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Mendel s Laws
The pair of chromosomes separates during meiosis Mother s gamete has one or the other of her 2 alleles for the trait (1 on each of her paired chromosomes) Father gives one or the other of his 2 alleles for the trait Mother Dd Father Dd Chromosomes/alleles separate D or d D or d There is a 50% chance the offspring will get either of these alleles Offspring DD or Dd or dd And a 50% chance the offspring will get either of these alleles
The distribution of one pair of alleles into gametes does not influence the distribution of another pair The genes controlling different traits are inherited independently of one another E.g. height and color of pea plants sort independently and one is not dependent on the other
Mendel s Law of Independent Assortment The segregation of one pair of chromosomes does not affect the segregation of any other pair of chromosomes.
The chance distribution of chromosomes to daughter cells during meiosis; along with recombination, a source of genetic variation (but not new alleles) from meiosis
Illustrations usually show only one or two chromosomes in meiosis, but remember there are 23 chromosomes in a human gamete. That means there are 2 23 possible combinations (8,388,608 combinations).
Characteristics that are influenced by alleles at only one genetic locus Examples include many blood types, such as ABO Many genetic disorders, including sickle- cell anemia and Tay-Sachs disease Over 19,000 human traits are known to be inherited according to Mendelian principles
Large molecules found on the surface of cells Several different loci govern various antigens on red and white blood cells Foreign antigens provoke an immune response
Polygenic traits, or continuous traits, are governed by alleles at two or more loci, and each locus has some influence on the phenotype Hair, eye, and skin color are polygenic traits
Polygenic Inheritance
Coloration is determined by pigment produced by specialized cells called melanocytes The amount of melanin produced determines how dark or light skin will be Melanin production is influenced by interactions between several different loci that, until recently, had not been identified
Polygenic traits account for most of the readily observable phenotypic variation seen in humans skin color, hair color, stature, shape of face, fingerprint patterns Some phenotypes serve as a basis for racial classification which have more to do with socially and culturally constructed ideas than they do with biology
Distribution of ABO blood type in a hypothetical population is not continuous The expression of the trait is described in terms of frequencies
Height in this population shows the continuous expression of height in a large group of people
A group of male students arranged according to height The most common height is 70 inches, which is the mean, or average, for this group
All cells contain mitochondria that convert energy into a form that can be used by the cell Each mitochondrion contains copies of a ringshaped DNA molecule, or chromosome Animals of both sexes inherit their mtdna, and all mitochondrial traits, from their mothers
All the variation in mtdna is caused by mutation, which makes it very useful for studying genetic change over time Mitochondrial DNA is passed from mother to daughter; it does not combine male and female chromosomes or go through meiosis Studying mutation patterns in mtdna helps us to understand human ancestry
What are the four evolutionary forces and how can they lead to changes in alleles frequencies from one generation to the next in breeding populations?
Evolution is a two-stage process: 1. The production and redistribution of variation (inherited differences among organisms) 2. Natural selection acts on this variation so the inherited differences among individuals differentially affect their ability to successfully reproduce (produce more offspring in the next generation)
From a modern genetic perspective, we define evolution as a change in allele frequency from one generation to the next Allele frequencies are indicators of the genetic makeup of a population - individuals that share a common gene pool In a population, allele frequencies refer to the percentage of all the alleles at a locus accounted for by one specific allele
Genotype refers to the type of genes you have Phenotype is the observable result of the genotype Gene Pp Allele P Allele p
Let s look at our pea plants There are three possible genotypes: PP, Pp, pp There are two possible phenotypes: P or p (Purple or White) p is the allele frequency for the dominant allele and q is the frequency of the recessive allele The use of p here is incidental and has nothing to do with the allele designation
Population of Pea Plants Gene Pool a collection of all the alleles in a population Key: Purple is Dominant White is Recessive
p = frequency of dominant alleles q = frequency of recessive alleles Key: Purple is Dominant White is Recessive
p = 14/20 =.7 q = 6/20 =.3 p+q = 1 therefore.7+.3 = 1 Key: Purple is Dominant White is Recessive
1. Count the number of alleles for each genotype for a specific trait (like ABO blood type) 2. Divide that number by the total number of alleles in the population number of alleles in population for a trait / total number of alleles in population There are six possible genotypes: AA, BB, AB, AO, BO, OO There are four possible phenotypes: A, B, AB, O Remember A and B are co-dominant so both will be expressed
In a total population of 200 individuals, each person has 2 alleles = 200 people x 2 alleles each = 400 total alleles in population Calculate the total number of A alleles in this population of 200: 84 people have genotype AA = 84 x 2 = 168 A alleles 6 people have genotype BB = 0 = 0 A alleles 12 people have genotype AB = 12 x 1 = 12 A alleles 60 people have genotype AO = 60 x 1 = 60 A alleles 16 people have genotype BO = 0 = 0 A alleles 22 people have genotype OO = 0 = 0 A alleles Total A alleles is 168 + 12 + 60 = 240 A alleles
If there are 240 alleles in this population of 200 (400 total alleles) what is the allele frequency of A alleles in this population? Calculate the allele frequency of the A allele in a population of 200 people remember everyone has two alleles 240/400 = 0.6 (60%) frequency of A alleles
What would the total frequency of B alleles in this population be? 12+12+16 = 40 40/400 = 0.1 (10%) allele frequency of B alleles What would the total frequency of O alleles in this population be? 60+16+44 = 120 120/400 = 0.3 (30%) allele frequency of O alleles
Allele Frequency Notice: adding the allele frequency of A (0.6) and the allele frequency of B (0.1) and the allele frequency for O equals 1. 0.6 + 0.1 + 0.3 = 1 or 60% + 10% + 30% = 100%
Can refer only to populations or groups of individuals, but never to an individual An individual has either A, AB, or O blood type (phenotype) or AA, BB, AB, AO, BO, or OO genotype A person s genotype is fixed and does not change over the lifetime of the individual, unlike allele frequencies of a population which do change over time Remember, only populations can evolve; individuals cannot
Studying genetic change in populations.
States that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences
AA (Homozygous Dominant) frequency= p² aa (Homozygous Recessive) frequency=q² Aa (Heterozygous) frequency= 2pq Equation: p²+2pq+q²=1 If you know one allele s frequency, you can use the equation to find out all frequencies
https://youtu.be/oebnom3k9cq
12% of the population of Ireland have red hair, which is a recessive phenotype Equation: p² + 2pq + q² = 1 q²=.12 q=.35 Therefore p =.65 Equation: (.65)² + 2 (.65)(.35) +.12 Equation:.42 +2(.65)(.35)+.12 =1 42% Homozygous Dominant for non-red hair 12% Homozygous Recessive for red hair 46% Heterozygous
When allele frequencies and genotype frequencies change, one or more evolutionary forces must be at work If evolutionary forces were not at work, the frequencies would remain constant
Macroevolution Large-scale changes that occur in populations after many generations, such as the appearance of a new species (speciation) Microevolution Small genetic changes that occur within a species E.g. science has documented many examples of the evolution of resistance of pests to pesticides, weeds to herbicides, and pathogens to medicines E.g. shift in allele frequency in peppered moths
Small Population Genetic Drift and Founder Effect Non-Random Mating consanguineous or selective mating Mutation Gene Flow Natural selection (consanguineous - relating to or denoting people descended from the same ancestor)
Mutation is a molecular alteration in genetic material: For a mutation to have evolutionary significance it must occur in a gamete (sex cell) and passed from one generation to the next Mutation is the only way to produce new genes
Mutations can involve changes in a single DNA base changes in large sections of DNA changes in entire chromosomes
Mutations are usually (but not always) recessive So, harmful effects are not expressed unless two of them are brought together into the homozygous condition This is more likely to occur with consanguineous mating (inbreeding)
One DNA base can be substituted for another as in the sickle cell allele Sickle cell mutation occurs when the base T is changed to an A which affects the entire structure of the red blood cells People with two alleles, have sickle cell anemia, are likely to die early
Mutation is rare at any given locus, but there is a high probability of at least one new mutation in each individual In a given population or species, mutation is common within a single generation We all carry at least one lethal recessive mutant allele
Two populations mix genetically and tend to become more similar over time Gene flow reduces differences between groups Amount of flow depends on environmental and cultural factors Geographic distance, ethnicity, religion, social class and education can limit gene flow
Gene flow can change the allele frequency in populations over time The allele frequency of red ball trait in population 1 Time 1 is 1.0 or 100% What is the allele frequency for population 1 Time 2 after gene flow? There are 19 red balls + 5 blue = 24 total balls or 19/24 =.79 or 79% red ball The allele frequency of population 1 changed from 100% to 79%
The result of random change in allele frequency from one generation to the next in a given population Change is more dramatic in small isolated populations
Drift occurs solely because the population is small and relatively isolated Allele frequencies may increase or decrease dramatically because of random chance with each new generation Genetic drift occurs in each generation Genetic drift leads to the reduction of variation within a population over time The larger the population size, the less change from one generation to the next
On the left is a small population, on the right a larger population. Notice how unpredictable and how dramatic the change in allele frequency is from one generation to the next in the small population. Notice how one or the other of the alleles can become fixed or disappear (individuals with that trait all die or cannot pass the trait on).
What happens to the frequency of the red trait from one generation to the next in this small population?
Frequency of red trait dramatically increases from generation to generation Change is due to chance, not natural selection or mutation
A specific kind of genetic drift Occurs when a small band of founders leaves its parent group and forms a colony elsewhere A new population is established and as long as mates are chosen within this population, all the members will be descended from the founders A once rare allele that was carried by even one of the founders can eventually become common
Dunkers, religious sect came from Germany to U.S. in 1700s Small population Founder effect - A kind of genetic drift caused when a small number of founders form a new population
Small closed population - descendent from a small number of German immigrants - about 200 individuals The Amish carry unusual concentrations of gene mutations that cause a number of otherwise rare inherited disorders, including forms of dwarfism and Polydactyly - extra fingers or sometimes toes
Population bottleneck - a rapid and radical decline in the population size This can lead to rapid changes in a population's allele frequency and a reduction in the population's genetic variation
Humans Have Spread Globally, and Evolved Locally Modern humans appeared 50,000 years ago, but genetic drift and natural selection have recently remolded the human clay
Gene flow - Movement of alleles from one population to another (migration). Genetic drift random change in allele frequencies in small relatively isolated populations
Mutation, gene flow, genetic drift, and recombination produce variation and distribute genes within and between populations But, there is no long-term direction of these factors For adaptation and evolution to occur, a population s gene pool needs to change in a specific direction, to increase or decrease allele frequencies Some alleles become more common, others less common in the population
Natural selection is the one factor that can cause directional change in allele frequency relative to specific environmental factors If the environment changes, selection pressures change, and allele frequencies also change If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually in each generation this is called adaptation
Natural selection may work to increase or decrease the frequency of certain alleles
An example of natural selection combined with mutation is the Sickle-cell trait The allele for this trait Hb s is rare (Normal hemoglobin is Hb a ) People who are homozygous Hb s Hb s have sickle-cell anemia which is fatal People who are heterozygous Hb s Hb a genotype, have some abnormal sickle shaped hemoglobin, but it is not fatal
People who are homozygous (Hb s Hb s ) have sickle cell anemia, are likely to die early We would expect frequencies of the sickle cell allele to become 0 over time But some populations (Africa, India, Mediterranean) show higher frequencies How can a harmful allele exist at high frequencies?
Distribution of Malaria Distribution of Sickle Cell Gene Researchers noticed that the same areas that had high rates of malaria also had a higher frequency of the sickle cell gene What as the connection?
In areas where malaria is common, the heterozygous condition Hb s Hb a is beneficial, because those with 1 allele for the sickle-cell trait are more likely to survive and reproduce Carriers (heterozygotes) for sickle cell, have none of the fatal symptoms of sickle cell, AND have an enormous resistance to malaria
Independently of one another, the following mechanisms of evolution can occur Mutation Sickle-Cell Natural selection Peppered moth Sickle-Cell Genetic drift (chance events) Founder Effect, Isolating Effects Gene flow Migration Mutation, Selection, Drift and Flow Mutation is the ONLY source of new genetic information
Microevolution: https://youtu.be/lk4_aiocyhc