Chapter 23: Population Geneitcs (Part 1, Week 2) Flashcards
What is a disease with a recessive pattern of inheritance and is caused by a mutation in a gene that encodes B-globin, a subunit of hemoglobin that carries oxygen in the red blood cells?
Sickle cell disease
Causes some red blood cells to take on a crescent or sickled shape. These sickled cells may block the flow of blood through his vessels.
What are heterozygotes?
An individual having two different alleles of a particular gene or genes, and so giving rise to varying offspring.
What is a perk of heterozygotes, who carry one copy of the sickle cell allele and a copy of the more common (non-disease-causing) allele?
Increased resistance to malaria
What is the study of genes and genotypes in a population?
Population genetics
Population is a group of individuals of the same species that occupy the same environment at the same time.
What is the central issue in population genetics?
Genetic variation
Its extent within populations, why it exists, how it is maintained, and how it changes over the course of many generations
What is the difference between genotype and phenotype?
Genotype is the entire genetic profile while phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
What does population genetics help us understand?
The relationship between genetic variation and phenotypic variation.
What is an extension of our understanding of Darwin’s theory of natural selection, Mendel’s laws of inheritance, and new studies in molecular genetics?
Population genetics
What are all of the alleles for every gene in a population called?
Gene pool.
Each member of the population receives its genes from its parents. Individuals that reproduce contribute to the gene pool of the next generation.
How can a certain species occupy a wide geographic range and are divided into discrete population due to geographic isolation?
May be located on either side of a physical barrier like a mountain range or very wide river.
What happens when the size and location of a population change?
The genetic composition generally changes as well.
What is an example of the genetic composition changing because the size and location of the population?
These changes may involve adaptation, better suited for its environment, making it more likely to survive and reproduce.
For example, a population of mammals may move from a warmer to a colder geographic location. Over the course of many generations, natural selection may change the population such that animals whose fur is thicker and provides better insulation against the colder temperatures become more prevalent.
What is polymorphism (greek meaning many forms)?
The presence of two or more variations of a character (trait) within a population.
Traits in the sense of colors, nose sizes, colored eyes, etc. Anything really.
Why does polymorphism exist?
Due to the existence of two or more alleles of a gene that influences the character or trait.
What is a gene that commonly exists as two or more alleles in a population?
Polymorphic gene.
What is the criteria for a gene to be polymorphic?
The gene MUST exist in at least two alleles, and each allele MUST occur at a frequency that is greater than 1%.
What is a gene that exists predominantely as a single allele in a population?
Monomorphic gene
What is the single criteria for a gene to be considered monomorphic?
When 99% or more of the alleles of a given gene are identical in a population.
What types of molecular changes cause genes to be polymorphic?
- deletions of a significant region of the gene
- duplication of a region
- change in a single nucleotide*****
What is the last type of variation called and is the SMALLEST type of genetic variation that can occur within a given gene AND is the most COMMON?
Single-nucleotide polymorphism (SNP) or “snips”
What is an example of a SNP?
Sickle cell disease.
The sickle cell allele involves a single-nucleotide change in the B-globin gene, which encodes a subunit of the oxygen-carrying protein called hemoglobin. (A and T switch strands)
What is the % SNPs of all variation in human gene sequences that occurs among different people?
99%
How many different, on average, SNPs occur in a gene that is 2,000 - 3,000 bp in length?
10
Polymorphism is the norm for relatively large, healthy population of nearly all species, as evidenced by the occurrence of SNPs within most genes.
So why do we care about SNPs? Give
- It affects the function of proteins encoded by affected genes therefore effecting the proteome, and influence how humans develop diseases.
- Variations in SNPs in the human population are associated with how people respond to viruses, drugs, and vaccines.
- This analysis of SNPs may be instrumental in the current and future development of personalized medicine doctored to a patient’s genotype.
- Understanding predispositions to certain diseases which then can initiate preventative measures to minimize changes of developing the diseases.
What is one approach to analyzing genetic variation in populations?
Consider the frequency of alleles and genotypes in a quantitative way.
What are the two fundamental calculations that are central to population genetics? And what are their equations?
Allele Frequency = # of copies of a specific allele in a population
DIVIDED BY
total # of all the alleles for that gene in the population
Genotype frequency = # of individuals with a particular genotype in a population
DIVIDED BY
Total number of individuals in the population
T/F Allele and genotype frequencies are related and they do not need to be clearly distinguised between them.
False. The frequencies need to be clearly distinguished between the two.
What must you remember in calculating an allele frequency for a diploid species who are homozygous and who are heterozygotes?
Homozygous individuals have two copies (one from the mother and one from the father) of a given allele whereas heterozygotes have only one.
Who independently derived a simple mathematical expression, called the ____-_____ equation, that describes the relationship between allele and genotype frequencies when a population is not evolving?
Godfrey Harold Hardy, an English mathematician, and Wilhelm Weinberg, a German physician.
The Hardy-Weinberg equation.
Practice - Calculate Allele Frequency and Genotype Frequency using the the equations.
49 red-flowered plants with the genotype CrCr (homozygote)
42 pink-flowered plants with the genotype CrCw (heterozygote)
9 white-flowered plants with the genotype CwCw (homozygote)
Calculate the frequency of the Cw allele!
Calculate the genotype frequnecy of CwCw!
Frequency of Cw =
(CrCw) + 2(CwCw)
DIVIDED BY
2(CrCr) + 2(CrCw) + 2(CwCw)
42 + (2)(9)
DIVIDED BY
(2)(49) + (2)(42) + (2)(9)
= 60/200 = 0.3, or 30% of the alleles for this gene in the population are the white color (Cw) allele.
Genotype Frequency
= 9
DIVIDED BY
49+42+9
= 9/100 = 0.09, or 9% of the individuals in this population have the white-flower genotype.
State te Hardy-Weinberg equation and explain its parts.
Using the previous example of four-o’clock plants, if the allele frequency of Cr is denoted by the symbol p and the allele frequency of Cw by q, then
p + q = 1
For example, if p=0.7, then q must be 0.3. In other words, if the allele frequency of Cr equals 70%, the remaining 30% of alleles much be Cw, because together they equal 100%. (FOR THIS, ALLELE FREQUENCIES MUST BE CALCULATED FIRST)
For a gene that exists in two alleles, the Hardy-Weinberg equation states that
p^2 + 2pq + q^2 = 1
If we apply this equation to our flower color gene, then
p^2 = the genotype frequency of CrCr homozygotes
2pq = the genotype frequency of CrCw heterozygotes
q^2 = the genotype frequency of CwCw homozygotes
If p = 0.7 and q = 0.3, then
Frequency of CrCr = p^2 = (0.7)^2 = 0.49
Frequency of CrCw = 2pq = 2(0.7)(0.3) = 0.42
Frequency of CwCw = q^2 = (0.3)^2 = 0.09
IN OTHER WORDS, if the allele frequnecy of Cr is 70% and the allele frequency of Cw is 30%, the expected genotype frequency of CrCr is 49%, of CrCw is 42%, and of CwCw is 9%.
What can be used to illustrate the relationship between allele frequencies and the way that gametes (reproductive or sex cells) combine to produce genotypes?
Punnett square
What does the validity of the Hardy-Weinberg equation rest on?
The assumption that two gametes combine RANDOMLY with each other to produce offspring.
In the population, what is the relationship between the frequency of a gamete carrying a particular allele and the allele frequency in that population?
They are the same.
If the allele frequency of Cr equals 0.7, the frequency of a gamete carrying the Cr allele also equals 0.7.
The probability of producing a CrCr homozygote with red flowers is 0.7 x 0.7 = 0.49, or 49%. (hence the p^2 and q^2 for homozygotes; CwCw and CrCr)
Since the probability of inheriting both Cw alleles, which produces white flowers, is 0.3 x 0.3 = 0.09, or 9%.
[INFO] Two different gamete combinations produce heterozygotes with pink flowers. An offspring could inherit the Cr allele from the pollen and Cw from the egg, OR Cr from the egg and Cw from the pollen.
Therefore, the frequency of heterozygotes is pq + pq, which equals 2pq. In our example, this is 2(0.7)(0.3) = .42, or 42%. Note that the frequencies for all three genotypes total 100%.
What is the Hardy-Weinberg equation, again?
What does each part of the equation designate?
What is the significance of this equation resulting in 1?
p^2 + 2pq + q^2 = 1
p^2 designates genotype frequency of CrCr homozygotes
2pq designates genotype frequency of CrCw heterozygotes
q^2 designates genotype frequency of CwCw
1 = 100% of all frequencies combined
What does the Hardy-Weinberg equation predict?
That allele and genotype frequencies will remain the same, generation after generation, provided that a population is in equilibrium.
How can a population be in equilibrium?
Name some of the following conditions that must be met. (5)
The population must not be affected by evolutionary mechanisms that can change allele and genotype frequencies.
- No new mutations occur to alter allele frequencies
- No natural selection occurs; that is, no survival or reproductive advantage exists for any of the genotypes
- The population is so large that allele frequencies do not change due to random chance.
- No migration occurs between different populations, altering the allele frequencies.
- Random mating occurs; that is, the members of the population mate with each other without regard to their genotype.
What causes microevolution to happen?
- The introduction of new genetic variation into a population is one essential aspect of microevolution. New alleles of pre-existing genes arise by random mutation. and new genes can be introduced into a population by gene duplication and horizontal gene transfer.
- One or more mechanisms that alter the PREVALENCE of a given allele or genotype in a population
In 1926, Russian geneticist Sergei Chetverikov was the first to suggest that what?
Hint: Raw material
Random mutations are the raw material for evolution
Due to their low rate of occurence, what does not play a major role in changing allele frequencies in a population over time AND does not significantly disrupt a Hardy-Weingberg equilibrium?
Mutations by themselves
EXAMPLE TIME:
What are (3) examples of new genetic variation that govern microevolution?
What are (4) examples of evolutionary mechanims that alter the PREVALENCE of a given allele or genotype
For GOVERNENCE (new genetic variation):
New Mutations
Gene Duplication
Horizontal Gene Transfer
For PREVALENCE:
Natural Selection
Genetic Drift
Migration
Nonrandom Mating
What source of genetic variation happens at a very low rate, can be neutral, deletrious, or beneficial and the changes from one generation to the next is very small?
New mutations within genes that produce new alleles
What source of genetic variation are abnormal crossover events and transposable elements that may increase the number of copies of a gene and over time, the additional copies accumulate random mutations adn constitute a gene family?
Gene Duplication
What source of genetic variation happens when a gene from one species may be introduced into another species?
Horizontal gene transfer
What evolutionary mechanism is a process by which individuals that possess certain traits are more likely to survive and reproduce than those without the traits?
Natural Selection
What evolutionary mechanism is a change in genetic variation from generation to generation due to random change? Allele frequencies many change as a matter of change from one generation to the next.
Genetic Drift
This has a much greater influence in a small population
What evolutionary mechanism introduces migrants into a recipient population that may change allele frequencies of that population?
Migration
What evolutionary mechanism do individuals select mates based on their genotypes or phenotypes? This alters the relative proportion of homo and heterozygotes that is predicted by the Hardy-Weinberg equation, but it does not change allele frequencies.
Nonrandom mating
