6.2 Flashcards
What is Discontinuous Variation?
discontinuous variation is genetic variation where phenotype classes are distinct and discrete.
There are no or very few intermediates (two or more non overlapping categories)
What are examples of Discontinuous Variation?
- Sex
- Blood group
- Ear lobes
What controls Discontinuous variation?
Characteristics that exhibit discontinuous variation are usually determined by the alleles of a single gene locus.
They are Monogenic (determined by a single gene)
In either case:
- Different alleles at a single gene locus have large effects on the phenotype
- Different gene loci have quite different effects on the characteristics
What is Continuous Variation?
Continuous Variation is variation that produces phenotypic variation where the quantitative traits vary by very small amounts between one group and the next
where a population shows a range with a smooth gradation between many intermediates
What are examples of Continuous Variation?
- Eye colour
- Height
- Skin colour
- Hair colour
- Foot size
What controls Continuous variations?
characteristics that exhibit continuous variation are usually determined by many genes
They are polygenic (many genes involved in determining characteristics)
The alleles of each gene may contribute a small amount to the phenotype, thus the alleles have an additive effect on the phenotype.
The greater the number of gene loci determining a characteristic, the more continuous the variation (the greater the range)
what is quantitative genetics
quantitative genetics is the study of genetics of such inherited characteristics. Plant breeders use this knowledge as crop plants are polygenic
what is a genotype
genotype is the genetic makeup of an organism
what is a phenotype
phenotype is the visible characteristics of an organism
what two factors is a phenotype influenced by
influenced by its environment and its genotype
what is a mutation
a change to genetic material, this may involve structures to the DNA or changes to the gross structure of the chromosomes
what are mutagens
mutagens are certain physical and chemical agents that can increase the rate of mutation
What are the three main Mutagenic Agents that cause Gene Mutations?
- Physical Agents
- Chemical Agents
- Biological Agents
What Physical agents can cause Gene Mutations?
- X-rays
- Gamma Rays
- UV light
What Chemical agents can cause Gene Mutations?
-Benzopyrene (In tobacco smoke)
- Mustard Gas
- Nitrous Acid
- Aromatic Amines
- Free Oxygen Radicals
- Colchicine
What Biological Agents can cause Gene Mutations?
- Some Viruses
- Viral Nucleic acid incorporated into our genome
- Food Contaminants
What are the characteristics of Mutations that arise from Gamete Formation?
-harmful, advantageous or neutral
Mutations that occur during Gamete formation are also:
- Persistent: they can be transmitted through many generations without change
- Random : they are not directed by a need on the part of the organism in which they occur
What is an Allele?
An allele is a version of a gene.
What does Heterozygous mean?
Heterozygous means having different alleles at a particular gene locus on a pair of homologous chromosomes.
What does Homozygous mean?
Homozygous means having identical alleles at a particular gene locus on a pair of homologous chromosomes.
What does Monogenic mean?
Monogenic means determined by a single gene.
What does Dihybrid mean?
Dihybrid means involving two gene loci
who layed out the foundation of genetics
Gregor Mandel (experimenting with pea plants)
What is the expected phenotypic ratio of crossing heterozygous parents in a Monohybrid cross?
3:1 is a Monohybrid Phenotypic ratio.
how can we know the Genotype of Phenotypically similar individuals?
Some tall plants will be heterozygous (Tt) and some will be homozygous dominant (TT).
To know which one a plant is, we would cross each one with a homozygous recessive (tt) small plant.
If any of the offspring were small plants, this would indicate the unknown genotype to be Tt. If all the plants were tall, this would indicate the unknown genotype to be TT.
What did Mendel conclude about Dihybrid inheritance?
Mendel concluded that:
- The alleles of the two genes are inherited independently of each other, so each gamete has one allele for each gene locus
- During fertilisation, any one of an allele pair can combine with any one of another allele pair.
what was the genotype and phenotype when mendel bred a green wrinkled pea with a yellow round seeded pea
a yellow round seeded pea
YyRr
What is the expected phenotypic ratio of crossing heterozygous parents in a Dihybrid cross?
(Assume that colour and seed shape are two monogenic characteristics )they do not influence one another)
The expected phenotypic ratio of crossing heterozygous parents would be 9:3:3:1 in which the alleles of two different genes assort independently into gametes.
what would it suggest about the chromosomes if a 9.3.3.1 ratio was produced
when investigating dihybrid inheritance, Mendel without knowing about genes or the process of meiosis, chose two characteristics, and the genes for which are on different chromosomes.
If the two genes were on the same chromosome, the inheritance pattern would be different .
What is Codominance?
Codominance is where both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype.
What does Multiple Alleles refer to?
Multiple alleles refer to characteristics for which there are three or more alleles at a specific gene locus in the population’s gene pool.
However, individuals can only possess two alleles, one on each gene locus, in a pair of homologous chromosomes. as humans are diploid
what is an example of codominance
ABO Human Blood groups
sickle cell anaemia
How is ABO Human Blood groups an example of Codominance and Multiple alleles?
The four blood groups are determined by three alleles of a single gene on chromosome 9.
This gene encodes for an isoagglutinogen on the surface of erythrocytes.
A and B are both dominant to O, which is recessive.
A and B are codominant, so can be expressed together
any individual will only have two of the 3 alleles within their genotype
How is Sickle Cell Anemia an example of Codominance and Multiple alleles?
Sickle cell anemia is caused by a mutation in the gene that codes for the B-globin chain of Hemoglobin.
The mutant allele is given the symbol Hb^s and the normal is Hb^N
In heterozygous people, half the Hemoglobin in their red blood is normal and half is abnormal. However, heterozygous people do not suffer from Sickle cell Anemia.
If we consider the type of Hemoglobin as the phenotype, then the alleles are considered codominant, but if we take sickle cell anemia as the phenotype, there is one dominant allele and one recessive allele.
How is Cow skin colour an example of Codominance?
Coat color in shorthorn cattle is an example of Codominant inheritance. The one gene for coat colour has two alleles. C^r (red) and C^w (white)
Cattle that are homozygous for red or white exhibit that colour. (C^rC^r) OR (C^wC^w)
Cattle that are heterozygous have both red and white hairs. (C^rC^w)
How is Plant colour an example of Codominance?
Plant color is determined by two alleles.
Plants that are homozygous for red or white exhibit that colour.
Plants that are heterozygous have both red and white colour.
how is blood group AB shown in offspring genotypes
shown as I^BI^A
a
a
learning tip: when we refer to multiple alleles and say that a particular gene has more than 3 alleles, we mean within the gene pool of the population or species. E very individual organism has, within their genome, only two of these alleles at a particular gene locus
What is Sex-linkage?
Sex-linkage is where a gene is present on one of the sex chromosomes, but not the other.
What determines Human Sex?
In humans, sex is determined by one of the 23 pairs of chromosomes, called the sex chromosomes.
What are Autosomes?
Autosomes are the remaining 22 chromosomes that are not sex chromosomes. Each of the autosomal pairs is fully homologous.
They match for length and contain the same genes at the same loci
Are the X and Y chromosomes in males homologous?
The X and Y chromosomes are not fully homologous.
Only a small part of one matches a small part of the other so that these chromosomes can pair up before meiosis.
what are the sex chromosomes in females
XX
what are the sex chromosomes in males
XY
What is the responsibility for the genes located on an X chromosome?
The X chromosome contains over 1000 genes that are involved in determining characteristics or metabolic functions.
They are not concerned with sex determination, and most of these have no partner alleles on the Y chromosome.
If a female has one abnormal allele on one of her X chromosomes, she will probably have a functioning allele of the same gene on her other X chromosome.
Why are males more susceptible to genetic diseases?
If a male inherits, from his mother, an X chromosome with a genetically abnormal allele for a particular gene, he will suffer from a genetic disease, as he will not have a functioning allele for that gene.
Males are functionally haploid, or hemizygous, for X-linked genes. They cannot be heterozygous or Homozygous for X-linked genes.
What are examples of sex linked characteristics
Hemophillia A and Colour blindness
What is Hemophillia A and how is it Sex-linked?
A person with Haemophilia A is unable to clot blood fast enough.
One of the genes on the non-homologous region of the X chromosome codes for a blood-clotting protein called factor 8. A mutated form of the allele codes for nonfunctioning factor 8.
A female with one abnormal allele and one functioning allele produces enough factor 8 to enable her blood to clot normally when required. However, this female is a carrier for the disease.
If such a female passes the X chromosome containing the faulty allele to her son, he will have no functioning allele for factor 8 on his Y chromosome.
As a result, he will suffer from haemophilia A.
what would the genotype look like for a male with haemophillia
X^h Y
what would a symptomless female look like who is recessive for hemophilia
X^H X^h
What is Colorblindness and how is it Sex-linked?
One of the genes involved in coding for a protein involved in colour vision is on the X chromosome, but not on the Y chromosome.
A mutated allele of this gene may result in colour blindness - an inability to distinguish between red and green.
A female with one abnormal allele and one functioning allele will not suffer from colour blindness, but a male with an abnormal allele on his X chromosome will not have a functioning allele on his Y chromosome and will therefore suffer from red-green colour blindness.
How is coat colour Sex-linked in cats?
One of the genes for coat colour in cats is sex-linked. It is on the non-homologous region of the X-chromosome.
The alleles O (organge) and B(black) are codiminant meaning cats with XCO XCB will appear a mixture of orange and black.
Male cats can only be one colour as they only have one X chromosome
Do females have twice as many X-linked genes?
In every female cell nucleus, one X chromosome is inactivated.
Determination of which member of the pair of X chromosomes becomes inactivated is random and happens during early embryonic development
A
A
A
define codominant
where both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype
What is Autosomal inheritance?
Inheritance of genes located on non-sex chromosomes
What is Autosomal linkage?
Autosomal linkage is when 2 Gene loci present on the same Autosome are often inherited together.
Why are Autosomal linked genes inherited together?
Autosomal linked genes are usually located near one another on an Autosome.
This means that when independent assortment occurs in prophase one, they are usually assorted together.
Linked genes are therefore not free to undergo independent assortment.
are linked genes free to undergo independent assortment?
no, the genes are not free to undergo independent assortment
how many pairs of autosomes do humans have
22
What are the characteristics of the phenotype of Autosomal linked traits?
- As genes are inherited together, there are fewer gamete combinations possible reducing variation in offspring
- A large proportion of offspring are usually phenotypically similar to their parents
What is the expected phenotypic ratio of crossing heterozygous parents which are Autosomally linked?
3:1
(if they were not linked the ratio would be 9:3:3:1)
why are linked genes always inherited as one unit
as linked genes are not affected by crossing over of non-sister chromatids
If genes are Autosomally linked, but four phenotypes are seen in offspring, what would have happened?
This only possible because crossing over must have occurred during Meiosis to make Recombinant gametes (new combinations of gametes).
You will still get a higher proportion of the non-crossed original linked gene
What are Recombinant Gametes?
Gametes with new combinations of alleles
What affects the likelihood of recombinant gametes forming?
The further apart the two gene loci are on a chromosome, the greater chance of recombinant gametes forming.
What is Epistasis?
Epistasis is the interaction of non-linked gene loci where one masks the expression of the other.
How does Epistasis affect Genetic Variation?
Epistasis reduces the number of phenotypes produced in the F2 generation of dihybrid crosses and therefore reduces genetic variation
how do the genes work in epistasis
the genes may work together antagonistically (against each other) or in a complimentary fashion
What is Recessive Epistasis?
The homozygous presence of a recessive allele at the first locus prevents the expression of another allele at a second locus.
The alleles at the first locus are epistatic to those at the second locus, which are hypostatic to those at the first locus.
define hypostatic
the gene whose expression is masked by an epistatic gene
give an example of recessive epistasis in flowers
the homozygous aa in plants is epistatic to both alleles of the B/b gene. Neither the allele B for purple nor the allele b for pink when in a homozygous state, can be expressed if no dominant A allele is present.
What is Dominant Epistasis?
The homozygous or heterozygous presence of a dominant allele at the first locus prevents the expression of another allele at a second locus.
The alleles at the first locus are epistatic to those at the second locus, which are hypostatic to those at the first locus.
give an example of dominant epistasis in chickens
individuals carrying at least one dominant allele, have white feathers even if they also have one dominant allele for coloured feathers. Birds also homozygous will appear white.
with science developing, what is epistasis more often explained as
epistasis is more often explained in terms of the genes working to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway.
go over p194-196
what ratio may suggest recessive epistasis
9:3:4
what ratios may suggest dominant epistasis
12:3:1
13:1
what ratios may suggest that epistasis is occurring by complimentary gene action
9:7
9:3:4
9:3:3:1
What is the Chi-Squared test?
Squared test?
The Chi-squared test is a statistical test designed to find out if the difference between observed and expected data is significant or due to chance
when is the Chi-squared taken
if we obtain results that are not quite as expected, we need to know whether the difference is just due to chance or whether the difference between what we observe and what we expect is significant. If it is significant, it may be that the inheritance pattern is different to what we thought and we need to re-think how to explain our observations
(by chance we got the same results…)
when is the chi squared test suitable for use
- Data is not continuous
- Sample size is large
- Raw data
- No Zeros in the raw data
what does the null hypothesis state
there is no statistically significant difference between the observed and expected data. Any difference is due to chance.
what is the number of degrees of freedom?
degrees of freedom= number of categories -1
How do we determine if the test is significant?
Find the value in the table associated with p=0.05 and the correct number of degrees of freedom.
If the value calculated is lower than the value in the table, we Accept the null hypothesis (any difference is due to chance and not significant)
what becomes more complicated as the number of gene loci increases
genetic analysis of inheritance becomes more complicated
does the environment have a greater effect on monogenic or polygenic traits and give an example explaining why
the environment has a greater effect on the expression of polygenic characteristics
for example, each person has a genetic potential for height and intelligence, but without proper nutrition and also, for intelligence, mental stimulation, these potentials will not be reached
what has the potential to introduce new alleles into populations
mutations
migration
what are allele frequencies
proportion of a particular allele in a gene pool
what is natural selection
where better adapted organisms are more likely to survive and pass on advantageous alleles, over time allele frequencies will change . natural selection can also maintain constancy of a species, as well as leading to a new species
What are the three main types of selection?
- Stabilising selection
- Directional selection
- Disruptive selection
what is stabilising selection
Stabilising Selection is natural selection leading to consistency within a population.
Intermediate phenotypes are favoured and extreme phenotypes selected against.
Alleles for extreme phenotypes may be removed from the population.
Stabilising selection reduces genetic variation within a population.
when does stabilising selection occur
normally occurs when the organisms environment remains unchanged
whats an example of stabilising selection
babies birth mass close to 3.5kg more likely to survive
What is Directional Selection?
Directional Selection is a type of natural selection that occurs when an environmental change favours a new phenotype and so results in a change in the population mean.
Over several generations, there is a gradual shift in the optimum value for the trait.
Directional selection is used by plant and animal breeders to produce desirable traits
in nature, in a population, periods of directional selection may alternate with periods of stabilising selection
when does directional selection occur
when environmental conditions change
What is Disruptive Selection?
Disruptive Selection is natural selection which favours both the extreme phenotypes, and the intermediate phenotype is selected against.
When does disruptive selection occur?
When the environment favours more than one phenotype
What is Genetic Drift?
Genetic drift is the loss of genetic diversity and loss of alleles as a result of a random/ catastrophic event.
It is not as a result of a selection pressure. Neither allele provides any advantage or disadvantage.
what are two ways in which genetic drift can arise
Genetic drift can arise after a Genetic Bottleneck or as the result of the Founder Effect.
What events can cause Genetic Drift?
- Earthquake
- Flood
- Outbreak of disease
- Food shortage
- Habitat destruction
- Overhunting
- Genocide
What is Genetic Bottleneck?
When a population size shrinks and then increases again, it is said to have gone through a genetic bottle neck.
After this event, the genetic diversity within the population will be reduced.
There will be a reduced gene pool, but this may contain advantageous or disadvantageous alleles.
-species that have been selectively bred for certain traits have also been through a genetic bottleneck
a genetic bottleneck causes population shrinkage, what could this lead to
population shrinks to such a small size that fertility is affected, leading to the species becoming endangered then extinct
What is the Founder Effect?
The Founder Effect occurs when a small sample of an original population establishes in a new area and becomes reproductively isolated.
Its gene pool will not be as diverse as that of the parent population. the new population is likely to exhibit a loss of genetic variation
The Founder Effect is a special case of Genetic Drift
Suggest how shrinkage of a population size may affect the reproductive rate of that population?
May be too few females/imbalance of the sexes so not all animals can mate - reduced number of offspring born.
May be hard for individual animals to find mates.
If very few males, then many offspring will be genetically related and if they interbred this could increase the frequency of harmful alleles.
What is a Population?
A population is a group of individuals that belong to the same species and live in the same area.
what does population genetics aim to study
population genetics attempts to study the changes in allele frequencies within a population over time.
population genetics studies the variation in alleles and genotypes within the gene pool and how their frequencies change over time
What factors affect Allele Frequencies in populations and hence the genetic diversity in a gene pool?
- Population Size
- Mutation Rate
- Migration
- Natural Selection (stabilising, disruptive, directional)
- Changes to the Environment- (can lead to a bottleneck)
- Isolation of a population (founder effect)
- Non random mating
- Genetic Drift
- Gene flow
what is speciation
when two genetic populations have become so genetically different that they can no longer interbreed to produce fertile offspring
what is the hardy- weinberg principle
its a fundamental concept in population genetics. it describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a breeding population. It can be used to determine the frequencies of those carrying a recessive allele ( heterozygotes) for a genetic disorder with a recessive inheritance pattern, we know the incidence of infected babies born each year in that population
What does the Hardy-Weinberg principle assume?
The principle assumes that:
- The population is large
- Random Mating
- No Selective advantage for any genotype thus no selection
- No mutation, migration or genetic drift
What is the Hardy-Weinberg Equation?
Dominant allele is p
Recessive allele is q
p + q = 1
p^2 + 2pq + q^2 = 1
Describe and Explain how an animal may evolve from its ancestor?
Natural Selection:
- Mutations
- Selection pressure
- Greater survival probability
- More likely to reproduce
- Beneficial alleles passed to the next generation
- Allele frequency for the relevant genes would have increased with each generation
Roles of regulatory genes:
- Regulatory genes control expression of other genes
- Genes switched on or off during development (Transcription factors)
- Epistasis
What is Speciation?
Speciation is the splitting of a genetically similar population into two or more populations that undergo genetic differentiation and eventually reproductive isolation, leading to the evolution of two or more new species.
how does speciation occur
the species must be split into two isolated populations. If this happens any mutations that occur in one population are not transmitted by interbreeding to the other population
What are sub-species?
Sub species are distinct groups of the same species that have different traits, but are still able to interbreed
formed during the evolutionary process of speciation
What are the two main types of isolating mechanism
Geographical
Reproductive
What is Allopatric speciation?
Allopatric speciation is the formation of two different species from one original species due to geographical isolation.
the genetic changes may be due to mutation, selection and genetic drift
What can act as a Geographical isolator?
- Lakes
- Rivers
- Oceans
- Mountains
How does Allopatric Speciation occur?
Geographic barriers can split populations
Isolated populations are subject to different selection pressures
Undergo independent changes to the allele frequencies and or chromosome arrangements within their gene pools
what type of speciation does geographic isolation lead to
leads to allopatric speciation
what leads to reproductive isolation
biological and behavioural changes within a species may lead to reproductive isolation of one population from another
what type of speciation does reproductive isolation lead to
sympatric speciation
What is Sympatric speciation?
Sympatric speciation is the formation of two different species from one original species due to reproductive isolation, while populations inhabit the same geographical location
What can act as a Reproductive isolator?
Biological and behavioural changes:
- Time of activity
- Courtship changes
- Changes in genitalia or plant flower structure
Genetic Changes:
- Mutations/ Changes in Chromosome numbers
in terms of reproductive isolation, what can a change in chromosome number cause:
-prevent gamete fusion
-make gametes less viable so they fail to develop
-lead to infertile hybrid offspring with an odd number of chromosomes, so that chromosome pairing during meiosis cannot occur
mating between members of the reproductively isolated populations may also be prevented by mutations leading to changes in:
-courtship behaviour e.g. time of year for mating or courtship rituals that precede mating
-animal genitalia or plant flower structure
The Chromosome number of a Horse is 64, the Chromosome number of a Donkey is 62, suggest why mules are infertile?
Mules will have 63 chromosomes (32 from mother and 31 from father) so there is an odd number of chromosomes; they cannot pair up; meiosis cannot take place; gametes cannot be made.
How does Human activity speed up evolution?
For example, habitat destruction, such as clearing forests for wood/space; climate change; pollution; agriculture; use of pesticides; antibiotics; changes may cause wildlife to migrate; may introduce a novel selection pressure.
What is Artificial Selection?
Artificial selection or Selective Breeding of organisms involves humans choosing the desired phenotypes and interbreeding those phenotypes individually.
This selects the genotypes that contribute to the gene pool of the next generation of these organisms
humans are the agents of selecction
who are the agents of selection in natural selection
the environment is the agent of selection
what are desirable characteristics in plants
-increased yield
-increased pest and disease resistance
what are desirable characteristics in animals
-docility
-placidity
-the ability to be trained
-accept human as pack leader
-tolerate being penned with other animals
what are some examples of organisms that humans have bred for artificial selection
-cats for pest control and companionship
-pigs for meat
-horses for transport
-
outline what selective breeding programmes are
- new breeds can be produced by selective breeding programmes
-breeders may grow many plants of a particular type under the conditions they wish these plants to withstand - such as low temperature. They will then select those individuals that grow best under these conditions and cross pollinate them, collect and sow the seeds and repeat this process over generations.
A selective programme takes about 20 years
What is Inbreeding Depression?
The increased chances of an individual inheriting two copies of a recessive harmful allele.
when related individuals are crossed
Lack of vigour, loss of fertility and reduction in population size, due to organisms being inbred and becoming homozygous at many gene loci.
What is Hybrid Vigour?
Hybrid Vigour is when breeders outcross individuals belonging to two different varieties to obtain individuals that are heterozygous at many gene loci.
what are the advantages of selective breeding in crops
- it develops bigger and better varieties of crop plants and animal breeds
what are the disadvantages of selective breeding in crops
-reduces genetic diversity therefore if a pathogen was introduced and one is affected then it is likely that all of them will be affected
What are Gene Banks?
Gene banks store genomes, maintained within their organism.
Gene banks are sources of alleles to introduce back into the artificially bred strains and increase hybrid vigour
What are examples of Gene Banks?
- Rare breed farms
- Wild populations of organisms
- Crops in cultivation
- Botanic gardens and zoos
- Seed bands
- Sperm bands
- Cells in tissue culture
- Frozen embryos.
Why is it important to maintain viable wild populations of crop plant species?
genetic variation
genetic resource / gene bank
source of useful alleles
can be cross bred with crop varieties
allows introduction of different traits
unknown future requirements
potentially useful in changing climate
prevention of inbreeding depression
promotion of hybrid vigour
prevent dwindling gene pool
source of replacement if cultivated population is in danger
What are the Ethical issues with Artificial Selection?
- Animal welfare issues
-animals are produced to have more lean meat and less fat so may not be able to cope with cold conditions
- Problems faced by dogs (cancer, heart disease, difficulty breathing) - high susceptibility to disease by inbreeding
-some selected coat colours do not camouflage in the wild
- Animals no longer able to survive in the wild because traits desired by humans may put them at a selective disadvantage.
