4 Genetics - How variability is produced in a population
Steve JH Lee
• Genetic Variability within a species measures how much variety of the genotype of organisms exists in a population This means how many types of a specific gene/trait exists in a population• DO NOT CONFUSE THIS WITH GENETIC DIVERSITY Genetic Diversity Genetic Variability Tendency of individual genotypes The amount of variation seen to vary from one another (phenotype from genotype) in a particular population Cause Effect
• Lets consider a hypothetical case: 2 men are trapped in a savannah. Each brought with him 5 survival items. Joe brings a knife, compass, water purifier, a pistol, and a sunglass Kevin brings 5 hunting rifles and unlimited ammunition to last his stayWho do you think has a better chance of surviving? Of course at first it may seem as if Kevin has the advantage because he will not have trouble feeding himself and Joe has brought a lot of useless stuff
• But what if there was a severe sandstorm and both were swept away and lost all sense of direction? Who would be more prepared? Of course Joe with his compass
• Having a variety of genes in a population is important because it gives BIODIVERSITY• Without variability, it becomes difficult for a population to adapt to unpredictable environmental changes Remember Kevin’s mistake!!• Genetic Variability Genetic Diversity Better Fitness
• For example, lets say a deadly virus/bacteria kills off organisms that have Gene A1 If all organisms were uniform and had only 1 type of the gene, A1, all organisms in the population would DIE• However if a diverse population has multiple versions of gene, or a VARIETY of genes: A1, A2, A3, A4, .. A99 then only A1 would die and the rest would survive.
• That means less Variability = more prone to extinction • Genetic variability is also the key factor in evolution, natural selection. • This is because it affects how much of a population are handle environmental stress and surviveNow we will look at the individual mechanisms that produce Genetic Variability
• In biology there are 3 main sources of Genetic Variability: 1. Meiosis Crossing Over (Homologous Recombination) Random Orientation Independent Assortment Random Fertilization 2. Mutations Point Mutations Insertions/Deletions Amplifications 3. Random Mating
• Homologous Recombination is one major source of genetic variety in animals• Remember in Interphase homologous chromosomes duplicate and associate during Prophase I• This formation is known as a Tetrad or a Bivalent• You generate variety when you guarantee infinite # of possible combinations
• Crossing Over takes place where alleles between non-sister chromatids are exchanged between homologous chromosomes. In a sense, the genes are mixed up together Knowing that there are millions of alleles on a single chromosome think of the number of the possibilities!!
The Steps of Crossing Over/HomologousRecombination• Synapsis: the homologous chromosomesassociate and join• Chiasma: alleles in neighboring non-sisterchromatids are exchanged via help of cuttingenzymes.• Recombination: after the exchange thealleles are swapped.
Step 1. Interphase, homologous pair replicates. 2copies of each pair held together by centromere andeach copy is called sister chromatidStep 2. Molecule cohesion occurs and homologouspairs are held together. Parallel “arms” between non-sister chromatids exchange alleles. Chiasmata= allele exchange sitesStep 3. cohesion breaks apart. Homologous pair stillheld at the chiasmata.Step 4. Anaphase finally breaks apart thehomologous pair with different gene combinations
Recall: More Variability = More Combination Possibilities• The next major source of variation is Metaphase I and Metaphase II when the homologous chromosomes (I)/sister chromatids (II) separate• Random orientation: there is no fixed designation of how homologous chromosomes separate Essentially this means homologous chromosomes are free to go which side they want Assuming that there are 2 possibilities for each chromosome (2 poles) that means there are 223 possible types of cells with different combinations of genes !!
• The 2nd Random Orientation occurs during Metaphase II Unlike Metaphase I, it is much less significant in terms of contributing to genetic diversity Metaphase I Metaphase II Entire homologous chromosomes separate, Sister chromatids separate which are not as both significantly different from each other in dramatically different except areas where terms of type and orientation of alleles crossing-over has taken place 223 223 Lots of variation in gametes Some variation in gametes
Random Orientation Independent Assortment in that the way chromosomes/chromatids randomly segregate form different combinations of chromosomes in each final haploid gameteVery helpful animation regarding Random Orientation/Independent AssortmentLINK: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
Mendel’s law of Independent Assortment states that : • Allele pairs separate independently during the formation of gametes (Random Orientation) • Traits are transmitted to offspring independent of one anotherEXCEPTION: Linked GenesLinked genes are genes that are inherited together because they are relativelyclose to each other on the same chromosome Because of their distance, it is unlikely for them to get recombinated
Cross-Over/Homologous Gene Recombination Increases the variety in genes through random allele rearrangement Even new combinations of linked genes Increases genetic diversity of pop Random Orientation/Independent Assortment 2n possible gametes For humans that is 8,388,608 possible gametes for one meiosis
Random Fertilization means that a random male gamete sperm will fertilize a random female game egg8.4 million possibilities x 8.4 million possibilities = 70,036,874,418,000 possibilities…. And this is NOT counting Crossing Over
1. Crossing Over: homologous gene recombination leads to rearrangement of alleles2. Random Orientation: homologous chromosomes/sister chromatids randomly segregate and assort with each other creating 2^n possibilities3. Random Fertilization: a randomly produced male gamete fertilizes a random female gamete, squaring the number of variable possibilities.Assessment Statements with Meiosis Variability: 10.1.1 10.1.2 10.1.3 10.1.4
Whereas meiosis is a more direct and short term mechanism for genetic variability, mutations are long-term and evolutionary mechanism for population diversity By long-term, takes multiple generation for the changes to get integrated
Genetic mutations are changes in a cell’s genome/DNA sequence withmultiple causes including radiation, viruses, and errors in meiosis and DNAreplication A mutation is passed on to the offspring stably who will eitherresemble that mutation or carry the allele. Acquired Mutations such as cancer however cannot be passed onto offspring
Point Mutations or base-substitutions occur when a single nucleotide is replaced with another Often a result of chemical damage or malfunction in DNA replicationResult?: the specific codon in the specific allele is misread, protein structure is altered In another words that protein is defunct
In insertions and deletions occur when one or more nucleotides are added or deleted from the DNA sequence. Usually caused by transposable elements or errors during replication (such as AT repeats)More significant than point mutations this is because the entire DNA sequence is shifted/moved and all genes in that DNA sequence are misread ALL GENES in that sequence cannot be read
Mutations can be achieved in 2 ways 1. Somatic Mutations (acquired mutations) NOT passed down Ex: UV light, cancer 2. Germ-line Mutations (mutation in a person’s DNA that is passed down through gamete formation)When a mutation is beneficial, such as pesticide-resistance, or harmful according to the situation, it fuels natural selection
List of all Assessment Statements4.1.3 Define Gene Mutation4.1.4 Explain the consequences of base-substitution in relation to transcription/translation using Sickle-cell anemia as an example10.1.1 Describe the behavior of chromosomes in phases of meiosis10.1.2 Outline the formation of chiasmata during cross-over10.1.3 Explain how meiosis results in effective infinite variety in gametes through crossing-over in Prophase I and random orientation in Metaphase I and II10.1.4 State Mendel’s Law of Independent Assortment