Biology 12

Pedigree problem 8

a) Is this trait dominant or recessive? Give reasons to justify your answer. Recessive : H = normal, h = hairless

b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. autosomal

c) Which individuals are definitely homozygous? Shaded onesd) Which individuals are definitely heterozygous? Unshaded ones with

children or parents who are shadede) What is the probability of individuals III1 and III9 having hairless

offspring? Show all working Hh x Hh = 25% hairless

H h

H

h Hh hh

Hh hh

Pedigree problem 9

a) Is this trait dominant or recessive? Give reasons to justify your answer. Dominant

b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. Autosomal

c) Which individuals are definitely homozygous? All the unshaded onesd) Which individuals are definitely heterozygous? Any shaded one with an

unshaded child or an unshaded parente) Individuals II8 and II9 are expecting another child. What is the probability it will

have astigmatism? Show all working Aa x aa = ½ Aa astigmatism ½ aa normal

A a

a

a

Aa

Aa

aa

aa

Pedigree problem 10

a) Is this trait dominant or recessive? Give reasons to justify your answer. Recessive

b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. Sex-linked

c) Which individuals are definitely homozygous? I4 (males can’t be homozygous – they only have 1 chromosome for this characteristic)

d) Which individuals are definitely heterozygous? II5, II9, III5, III7, III9 (males can’t be heterozygous – they only have 1 chromosome for this characteristic)

e) Individuals II4 and II5 are expecting another child. What is the probability it will have the condition? Show all working XnY x XNXn = 25%

XnXN

XN

Y

XNXnXNXN

XNY XnY

Evolution

• Evolution refers to changes in gene frequencies over time

• In Darwin's time, it was thought to involve the gradual change in organisms over a long period

• Now the theory looks at changes in gene pools and genetic mechanisms

Genotypes and gene pools

• The allele combination possessed by an individual is referred to as genotype

• The allele frequencies present in a population is referred to as the gene pool

Natural selection

• Variation• Overpopulation • ‘struggle for existence’ • ‘survival of the fittest’ • Inheritance of successful

variations • Change in gene

frequencies

Variation

Inheritable differences due to different allele combinations

These are due to• Mutations changes in DNA

• Mixing of alleles during sexual reproduction meiosis and fertilisation

Mutations

Changes in DNA

These can be• Beneficial eg disease resistance• Harmful eg haemophilia• Neutral eg tongue rolling

Sexual reproduction

• Crossing over – swapping of genetic material between homologous chromosomes

• Random segregation during meiosis – its random which combination of alleles ends up in each gamete

• Random combination of gametes at fertilisation – its random which gametes end up together

Struggle for existence• More offspring are produced than can survive so most will

die or be killed

• This leads to winners - survivors with more offspring and losers – those with less offspring

• Winners pass on their genes to their offspring, so more of the next generation will have the successful allele combinations

Selection pressures

Anything that can affect survival or reproduction. They include:

• Competition eg for food, water, shelter, mates, nesting sites, etc

• Predation – eg being eaten, also disease or parasites

• Environmental forces – eg heat, cold, drought, fire, flood, snow, high UV radiation, etc

Examples of natural selection• Insecticide resistance in insectsVariation exists in the population – so some are more resistant to chemicals than

othersWhen they are sprayed, the resistant forms are more likely to surviveThese will breed and pass on their resistant alleles to their offspringAfter several generations, the population will consist of mainly resistant insects• Antibiotic resistance in bacteriaVariation exists in the population – so some are more resistant to antibiotics than

othersWhen they are treated, the resistant forms are more likely to surviveThese will breed and pass on their resistant alleles to their offspringAfter several generations, the population will consist of mainly resistant bacteria• Peppered moths2 forms exist – dark and lightDark forms are more easily caught and eaten by their predators (birds) if they live

on light coloured trees or walls, light forms are more likely to be eaten if their surroundings are dark

In unpolluted areas, trees are pale and the dark forms decreaseIn polluted areas, trees are darkened with pollution and the pale forms decreaseIn this case the selection pressure (predation) is affected by the colour of their

surroundings

Artificial selection

This is – where humans provide the selection pressures and breed for certain characteristics

Examples include – domestic animals eg dogs, cats, pigeons, sheep, cattle, etc and crops eg wheat, rice, fruit, vegetables, etc

Random genetic driftThis is change in gene frequencies due to chance

processes, not selection pressures. The alleles affected are usually survival neutral.

It occurs most often in smaller populations because variation is usually lower to start with, and with fewer individuals, the loss of one or two can have large effects on the alleles remaining in the population

Founder effect refers to random genetic drift occurring in small isolated populations eg Dunkers in USA

Speciation• Speciation refers to development of new

species – population is so different to the original that they no longer can interbreed

• A species is a group of organisms that, under natural conditions, can breed to produce fertile offspring

• Races or subspecies are smaller breeding groups within a species that exhibit differences from other breeding groups. If separated they may develop into new species

• Problems with the species concept include fossils or extinct species – can’t breed these, ring species – breed with intermediate populations, but those at the ends won’t breed with each other

IsolationIsolation prevents gene flow, thus reducing the

inflow of alleles selected against – this intensifies the effect of natural selection

Barriers include:Geographical – rivers, oceans, mountains, deserts,

etcReproductive – features that prevent interbreeding• Mechanical – physical or anatomical differences

that prevent mating eg different copulatory organs

• Behavioural – different mating calls or dances• Ecological – different breeding locations, mating

seasons• Hybrid sterility – either no offspring are

produced, or offspring are sterile

Migration

Migration includes• immigration coming in to a population• emigration leaving a population Migration into new areas allows organisms to be subjected to

a new set of selection pressures, and hence a new round of evolution

In some cases, organisms unsuited to an environment can emigrate, thus increasing the effects of natural selection

In other cases, organisms unsuited to an environment can immigrate, thus decreasing the effects of natural selection

ExtinctionExtinction is the disappearance of a species

Organisms become extinct when the selection pressures become too great for species survival

Examples of extinction events and their causes include catastrophes eg volcanoes & meteor hits; climate change; sea level change; human activity

Examples of organisms made extinct due to human activity include: dodo & passenger pigeon – hunted to extinction

Examples of organisms at risk of extinction due to human activity include: rhino, giant panda, elephant, etc – includes hunting, habitat loss, effects of pollution