UNIT 7 - Inheritance and Evolution

gentoype

genetic constitution of an organism - refers to alleles present-BB/Bb-bb

phenotype

appearance of a characteristic due to the expression of the genotype and its interaction with the environment

alleles

different form of genes. can be dominant / recessive/ co dominant

dominant allele

always expressed in phenotype

recessive allele

only expressed when the genotype is homozygous recessive

codominant allele

both alleles are expressed in the phenotype- neither allele is dominant / equally dominant- with both contributing to the phenotype of the characteristic

locus

position of a gene on a chromosome. alleles occupy the same locus on each member of a pair of homologous chromosome.

in a diploid organism, the alleles at specific locus may be heterozygous or homozygous

homozygous

pair of homologous chromosomes carrying the same alleles for a single gene

heterozygous

pair of homologous chromosomes carrying two different alleles for a single gene

how are these types of inheritances presented via genetic diagram coding-

monohybrid

codominant

multiple alleles

sex- linkage

autosomal linkage

epistasis

monohybrid - single letter, capital/ lower case - B or b

codominant- Gene^ allele - I^B I^A etc

multiple alleles - Gene ^allele- I^B I^O etc

Sex - linkage - Chromosome^ allele- X^ R X^r / X^R Y

autosomal linkage - single letter, capital/ lower case - Aa Bb

epistasis- single letter, capital/ lower case EE Bb

multiple alleles

there are more than two alleles of a particular gene. however, only two alleles can be present in a genotype - one on each member of a pair of homologous chromosomes -

an example of this is ABO Blood Groups where the O is recessive- In one genotype there will be two alleles present in any one genotype- I^A = I^B these alleles are codominant so the blood group would be AB

sex linked gene

where the gene is carried on only one type of sex chromosome- usually X

autosomal linkage

genes that are licated on the same chromosome- not the sex chromosome

epistasis

when one gene modifies or masks the expression of a different gene at a different locus

what is monohybrid inheritance

inheritance of one particular characteristic- genetic inheritance cross of a characteristics determined by one gene

what is dihybrid inheritance?

inheritance of two different characteristics. each characteristic is controlled by a different gene and found on different chromosomes.

why is a test cross used?

to determine the genotype of an organism with a dominant phenotype. involves crossing the organism with the homozygous recessive

what does pure breed mean?

homozygous for each allele

sex-linked genes

the gene is carried on only one type of sex chromosome, usually the X (has more alleles in the extra tail that the Y chromosome does not have)

what are the non-sex chromosomes called

autosomes

what is colour blindness caused by

caused by a recessive allele carried on the X chromosome

the allele B for normal colour vision is dominant, allele b for colour blindness is recessive

the Y chromosome does not carry the allele

males cannot be heterozygous as they can carry only one allele on the X chromosome - X^BY (normal) or X^bY (colour blind)

what is haemophilia and what is it caused by?

draw a genetic cross diagram for a woman who is heterozygous and man who is “normal”

recessive condition in which the blood doesnt clot and sufferers can bleed to death if its not treated- it is sex linked and carried on the X chromosome

why are X linked genes guaranteed to he passed from mother to son?

male children must inherit their X chromosome from their mother- so if there are any x linked are guaranteed to be passed from mother to son

why can a man not pass a sex linked gene to his son?

males have a Y chromosome and this could only have been passed on from their father so the X chromosome must have come from their mother

why can a gene, if sex linked, be assumed to be X- linked?

there are no known examples of genes carried only on the Y chromosome

why does a recessive sex linked trait tend to occur most often in males?

there is no other allele on the Y chromosome, so the recessive allele is always expressed. but doesnt mean that females dont get disease- it is just less likely as they have to inherit two recessive alleles

how can you prove if the allele is recessive?

• 2 unaffected parents who have an affected child

• the parents musr therefore be heterozygous

how can you prove an allele is not sex linked (when known to be recessive)

• 2 unaffected parents have an affected daughter

• the father would pass on the dominant allele on the X chromosome so all daughters should be unaffected however there is a daughter with the allele therefore - not sex linked ( has to be recessive)

how can you prove an allele is dominant

both parents must be heterozygous and pass on their recessive alleles

when can you use the chi- squared test in relation to genes

to compare the goodness of fit of observed phenotypic ratios with expected ratios

what is dihybrid inheritance?

This usually refers to the inheritance of two different characteristics. Each characteristic is controlled by a different gene.

The two genes are carried out on different homologous chromosomes

how can you use this example to explain the dihybrid inheritance for F1 and F2

• The allele for round seeds, R is dominant to the allele for wrinkled seeds, r.

• The allele for yellow seeds, Y is dominant to the allele for green seeds, y.

(both are pure breed initially)

• pure breeding pea plants (RRYY) which always produced round-shaped, yellow coloured seeds

• pure breeding pea plants (rryy) which always produced wrinkled-shaped, green coloured seed

if these are bred with each other - then all the off spring will be still expressing the dominant allele as the new formation will be RrYy

• if this offspring is further inbred with each other the following gametes will be produced

• RY, Ry, rY and ry

during dihybrid inheritance why are 4 different gametes produced?

• two genes are on separate pairs of homologous chromosomes and each pair aligns independently on the spindle during meiosis 1.

• so any one of the two alleles for the gene for seed shape (R and r) can combine with either of the alleles for seed colour (Y and y)

what are the ratios of F1 offspring (F2) when undergone dihybrid inheritance - formed from 4 different gametes

16 offspring characteristic can be formed

• 9:3:3:1

majority being effected by the dominant allele

why are the observed and expected ratios different in dihybrid inheritance?

• Small sample size - if there is a low number of offspring then the sampling error is greater and the less likely that the observed ratio will be the same as the expected ratio.

• The fusion of gametes at fertilisation is random.

• Epistasis

• Linked genes.

when is the chi squared test used?

• compare the goodness of fit

• between observed phenotypic ratios and comparing to expected

• to determine if there is a statistical difference between expected and observed

what are differences between linked and unlinked genes?

unlinked: the genes controlling these factors are on two different non-sex chromosomes

linked: genes carried on the same pair of homologous chromosome

what are autosomal chromosomes?

• non- sex chromosomes

• in humans - 22 pairs of non sex chromosomes

how do autosomal linked genes work?

present on the same chromosome at different loci rather than on separate chromosomes as previously considered in dihybrid inheritance.

• linked genes are usually inherited together

  • fewer genetic combinations present of their alleles as there is no independent assortment

  • leading to reduced variety of gametes produced and so reduces the variety of offspring produced

show how linked genes (non sex) work with this example

diagram for dihybrid and monohybrid inheritance describe what is seen + understand

If the two genes were unlinked the expected ratio would be 9:3:3:1

9 (Long-wings, broad) :3 (Long-wings, narrow) : 3 (Short-wings, broad) : 1 (short-wings, narrow)

If the two genes were linked the expected ratio would be 3:1

3 (Long-wings, broad) : 1 (short wings, narrow)

Note: The expected ratio for linked genes assumes that no crossing over (recombination) occurred between the pair of homologous chromosome carrying the linked genes. Crossing over during meiosis can separate the alleles of linked genes if alleles are exchanged between the homologous pair of chromosomes. This can lead to new combinations of alleles and the formation of recombinants or cross-over gametes, e.g. Ab and aB.

how many gametes can be made with and without crossing over of the homologous chromosomes?

crossing over results in additional gametes to form - multiple combinations can be formed

why do the recombinant gametes occur in lower numbers than the parental gametes?

• it is a lot more rare (crossing over generally is rarer)

what is epistasis?

when two or more genes interact to contribute to a phenotype. The allele of one gene affects or masks the expression of another gene at another locus.

when is epistasis generally observed in the body?

metabolic pathways which use enzymes

• each enzyme is dependant on the previous enzyme for its substrate

• If anyone of these enzymes is non-functional the pathway comes to a halt

why is this epistasis?

expression of one gene masks the expression of another

what is a population

A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.

what is a gene pool?

refers to all the alleles of all the genes of all the individuals in a population at any one time.

what is the allelic frequency?

is the number of times an allele of a particular gene occurs within the gene pool

what are some factors for hardy weinberg to work?

• population is large and isolated

• Mating within the population is random

• No mutations of the gene occur

• There is no selection i.e. all alleles are likely to be passed to the next generation

what are the two equations used in the hardy weinberg principles and what do each variables correlate to?

p = dominant

q = recessive

p+q=1

p²+2pq+q²=1

how is variation caused by in populations?

• environmental factors

• genetic factors

what are some genetic factors that cause variation?

• Gene mutations - changes in the base sequence of DNA often results in the production of different polypeptides/enzymes.

• Crossing over - during meiosis the exchange of alleles of the same genes (between the chromosomes of a homologous pair) results in new combinations of alleles.

• Independent segregation of chromosomes - during the first meiotic division separation of the two members of a homologous pair of chromosomes occurs independently of the separation of other homologous pairs.

• Random fertilisation of gametes occurs during sexual reproduction

what are some properties of characteristics mainly influenced by genetic factors?

· controlled by one or two genes i.e. one or two pairs of alleles.

· expressed as distinct phenotypes with no intermediates e.g. tongue roller or non-roller.

· are represented as distinct groups on a bar chart or pie graph

what are properties of characteristics which are significantly influenced by the environment?

· are controlled by many genes (polygenic)

· have no separate categories or types, but have a range of intermediates between the two extremes

· produce a curve of normal distribution when plotted on a graph

what are some examples of selection pressures effecting organisms?

• predation

• disease

• competition

describe the process of natural selection

• Organisms produce large number of offspring. Selection is due to the environmental conditions favouring particular phenotypes.

• A change in the environment results in organisms with a particular phenotype being better adapted to this environment surviving and reproducing while those that are less adapted do not.

• The survival of only some organisms of a population to go on to reproduce is known as differential survival and differential reproductive success.

• Organisms with alleles for the selected phenotypes will have a greater chance of surviving and reproducing. They will then pass on these alleles to the next generation.

• The frequency of the selected phenotype increases as does the frequency of the favourable allele/s in the gene pool.

• This continues from generation to generation as selection for these organisms continues. This is known as natural selection.

what are the different types of natural selection present?

• stabilising selection

• directional selection

• disruptive selection

describe and explain stabilising selection

and draw a brief graph

• environment is not changing (stable)

• Natural selection favours those organisms with alleles for characteristics towards the middle of the range e.g. fur length. These organisms survive to breed and pass on their alleles to the next generation.

• Organisms with alleles for the extremes of this range i.e. short or long fur length are less likely to survive and breed and so are less likely to pass on their alleles to the next generation.

• Therefore a similar range of fur length with the same mean fur is maintained generation after generation i.e. stabilising selection.

describe and explain directional selection

briefly draw the graphs

• occurs when the environment is changing

• change in the environment selects for those organisms with alleles for a phenotype towards the extreme of the range

• For example, a decrease in environmental temperature may select for individuals of a species of mammal that possesses longer fur than the mean fur length.

• Organisms with alleles for longer fur length are more likely to survive and breed passing on alleles for longer fur length to the next generation. The frequency of these alleles increases and the mean fur length of following generations increases until an optimum mean fur length is established.

• Therefore directional selection results in a change in the range of phenotypes until a new optimum mean fur length is established

describe and explain disruptive selection

briefly draw the graph

• opposite of stabilising selection

• involves phenotypes at the two extremes instead of the intermediate one

• least common but most important in bringing about evolutionary change

• the environment might become favoured to the two extreme fur length at different times of the year

• The population may then become divided into organisms with longer fur that are only active in the winter and organisms with shorter fur length that are only active in the summer. After many generations, distinct separate populations would form.

what is speciation?

evolution of new species from existing ones

what is a species?

group of individuals that are capable of breeding with one another to produce fertile offspring. members of one species are reproductively isolated from another species

how can evolution arise?

• result of a change in allele frequencies in a population

• reproductive isolation of two populations can result in the accumulation of difference in their gene pools

• new species arise when these genetic differences lead to inability of members of the population to interbreed and produce fertile offspring

• and so new species arise from existing ones

what are the two types of speciation

• allopatric speciation

• sympatric speciation

describe and explain allopatric speciation

• geographical isolation

• population with variation already present (mutations and meiosis) is split into separate groups

• due to a physical barrier separating population and prevents breeding between the two population so that they are then reproductively isolated

• large/ small scale

• populations are isolated with no gene flow between the gene pools

• different selection pressures will operate in the different environments resulting in selection for different phenotypes

• organisms possessing the selected phenotypes survive and breed, passing on their advantageous alleles to successive generations

• frequency of different selected phenotypes and so alleles in each isolated population increases

• allelic frequencies will also change as mutations may occur independently in each population

• over a long period of time the alleles in the separated gene pools become so different that different species arise and can no longer interbreed to produce fertile offspring

describe and explain sympatric speciation

• reproductive isolation in the same habitat

• could occur by random mutations within the population

• reproductive isolation may be the result of different flowering or mating seasons or courtship behaviours as a result of mutation

• sympatric speciation often involves disruptive selection

• Although living in the same habitat, the populations are reproductively isolated with no gene flow between the gene pools of the populations.

• Allelic frequencies will also change as mutations may occur independently in each population.

• Over a long period of time the allelic frequencies in the separated gene pools become so different that different species develop i.e. organisms from the different populations cannot interbreed to produce fertile offspring.

how can genetic drift lead to speciation?

• Genetic drift can lead to speciation.

• Genetic drift does not rely on natural selection but occurs by chance.

• By chance, the allele of a particular gene is passed on to the offspring more often than other alleles of the same gene.

• The frequency of this allele increases in successive generations.

• Genetic drift has a much greater effect in small populations where there is likely to be a smaller variety of alleles in the gene pool, i.e. genetic diversity is less.

• This can lead to speciation occurring more rapidly than in a large population where the frequency of the mutant allele is likely to be much less and thus has less of an effect.

what is an ecosystem?

a natural unit consisting of a community of living organisms their interactions with each other and with the non-living components of the habitat. Ecosystems can vary in size from very small to very large.

what are some biotic components of an ecosystem?

producers:

• plants convert light energy into chemical energy during photosynthesis to produce organic compounds

• act as basis to the food chain

consumers:

• break down large insoluble compounds into smaller soluble ones used to provide energy for growth

saprobionts:

• consist of bacteria and fungi that break down dead organisms and are essential for recycling of nutrients in the environment

what are abiotic components of an ecosystem?

• physical aspects

  • light

  • pH

  • temperature

  • water

  • oxygen

  • carbon dioxide

  • humidity

  • pollution etcwhawhat

what is a population?

group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed

what is the carrying capacity

• certain size of population of a species

what is an organism’s niche

an organism’s abiotic requirements in the habitat and its position in the food web and how it fits into the ecosystem

what are some sampling techniques present to estimate population size/ abundance of slow moving/ non- motile species

• Random Sampling

• systematic sampling along transects

  • belt transect

what techniques can be used to measure the population of mobile animals

• mark release and recapture method