Week 14 Flashcards
(92 cards)
1
Q
How can evidence of the seeds phenotypes be identified in this practical?
A
Evidence of their phenotypes can be gained from comparing their root hairs.
2
Q
Why is it important to keep the seeds on plates?
A
Stop contamination
3
Q
What is crossing over?
A
During meiosis, chromosomes can overlap. If they sections which overlap swap sections of DNA they can create new combinations of alleles
4
Q
What is recombination?
A
This is the result of crossing over and swapping DNA between pairs of chromosomes. This creates new combinations of alleles and can result in offspring inheriting different combinations of alleles than those observed in their parents.
5
Q
What are true or pure breeding line?
A
These produce generations which look identical because all offspring have the same genotype as the parents. Both parents are homozygous with both alleles being the same e.g. AA BB x AA BB or aa BB x aa BB.
6
Q
What are marker line chromosomes?
A
Created from the wildtype chromosomes by intoducing mutations. These have known mutations on each chromosome and can then be used in crosses to determine the location of particular genes. This is only possible because some chromosomes wil go through crossing over and recombination, resulting in different allele combinations.
7
Q
What is centiMorgans (cM)?
A
These are the units for measuring genetic linkage by helping to map genes on chromosomes defined by position relative to other gene loci.
8
Q
What is backcrossing?
A
Involves crossing the offspring with one of the parents.
9
Q
What happens when two gene loci are crossed?
A
If two gene loci are used in a cross there are 16 possible combinations from the 4 alleles from each parent.
10
Q
What does Mendel’s second law state?
A
The segregation of alleles of one pair is independent of segregation of another allele pair.
11
Q
Which of these statements about recombination of chromosomes is FALSE?
It can cause mutations which create new alleles.
It can create new combinations of alleles.
Involves movement of genetic material between chromosomes.
A
It can cause mutations which create new alleles.
12
Q
In an attempt to map some genes, you first cross 2 pure-breeding lines:
a homozygous recessive mutant for gene a ;
and a chromosome marker line which is homozygous recessive for two mutations b and c .
a) Which of the following shows the correct genotype of the recessive mutant for gene a ?
A
aa BB CC
Recessive alleles are usually represented as lower case letters and remember that homozygous pairs have two of the same allele.
13
Q
which of the following shows the genotype for the chromosome marker line?
A
bb CC
Chromosome marker lines can be used to determine the location of genes on chromosomes.
14
Q
which of these shows the genotype of the first generation (F1) cross between these two lines?
A
Aa Bb Cc
The F1 generation is the first generation resulting from a genetic cross.
In this cross, all the offspring would be heterozygous for all three genes.
15
Q
Genome size and ______
number are typically not correlated because potentially large amounts of DNA may be noncoding.
A
gene
16
Q
Organisms that exhibit polyploidy have how many sets of chromosomes?
A
Three or more
17
Q
Synthetic polyploids are created in the lab by ______ closely related plant species and then chemically inducing chromosome ______
A
Blank 1: crossing, hybridizing, or crossbreeding
Blank 2: duplication or doubling
18
Q
Sometimes two different species can hybridize. If the hybrid undergoes a genome duplication, the result is an organism called
A
an allopolyploid
19
Q
Modern tobacco arose from the ______of two Nicotiana species and subsequent genome _____.
A
Blank 1: hybridization or cross
Blank 2: duplication
20
Q
Each species has a characteristic genome size and gene number, however, the two do not appear to be correlated, as some species have very large genomes, but not a correspondingly large number of genes. Why are genome size and gene number not correlated?
A
Not all DNA is coding.
21
Q
The presence of three or more chromosome sets is called
A
polyploidy
22
Q
Ancient polyploids are known as _____
, while polyploid organisms generated by scientists in the lab are called ______
polyploids.
A
Blank 1: paleopolyploids
Blank 2: synthetic
23
Q
Which of the following processes are involved in making synthetic polyploids?
A
Crossing closely related plant species in the lab
Chemically inducting chromosome duplication in hybrids
24
Q
In _______, the genome of one species is duplicated because of a meiotic error leading to four copies of each chromosome, while ______ results from a hybridization of two species followed by genome duplication.
A
Autopolyploids
Allopolyploids
25
The average number of gene pairs of a hypothetical species is shown here as the Y axis, while time is shown on the X axis. About 75 million years are represented. The arrows pointing to the blue line indicate ______.
times of gene loss after polyploidy
26
How did modern tobacco arise?
From a cross between two Nicotiana species with subsequent genome duplication
27
In order to be fertile, a polyploid must have a(n) ______ number of chromosomes.
even
28
What is the difference between paleo and synthetic polyploids?
Paleopolyploids are ancient, while synthetic polyploids are generated by scientists in the lab.
29
True or false: It appears that polyploidy has occurred numerous times during the evolution of plants.
True
30
In many cases, polyploidy is followed by loss of duplicated _____ or even whole ______.
genes
| Chromosomes
31
The following diagram shows the average number of gene pairs (Y axis) over time (X axis, about 75 million years are represented) in a hypothetical paleopolyploid. The red dots on the diagram indicated by arrows, represent ______.
polyploidy events
32
As a result of polyploidization, the _____
| of genes can be changed.
expression
33
What is the predicted response of transposons to genome shock caused by polyploidization?
They jump to a new position in the genome.
34
Fertile polyploids must have ______.
an even number of chromosomes
35
True or false: Polyploidy accounts for all genome size variation.
False- Other factors, such as ncDNA, are incorporated also.
36
Evidence shows that polyploidy has occurred
relatively often during the evolution of plants.
37
Formation of a polyploid from two different species is usually followed by a rapid _____of genes.
loss
38
Increased DNA ______
| may cause short-term silencing of many genes after polyploidzation.
methylation
39
Barbara McClintock hypothesized that transposons could respond to genome _______
by jumping into a new position in the genome.
Blank 1: shock or duplication
40
What proportion of genome size variation can be accounted for by polyploidy?
Some genome size variation
41
What is one of the reasons for the much higher genome size of humans when compared to pufferfish?
Humans have many more introns which are also substantially larger.
42
One of the reasons for the large human genome is the presence of many relatively large ______ in our gene sequences.
introns
43
Natural selection-
Common descent-
Natural selection (microevolution, when referring to evolutionary processes taking place at population level, under the species level).
Common descent (macroevolution, when those processes are taking place above the species level, like when we are comparing different species or different families)
44
What is natural selection?
For a trait to evolve in a population, we need a series of conditions. First we need a population, a population is a group of organisms from the same species that are in a specific geographical area. Same species, means that they are able to reproduce, and have fertile offspring. So if they cannot reproduce then they cannot be changed overtime. Then when we have this population we need that for this given trait we need variation in the population. We need different members of the population to display different phenotypes. Secondly, you need this trait to have a genetic basis. So now you have all those conditions, you have a trait that is found in a population, this trait has a genetic basis and shows variation. What going to happen is that in nature, organisms cannot have as much offspring as they want. There is a fight for resources. Predators need to hunt, plants need to fight for the sun etc. so basically there is a constraint in the ecology of organisms that prevent them having as many babies as they would like to have. Some of these variations are going to affect how individuals interact with this ecology, so those variations are going to impact the reproductive success of some of the members of the population.
So those individuals that have advantageous traits, they will have more offspring and as those traits have a genetic basis, the offspring will inherit those traits and then in the next generation this advantageous trait will be more predominant in the population. There will be more individuals in the population with this favourable trait. Then if you keep repeating this process over time or generations we will have that, those traits will become more and more abundant in the population. In some cases replacing all the other alternatives.
45
What were Darwins Observations?
* Members of a population show variation in their traits
* Traits are inherited by offspring from their parents
* Populations produce more offspring than the environment can support
* Those inherited variations affect survival and reproduction
* Individuals whose inherited traits increase their survival and reproduction are likely to leave more offspring than other individuals
* Over generations, this will lead to the accumulation of favourable characteristics in the population
46
What are Darwins giraffes?
Prerequisites: The neck length is variable and inheritable trait.
1) Giraffes with longer necks to get the best food. Thus, they are more likely to reproduce and/or have more offspring.
2) The children of long-necked giraffes inherit the long neck of their parents.
3) Over the generations, short-necked giraffes disappear in favour of the long neck ones.
47
What are mutations?
One of the preconditions for evolution to happen in a given trait is the need of variation. You need to have people with different colour hair or giraffes with different neck lengths. Otherwise there is nothing to select.
Mutation is the change in the DNA is one of the main mechanisms to introduce new alleles into a population. Natural selection is working on phenotypes, not genotypes. We are selecting people with that hair or giraffes with long necks. We’re not selecting it directly, the genotypes producing those phenotypes. So when we talk about variation, it may mean that some population members have alleles that are absent in others. So for example, the allele for longer necks in giraffes can show variation in a population, some members have the allele for longer necks, and others for short, shorter necks but sexual recombination is another mechanism that is also able to introduce variation in the population. Because of sexual recombination you are able to put together alleles, combinations of traits that were not found in the same organism, the same in the previous generation. Therefore sexual recombination can also introduce phenotypic variation in the population.
48
What are some additional traits observed by Darwin?
* Members of a population show variation in their traits
* Natural selection selects phenotypes, no genotypes
* Variation may mean that some population members have alleles that are absent in others (e.g. allele for longer necks in giraffes)
* Variation may also mean to have a combination of different traits not present in other members (e.g. longer necks and better teeth to eat leaves)
Mutation and sexual recombination produce the variation that makes evolution possible
Mutations, changes in the nucleotide sequence of DNA, cause new genes and alleles to arise
Recombination produces new combinations of genes in the offspring.
49
What are point mutations?
* Change in one base in a gene
* Can have a significant impact on phenotype
You have small and large scale mutations, large ones that involve chromosomes or the whole genome. Point mutations for example, have different impacts on the AA sequence depending on the nucleotide change, so this is the original version of the codon in the DNA. You can have a silent mutation, where you change one nucleotide but because the genetic code is redundant, when it generates, you are not going to have a change of the amino acid here, so natural selection would do nothing against this mutation because there is no difference in phenotype. We have nonsense mutations that may introduce an early stop codon, that means we have a shorter protein and therefore this protein is most likely not going to be able to perform its function normally and what's going to happen is that natural selection is going to go against this mutation, it's going to remove it from populations because individuals with this non-functional protein won't be able to reproduce at the same level as others in the population. We have the missense which means there's a change of AA, some of those AA changes will make the protein more effective or work better, and natural selection will select those. Some other AA changes will make that protein work worse and therefore, natural selection will select against those mutations.
50
What are mutation rates like?
•Tend to be low in animals and plants compared to microbes
•Are more rapid in microorganisms (shorter generation time) because mutation happen when you are duplicating your DNA for the next generation, and therefore if you are having more generations in the same period of time you’re going to accumulate more mutations and this is what happens. Microorganisms are short lived compared to animals and plants.
Some estimations say that in humans, a baby has ~140 new mutations when compared to the parents, sperm contributes with ~130 mutations and ovule with ~10
51
•Mutation generates
variability
52
•3 mechanisms act on this variability:
* Natural selection
* Genetic drift
* Gene flow (migration)
53
•Only natural selection causes _____ _____- therefore making the organisms ____ adaptive in the ecosystem.
adaptive evolution
| more
54
What does natural selection cause and how?
Natural selection is the only mechanism that consistently causes adaptive evolution and impacts the phenotypes in a population.
* Evolution by natural selection involves both chance (variations) and “filtering” of this variation
* Only natural selection consistently increases the frequencies of alleles that provide reproductive benefit or survival advantage
When we talk about evolution by NS, we have a first step which is chance, its randomness. We have those DNA mutations that happen randomly in the genome. We have those sexual recombination events that are also random, and now this variation generated by random events is going to be filtered by NS. And NS, what its going to do is to select those variations that increase the reproductive success of organisms. So those organism that have variance that allow them to have more offspring are the ones that are going to be selected by NS.
55
What does natural selection increase?
* Natural selection increases the frequencies of alleles that enhance survival and reproduction -> Adaptation
* Adaptive evolution occurs as the match between a species and its environment increases
NS is going to increase the frequencies of those alleles, how abundant those alleles are in the population. The frequency of alleles that are beneficial for survival and reproduction, and this is what we call adaption.
56
Assumptions of hardy-Weinberg equation-
```
No selection
No mutation
Large population
Random mating
No migration
```
57
The five conditions for non-evolving populations:
* Extremely large population size (no genetic drift- acts when pop is small)
* No gene flow (no migration- no new alleles from external populations)
* No mutations- no new alleles
* Random mating (no sexual selection- when some mates are favoured over others.)
* No natural selection (non differential reproduction)
58
What is relative fitness?
* Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to other individuals
* For example, organism A has 10 descendants and organism B has 20. The fitness of organism B is double of A
* Selection favours certain genotypes by acting on the phenotypes of individuals
59
What does fitness measure?
Fitness is a measure of reproduction, the contribution that an individual is making to the gene pool of the next generation. For example, you have organism A that has 10 descendants and organism B that has 20. So the fitness of B is doubled. Phenotypes are going to be selected that increase the fitness of the individuals.
60
For a given _______ you have a _______ distribution, a _______ distribution. A big proportion of the population are going to be _______ range of the phenotype range. While you have less and less individuals when you go towards _____ of the distribution.
```
phenotype
normal
bell
middle
extremes
```
61
* Directional selection favours
* Disruptive selection favours
* Stabilising selection favours
* Directional selection favours individuals at one end of the phenotypic range
* Disruptive selection favours individuals at both extremes of the phenotypic range
* Stabilising selection favours intermediate variants and acts against extreme phenotypes
62
What is an example of directional, disruptive and stabilising selection?
E.g. fur colour of mice
You have these mice living in the countryside and then depending on the environment, on the ecology, we’re going to have that some mice are going to thrive and others are not going to have a very good time. You have the original population, with this phenotypic range displayed with the black graph. In the case of directional selection, we have that lets say that is the middle of the summer, so the white mice stand out very much at night, and they when you have predators like owls, they just go and eat the poor white mice, while the darker mice are going very well because they are less conspicuous by night. So they would survive better. So that means overtime darker mice will have more babies than fair mice, and then the next generation you will have a higher proportion of mice with darker fur. Distribution has moved. When its winter the white mice are more likely to survive, predators pick the darker mice. Disruptive selection, in which basically the extreme phenotypes are favoured against the others. Stabilising selection- situation in which the extreme phenotypes are being selected against and its the phenotype in the middle of the range that is being selected.
63
* Because the environment can ______, adaptive evolution is a ______ process
* Genetic drift and gene flow do ____ consistently lead to adaptation as they can ____ or _____ the match between organisms and their environment.
change
continuous
NOT
increase or decrease
64
What are some examples of selection?
Antibiotic resistance in bacteria-
This is pure natural selection at work. So you have a population of bacteria, the yellow ones are sensitive to antibiotics, they die if you treat them with antibiotics, while those two red ones are resistant. They have a gene that allows them to be antibiotic resistant. Means you have variation in the population for this trait and it has a genetic basis. Now you do a treatment with antibiotics, which is going to kill the yellow bacteria. Not all of them but they will have less offspring in the next generation, but there will be less yellow bacteria in the next generation. Because the bacteria that are resistant will divide more and better, so we have more and more bacteria over time.
65
Virus and vaccines--
•Viruses mutate very quickly -> they can generate huge diversity in short periods of time
-> some variants may escape immune system
•For example, the flu vaccine is recommended every year if you belong to a risk group.
Pure NS
66
What is sickle-cell anemia?
Impacts the haemoglobin-
Left hand side is normal haemoglobin- you have the beta globin gene. It is one of the two genes involved in making haemoglobin, the protein that is transporting oxygen in your body. You also have the RNA sequence, and the AA sequence in a healthy individual. You have that the two beta globins can pair with Alpha Globins and then you have a regular healthy haemoglobin that is found within a red blood cell. However, in ppl with SCA they have at least one allele that is mutated and there's a mutation in this nucleotide that is going to change this glutamine to valine and what is going to happen is that those hemoglobins are not going to polymerise correctly and actually have to form other versions of haemoglobin.
PPL with sickle cells are going to have different physical chemical properties to normal cells and what's going to happen is that they will clog some capillaries causing anaemia. So they are going to have a lack of blood in some parts of their body.
Individuals that have two alleles for a healthy haemoglobin, they are going to have fantastic red blood cells, but they are going to die because of malaria, so they will reproduce less. You will have individuals that have two copies for the sickle cell version of haemoglobin (aa). And those are the ones who are going to be resistant to malaria, but they are going to suffer sickle cell disease and they are going to reproduce less, however, the heterozygotes are the ones having advantage in this scenario, they have some red blood cells that are normal and others that look like sickle disease, this middle ground is very advantageous as they are not sick enough of sickle cell disease to die out of it or reproduce less. But we have enough sickle cells to do very well if they have a malaria attack. Homozygous- disadvantage, heterozygous- advantageous.
67
What is artificial selection?
Is induced by humans and we are acting as selective pressure. In those cases we have applied artificial selection to a number of animals and plants for our benefit.
Cows producing milk
Wolves- dogs
68
What is genetic drift?
* Genetic drift is the change in the frequency of alleles due to random sampling of organisms.
* Random means unpredictability
* Random means independent of the phenotype, thus non-adaptive (its not selecting genes/alleles based on how beneficial they are from the production its just a flip of a coin).
* Genetic drift tends to reduce genetic variation through losses of allele
69
What are some examples of genetic drift?
Two examples of genetic drift:
```
Bottleneck effect (time)
Founder effect (space)
```
Original population - colours show the genotype or phenotype. There is a bottleneck event which strikes the population and now in the next generation, only a few individuals will pass. So we only have a few variants, from this big large original population, only a very few variants are going to pass to the next generation. As this filtering is totally random it means that the final product is not going to be more adapted and we have reduced genetic variation.
* The bottleneck effect: A reduction in population size due to a non-selective change in the environment (e.g. a natural disaster kills most of population)
* The resulting gene pool may no longer be representative of the original population’s gene pool (e.g. some giraffes survive, but they all have short neck alleles)
* If the population remains small, it may be further affected by genetic drift.
70
Toba catastrophe hypothesis-
•Supervolcanic eruption 70k years ago in Indonesia
•10 years volcanic winter , 1,000-year-long cooling episode.
Some estimates show human species reduced to 2,000 individuals.
71
The Founder Effect-
* The founder effect occurs when a few individuals become isolated from a larger population (e.g. island formation, migration, etc.)
* Allele frequencies in the small founder population can be different from those in the larger parent population
Happens in space. The smaller population is going to be different from the parental population. Random event in which genetic diversity is reduced.
E.G. zoo populations
72
For genetic drift to work-
1. Significant in small populations
2. It can cause allele frequencies to change at random- causes selective pressure.
3. Individuals selection not based on phenotypes (no adaptation)
4. Genetic drift can lead to a loss of genetic variation within populations- same with NS if you keep having directional selection towards one allele, the competing allele might disappear in the population but in the case of genetic drift this can happen very fat and in a non adaptive way so it can be very dangerous if the alleles have become fixed in the population.
5. Genetic drift can cause harmful alleles to become fixed
73
Gene flow-
* Gene flow consists of the movement of alleles among populations (e.g. migration)
* Movement of fertile individuals or gametes from one population to another (for example, pollen)
* Tends to homogenize (reduce variation- so you have two populations that are different and as time goes by and gene flow among them, they become more similar, more similar at the level of gene frequencies or allele frequencies) the genetic make up of different populations
•Gene flow can decrease the adaptation of a population
•The great tit (Parus major) on the Dutch island of Vlieland
–Birds in the island highly adapted to island environment
–Birds migrating from mainland introduces alleles that decrease adaptation on the island
•Gene flow can increase the adaptation of a population
•For example: the spread of alleles for resistance
–Insecticides have been used to target mosquitoes that carry West Nile virus and malaria, but some other populations have insecticide resistance
–Similarly, bacteria and antibiotic resistance
74
Agriculture: pesticides-
On the left you have an agricultural field with water refuge. Lets say that you have some beetles that are feeding on your crops, the yellow ones are not resistant to pesticides they will die and the ones in blue are resistant and they don't care about the pesticides. So now you have this field and the one on the left is treated with pesticides and then in the next generation you will be left with resistant beetles and then the following generation you will only have pesticide resistant beetles. This is very similar to antibiotic resistance. This is bad for the farmer- you can now have a part of the field which you don't use pesticides, you only treat this part with pesticides. In the next generation you will have a few resistant beetles, but then they will mix. They will breed with the non-resistant ones from the refuge and they will in the next generation you'll have families of resistant and non-resistant ones but as the non-resistant ones will die overall you will have much fewer resistant beetles, which is good.
75
What is the ring is species?
* Gene flow between different species/populations that usually cannot reproduce due to geographical isolation
* Can produce introgression: insertion of an allele specific to one species in another one.
Population A4 and B3 are not able to reproduce, they are so divergent that there is some kind of barrier preventing them from reproducing. However, genes from population A4 can travel to A3, then A2, A1, parental one and then to B1, B2 and B3 so we can find genes, alleles from population A4 in the population B3 and this is not because there is direct gene flow between them, but because there's a ring of species, of different species that allows for those alleles to travel to introgress from one population to another one, and a classical example of a species ring is these gulls that are found around the northern Pole.
76
What are species?
•Species: group of organisms where two individuals are capable of reproducing fertile offspring, typically using sexual reproduction
77
What are the problems with the definition of species?
Problems with this definition-
•Species: group of organisms where two individuals are capable of reproducing fertile offspring, typically using sexual reproduction
Capable- mechanical constraints
Fertile offspring- a horse and donkey produce a mule which can not reproduce- no gene flow.
Sexual reproduction- most of life on earth doesnt have sexual reproduction.
78
A population-
•Population genetics:
•Microevolution:
•Macroevolution:
A population is a localised group of individuals that are capable of interbreeding and producing fertile offspring
•Population genetics: how populations change genetically over time
•Microevolution: change under species level, the change in the genetic makeup of a population from generation to generation (typically described as changes in allele frequencies)
•Macroevolution: change that occurs above species level (typically described with phylogenies)
79
What is a gene pool?
* The gene pool is the total aggregate of genes in a population at any one time
* Consists of all gene loci in all individuals of the population
* Remember! 1 loci has 1, 2 or more alleles (i.e. hair colour has alleles for black, blonde, red, brown, etc.) one from mother and one from dad. A single person only has two alleles but the population as a whole for a given gene may have more than one allele.
80
What is an example of mendelian genetics?
Is an example of mendelian genetics- intermediate dominance. Homozygotes have two phenotypes, different phenotypes and the heterozygotes has an intermediate phenotype.
81
What are the frequencies from a population of 100 plants?
Say you have a population that has 100 plants, 64 are red, 32 are pink and 4 are white. Numbers are the phenotypic frequencies. We know that the red plants have to be homozygous for the red allele so we can infer that 64 plants are red and that each of them have two alleles, but we also know that the pink plants are heterozygous and therefore we have each of them have at least one allele. In this population there are a total of 160 red alleles. Allelic frequencies.
82
All the alleles in a population have to add up to __.
1
83
Frequency of alleles designated by letters
p, q, r…
84
if you only have 2 alleles, p + q = 1
| •Thus, if p = 0.8, then q is…
0.2
85
If we know that the frequency of the red allele in the population is 0.8 then the gametes that are going to be producing the total population is going to be of gametes containing the red allele is going to be ___ and the same for the white. So there is a _____ correlation in the case that you have sexual organisms reproducing that are _____ and then you have a direct correlation between the _____ frequencies and the ______frequencies.
```
80%
direct
diploid
allele
gamete
```
86
How can we see how many individuals are going to be either homozygote, homozygous for the red allele in the next population?
So if you want to see how many individuals are going to be for example, homozygote, homozygous for the red allele in the next population, then we need to multiply the 0.8 of the X by the 0.8 of the 0.8 of the sperm cells and that gives you a value of 0.64. Can do the same with the rest.
87
Compare the first and second generation of red alleles in the next population?
Comparison between the first generation and the second one. So the 1st generation have those frequencies p=0.8 and q=0.2 and now if try to infer the allele frequencies in the first generation based on those crossings, what we are going to get is that there is no change for the phenotypic frequencies over time. No change over generations- bc we have not introduced any disturbance in the system. There are no mutations and there has been no NS. if there is nothing happening to the population there is no change in the allele frequencies and they remain stable.
88
How can we calculate genotype frequencies?
We can calculate the genotype frequencies by multiplying the two allele frequencies for the alleles in this genotype, in this case P.
If p and q represent the frequencies of two alleles in a pop at a particular locus, then
p2 + 2pq + q2 = 1
p2 and q2 represent the frequencies of the homozygous genotypes
2pq represents the frequency of the heterozygous genotype
89
What is the hardy- weinberg theorem?
* Newton's First Law of Motion: Every object in a state of uniform motion tends to remain in that state of motion in the absence of external forces
* Hardy–Weinberg states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary forces
* AKA as Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law
* Describes a theoretical population that is not evolving
* Frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation
* Provided that only Mendelian segregation and recombination of alleles are at work
90
Explain these-
```
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
```
Extremely large population size
•In a small population, genetic drift will randomly change the allele frequencies from one generation to the next
No gene flow
•Gene flow is the transfer of genes between populations (e.g. migration), can change allele frequencies is the two populations are very different
No mutations
Mutations, although rare events, can alter the gene pool
Random mating
•If individuals select their mates, preferring certain genotypes, alleles will not mix randomly
No natural selection
if individuals with certain genotypes differ in their reproductive success this will alter allele frequencies
91
What can the hardy weinberg equation help estimate?
•Can be used to estimate the percentage of the human population carrying the allele for an inherited disease.
• For example, phenylketonuria is a metabolic genetic disease of the amino acid phenylalanine
–Phenylalanine can build up brain to toxic levels, affecting brain development and function
–The loci is autosomic
–The disease is recessive
–Disease can be minimized with phenylalanine-restricted diet
•We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that
1. The PKU gene mutation rate is low
2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
4. The population is large
5. Migration has no effect as many other populations have similar allele frequencies
92
What is the frequency of carriers?
2pq - is a genotype frequency.
Frequency of carriers is 2% of PKU gene.
•Why do some diseases with a genetic base and associated with old age (e.g. cancer, Alzheimer, Parkinson, etc.) do not disappear from the genetic pool?
So we have seen scenarios in which NS is able to remove genes very strongly from the population but we have also seen scenarios in which you can keep the disease such as sickle cell, malaria. They are usually associated with all ages. Why do these diseases not disappear from the gene pool- why is NS not reducing the presence of those alleles in this genetic pool of human populations.
