder effect;Or chance means some genotypes reproduce more than

others – large effect on phenotypes as small number of individuals

3) Difficult!Allow small populations to breed for several generations in

lab and record resultsIn field could investigate environmental factors which might

affect breeding e.g. temperature, shelter, camouflage

Natural selection – stabilising and directional

Speciation – development of a new species

Needs reproductive isolation of a population so individuals cannot interbreed

Alleles do not pass from one gene pool to the other

Allopatric speciation = geographical isolation

Sympatric speciation = behavioural / seasonal isolation

Reproductive isolation by courtship behaviour = sympatric

Reproductive isolationby time of flowering =

sympatric

Peppered moths – evolution in action http://www.biologycorner.com/worksheets/pepperedmoth.html

Which moths survive? Why are the moths

Why? different colours in different areas?

The peppered moth Biston betulariaStabilising and directional selection p10

Definition of a species p11

Biological definition

Phylogenetic / evolutionary / cladistic definition

Why have 2 definitions?

Are they equally useful?

Artificial selection and natural selection Similarities and differences p13

Both select the organisms to reproduce

Humans choose the characteristics in artificial selection and it usually produces lower genetic diversity in the gene pool

Selection pressures in natural selection are less likely to reduce genetic diversity of the

species

Artificial selection

Selective breeding of cattle

Artificial selection – dairy cows

Selective breeding = humans select the organisms that will reproduce

(this is not genetic engineering)

• Choose parents which show the desired characteristic (animals or plants)

• e.g. resistance to disease, speed of growth, height, milk yield

• Breed the parents • Select the offspring with the desired

characteristic• Use them for breeding• Continue for many generations

(detailed records needed)

• Result - desired features are more obvious but genetic diversity is decreased by the inbreeding

Improving food production by selective breeding

Recent selective breeding of wheat

Producing bread wheat Triticum aestivum by artificial selection

Producing bread wheat Triticum aestivum by artificial selection

1) Wild einkorn wheat 2N = 14 chromosomes = AA genome (haploid number = 7)

2) Einkorn wheat – artificial selection of desired characters by early farmers 2N = 14 chromosomes = AA genome

3) Crossed with wild grass 2N = 14 chromosomes = BB genome (happened by chance)Gametes fusing = A and B genomesProduced sterile hybrid 14 chromosomes = AB genome (no homologous pairs so meiosis cannot produce gametes)

Producing bread wheat Triticum aestivum by artificial selection

4) Random mutation chromosome number doubles tetraploid 4N = 28

Emmer wheat has genome AABB

5) Chance cross with goat grass 2N = 14 chromosomes = DD genome

Produced sterile hybrid – gametes fusing are AB and D – meiosis unsuccessful

6) Random mutation chromosome number doubles hexaploid 6N = 42

Modern bread wheat Triticum aestivum has genome AABBDD

Spartina grass on edge of sea

How are these 2 species reproductively isolated?

Cloning in animals whole organism or stem cells(reproductive or non-reproductive cloning) p6 - 8

Dolly the Sheep – a product of nuclear transfer

Where are the enucleate egg and the surrogate mother?

Cloning animals – advantages or disadvantages?

• Many animals with desired characters produced

• Infertile animals can reproduce• Rare animals conserved

• Animals genetically identical• Very labour intensive and needs sterile

facilities and highly trained staff• Breeding can occur at any time of year

• Genetically modified animals can be used to produce pharmaceutical chemicals