Genetic Variation and Change
Demonstrate understanding of genetic variation and change
Introduction and key words
Genetic variation describes naturally occurring genetic differences among individuals of the same species. All organisms are slightly or greatly different. This variation permits flexibility and survival of a population in the face of changing environmental circumstances and can also produce variation in the gene pools. This variation is important, especially in New Zealand as the habitat is constantly changing living (biotic) and non living (abiotic factors) change the populations gene pools and pressures.
This standard is about what brings on this variation in populations and how this leads to different frequencies of traits and eventually natural selection. What leads to the variation in a species? Make sure you watch the videos and look at the animations, they will all help.
Full Topic Posters
These have been made by Benjamin Himme from https://www.pathwayz.org/ (Another great learning site)
DNA from the Beginning Good introduction to DNA and genetics
Populations and genetics
Population genetics is the study of genetic variation within populations, and involves the examination and modelling of changes in the frequencies of genes and alleles inpopulations over space and time. Many of the genes found within a population will bepolymorphic - that is, they will occur in a number of different forms (or alleles). Mathematical models are used to investigate and predict the occurrence of specific alleles or combinations of alleles in populations, based on developments in the molecular understanding of genetics, Mendel's laws of inheritance and modern evolutionary theory. The focus is the population or the species - not the individual.
A good starter video.
Dna Structure and Variation
Look at the level 2 Gene expression page for notes, animations and videos explaining the structure of DNA.
Genetic variation and change
sources of variation within a gene pool
A gene pool is the complete set of unique alleles in a population.
Genetic drift is the change in the relative frequency in which an allele occurs in a population due to random sampling and chance.
Migration is the transfer of alleles of genes from one population to another.
The gene pool changes in allele frequencies due to random events not related to the fitness of the allele relative to that environment.
The gene pool changes in allele frequencies due to new alleles being brought into the population (immigration) or being lost from the population due to emigration.
The effects of both genetic drift and migration are particularly apparent in a small population where the relatively small changes in allele numbers can have a bigger impact on the ratio of those alleles in the population.
Biological ideas and processes relating to sources of variation within a gene pool
Mutation as a source of new alleles
Mutation is a permanent / random changes in the DNA/ genetic material. Mutation must occur in gamete-producing cells to enter the gene pool of the population.
is can also be defined as a permanent change in the nucleotide sequence in a gene or a chromosome.
A mutation is a permanent (unrepaired) change in an organisms DNA.
They introduce new alleles into a population. Most mutations are harmful.
Mutations are caused by mutagens.
Beneficial ones tend to occur more often in organisms with short generation times.
Many may be silent – not observed – and may only be selected for or against at a later date.
Neutral mutations make no change at all.
Mutations must happen in gamete producing cells to enter the gene pool of a population. This is important!!
Rates of Mutation
Genes mutate at known rates. This rate varies depending on the gene involved. Some genes have high spontaneous mutation rates.
Mutation rates for genes within a species are probably similar, but the viability of mutations varies greatly. Mutant genes in the human population:
With approximately 30,000 genes in the human genome and two copies of each gene, each cell has a total of 60,000 genes.
In higher organisms, a mutation for a specific gene will occur in one gamete in 300,000.
Types of Mutations
A good introduction animation here
Somatic (Body cell mutations)
· somatic mutations occur in any cells of the body other than in the gametes
And at merit level:
Alterations in DNA that occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to the offspring.
Gametic (sex cell mutations)
· gametic mutations only occur in gametes, eg, sperm / eggs (accept pollen).
· somatic mutations are not passed on from one generation to the next
· somatic mutations only affect the individual organism in which the cells have mutated
· gametic mutations are (heritable) transferred to the next (& possibly subsequent) generations
· gametic mutations are not limited to the individual in which the original mutations has occurred
the new alleles created by gametic mutation are available to the gene pool and may become established in that gene pool.
So at merit level:
Gametic: (may be called germ line, which is acceptable). A heritable change in the DNA that occurred in a gamete (germ cell) – a cell destined to become an egg or sperm. When transmitted to the offspring, a gametic mutation is incorporated in every cell of their body.
Substitution of a single base, e.g. A → G. Affects 1 gene.
Addition or subtraction of a single base - causes a frame shift. Seriously affects one gene
Chromosome mutations -A chunk of chromosome can be deleted, added or moved to a different chromosome. Affects a number of genes
Aneuploidy -A whole chromosome, or whole set of chromosomes are added or lost.
If a mutation occurs in a gamete it will affect the entire organism produced (they are inherited) = Gametic mutation.
If the mutation occur in a body cell it will only affect one area (it is not inherited) = Somatic mutation.
Fitness of mutations
The fitness of a mutation describes its value to the survival and reproductive success of the organism.
A mutation may turn out to be:
Harmful: Non-lethal mutations, e.g. Down syndrome and sickle cell disease, may be expressed as effects that lower fitness.
Lethal: Many mutations are lethal and embryos are non-viable.
Silent (neutral): Most point mutations are probably harmless, with no noticeable effect on the phenotype.
Beneficial (useful): Occasionally mutations may be useful, particularly in a new environment, e.g. insecticide resistance in insects, antibiotic resistance in bacteria.
Some links of help
Excellent Nature magazine article on mutations and evolution link
Types of mutations link
Wiki article link
Mutation animation link
Independent assortment, segregation and crossing over during meiosis
Why is meiosis so important?
Meiosis produces four haploid cells that develop into gametes so that when fertilisation occurs, a new individual with the full number of genes results, maintaining the chromosomal number from generation to generation while promoting genetic diversity and variability within the population.
Meiosis is a vital process because it reduces the original number of chromosomes to half and allows genetic variability by genetic recombination and independent assortment.
Recombination is where there is an exchange of genetic material between adjacent chromatids of homologous chromosomes. This ‘random’ exchange of DNA results in novel combinations of alleles on the chromosomes, creating almost infinite potential for variation.
Independent assortment is when each of the chromosome pairs separate (segregate) in the first division. They do this independently of each other creating a huge amount of variation in the nature of the gametes produced.
Eg, in humans there are 23 pairs of chromosomes, so 223 kinds of gamete could be produced (>8 million)
Independent assortment A pair of chromosomes separates randomly, which results in each daughter cell having a unique set of chromatids/ chromosomes. In another exam they said it as: Allele pairs separate independently during the formation of gametes.
NOTE: distribution does not mean pair separation.
Crossing over / recombination The result of this process is an exchange of alleles/ sections or segments of chromosomes (not genes or information) – AND different allele combinations/ making chromatids/ chromosomes unique.
If you draw a diagram of this, must show homologous pair with cross over
Segregation:Each gamete / daughter cell receives a single allele / copy of each gene OR results in unique combination of alleles A great starter powerpoint
With independent assortment, the chromosomes that end up in a newly formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes endup with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency.
effect of co-dominance, incomplete dominance, lethal alleles, and multiple alleles
Linked genes (Genetic Linkage)
When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50%, linked genes have a recombination frequency that is less than 50%. Some exam questions talk about unlinked genes. What this is referring to is examples when genes are far apart and are less likely to be inherited together during homologous recombination. At level two you do not need to go into the recombination frequencies.
Biological ideas and processes relating to factors affecting allele frequencies in a gene pool
First you should understand what gene flow is.
Animation on Gene flow Gene flow
gene flow link
Notes with links to Wikipedia: In population genetics, gene flow (also known as gene migration) is the transfer of alleles or genes from one population to another.A great starter animation for this section of the topic.
There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.
An allele that is harmful is unlikely to become established, as it will be selected against, due to the individual’s chances of survival AND successful reproduction being reduced.
From understanding evolution, Berkley University
Darwin's grand idea of evolution by natural selection is relatively simple but often misunderstood. To find out how it works, imagine a population of beetles:
- There is variation in traits.
- For example, some beetles are green and some are brown.
- There is differential reproduction.
- Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
- There is heredity.
- The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
- End result:
- The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
The five fingers of evolution
natural selection An excellent animation that explains the concept well.
Migration is when Individuals may enter / leave a population. It is the transfer of alleles of genes from one population to another
Migration into or out of a population may be responsible for a marked change in allele frequencies(the proportion of members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population.
What is genetic drift?
Genetic drift—along with natural selection, mutation, and migration—is one of the basic mechanisms of evolution.
Genetic drift is the change in the relative frequency in which an allele occurs in a population due to random sampling and chance.
In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. That, in a nutshell, isgenetic drift. It happens to ALL populations—there’s no avoiding the vagaries of chance.
Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.
- Founder effect Occurs when a new colony is started by a few members of the original population. A founder population is likely to be small.
- This means that the colony may have EITHER reduced genetic variation from the original population OR non-random sample of the alleles in the original population.
A cool LEGO!! video showing Founder effect and other theories.