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genome complete set of genes or DNA of species Species is defined by its particular genome Genome of individuals within that species will vary, depending on their unique combination of alleles and their non-coding portion of DNA
Homologous Chromosomes Matching pairs of chromosomes in a diploid organism (human 46) Same genes are found at the same locations (loci) on the two chromosomes in a pair Male sex chromosomes as the X and Y contain different gene set not homologous
Genotype The genotype is the particular allelic combination present at a particular gene locus
Autosomes Homozygous- an organism has two copies of the same allele (AA or aa) Heterozygous- an organism carries two different alleles (Aa)
Sex chromosomes Females have two copies of the X chromosome they can be homozygous (XBXB or XbXb) or heterozygous (XBXb). Males only have one copy of the X chromosome so they are referred to as being hemizygous (hemi meaning half)
Phenotype observable form of a characteristic or trait. The phenotype includes any distinct property of an organism- physical, chemical, physiological or behavioural
Identical twins Genetically identical and therefore have the same genotype Possible to distinguish between such twins based on subtle differences in appearance This can be attributed to differences in environment, for example diets, exercise patterns and climates
gene Sequence of DNA nucleotides that codes for a particular characteristic or trait Slight variations in the code of a gene caused by mutations results in different forms of that trait (alleles) Combinations of alleles in an individual make up its genotype
monogenic Traits that are controlled by one gene Tend to have discontinuous (or discrete) variation e.g. ABO blood group
Polygenic Traits are controlled by multiple interacting genes Results in continuous variation e.g. height, dog and cat coat colour
Gene pool Complete set of alleles present in a population The relative proportion of a particular allele in a population is referred to as the allele frequency, and is often expressed as a percentage or as a decimal
Hardy-Weinberg principle Concept that allele frequencies in a population remain constant from one generation to the next unless an agent of change acts on the population
Conditions of Hardy-Weinberg principle part 1 • populations must be large • members of the population must mate at random: contrasting nonrandom mating such as restrictions to like phenotypes such as colour. • All matings are equally fertile, producing equal numbers of viable offspring
Conditions of Hardy-Weinberg principle part 2 • The population is closed, with no migration either into or out of the population. • The population is closed, with no migration either into or out of the population.
Mutations Changes in DNA Can have a beneficial or harmful effect, or no effect at all, on the organism Can occur randomly during replication May affect a single gene, multiple genes or may involve entire chromosomes
Mutagens Factors that induce mutation Common mutagens include different forms of radiation Somatic mutations occur in body cells and only affect that individual Germline mutations are heritable, they affect gametes and can therefore be passed on to offspring
Point mutations A mutation that alters, adds or removes a single nucleotide from a sequence of DNA or RNA is called a point mutation These include: • substitution mutations and • Frameshift mutations
Substitution mutations A substitution mutation is a point mutation in which one nucleotide is replaced by another type of nucleotide. There are different types of substitution mutations including: • silent • missense • nonsense
Silent mutation involves a substitution that results in no change in the amino acid. As the triplet genetic code is redundant, more than one code encodes for the same amino acid. The redundancy is in the third nucleotide
Missense mutation Point mutation involving a substitution that results in an amino acid replacement. The impact of this type of mutation on the characteristic (phenotype) of the organism depends upon the importance of the affected amino acid to the function of the protein
Nonsense mutation mutations that result in the generation of a 'stop' codon and no further amino acids are added after the site of mutation, such as UAU (tyrosine) to UAA (stop). These stop translation and can have serious effects, particularly if it occurs early in a gene
Frameshift mutations involve one or two nucleotides being either added or removed from a nucleotide sequence, altering every codon in that sequence from that point onwards and therefore every amino acid they code for Can have significant effects on the polypeptide
Block mutations Affect large sections of a chromosome, typically multiple genes Occur during meiosis in eukaryotic cells Can also be caused by mutagens There are four main forms of block mutations: • Duplication • Deletion • Inversion • Translocation
Duplication mutations involve the replication of a section of a chromosome that results in multiple copies of the same genes on that chromosome Can be thousands of repeats Often increases gene expression, which can be harmful or beneficial depending on the gene involved
Deletion mutations remove sections of a chromosome. Deletions lead to disrupted or missing genes, which can have serious effects on growth and development. Chromosomal deletions are often fatal
Inversion mutation a section of the sequence breaks o the chromosome, rotates 180° and reattaches to the same chromosome. Inversions may involve as few as two bases or they may involve several genes
Translocation mutations Whole chromosome or a segment of a chromosome becomes attached to or exchanged with another chromosome or segment eg. two non-homologous chromosomes may break off at the same time They may reattach to the other chromosome, swapping genetic material
Chromosomal abnormalities Mutation involving whole chromosomes, or a number of chromosomes Easily detected with a karyotype (technique of staining and photographing chromosomes to help classify them and detect chromosomal mutations) Two main forms: aneuploidy & polyploidy
Aneuploidy presence of an abnormal number of a particular chromosome, either an extra chromosome (trisomy) or a missing chromosome (monosomy) Usually caused by non- disjunction during meiosis
Polyploidy condition that results from cells and organisms that contain more than two full sets of chromosomes; that is, more than two of every chromosome in the set. • Diploid (two copies, 2n) • Triploid (three copies, 3n) • Tetraploid (four copies, 4n)
Polyploidy in plants more common in plants as they can survive by asexual reproduction • 3n- some banana varieties • 4n- some cultivated potatoes and cotton. • 6n- bread wheats • 8n- strawberries.
Environmental selection pressures Factor in an organism’s environment that removes unsuited individuals, and therefore alleles from a gene pool Act on phenotypes Can be natural environmental pressures or artificial pressures brought about by humans through selective breeding
Examples of environmental selection pressures • climatic conditions such as extreme temperature changes and drought • competition for resources such as the availability of food and water, as well as competition for shelter • mate availability • predator abundance
Natural Selection Environmental selective agents acting on a wild population whereby individuals with one phenotype in a population are more likely to survive and reproduce than other varieties, thus increasing the allele frequency in the gene pool
Process of natural selection part 1 - variation already exists in the population - Presence of an environmental selection pressure. - Individuals with the advantageous phenotype are more likely to survive and produce viable offspring...
Process of natural selection part 2 ...passing the alleles for the favourable trait onto offspring - Over generations the allele frequency and therefore number of individuals in the population with favourable trait increases
Gene flow movement of alleles through genetic exchange between individuals of different populations, resulting in an increase in genetic diversity In animals, this can result by their movement from one population to another (migration)
Genetic drift Random changes to allele frequencies in a gene pool as a result of chance events, resulting in a reduction in genetic diversity More clearly seen in small populations with little to no gene flow Unrepresentative sample of the original population
Bottleneck effect results in a drastic reduction in population size caused by a chance event such as a natural disaster and as a consequence, allele frequencies may change, reducing genetic diversity
Founder effect occurs when a small group of individuals from a larger population move to a new location and establish a new population The allele frequency is unlikely to be a representative sample of the original population Reduction in genetic diversity
Evolution change in the genetic composition of populations over time. This can be observed as changes in allele frequencies in a population over time. Genetic change in populations of an ancestral species can lead to new species
Processes of evolution Natural selection is a driving force of evolution Environmental conditions change over time and so too do the environmental selective pressures on a population Genetic drift can result in evolutionary change
Species group of individuals that are genetically similar enough to produce fertile viable offspring when interbreeding
Speciation evolution of new species from an ancestral species
Allopatric speciation occurs when a population becomes divided by a geographical barrier. The isolation prevents individuals of the separated sub-populations from interbreeding (prevention of gene flow)
Process of allopatric speciation part 1 • population is separated, no gene flow (due to geographical barrier) • accumulation of differences due to mutations occurring independently in populations.
Process of allopatric speciation part 2 • Each population experiences different selection pressures and through natural selection individuals of characteristics for the different environments survive and reproduce • when reunited, they are unable to produce viable/fertile offspring
Selective Breeding Process by which humans decide which individuals may breed and leave offspring to the next generation Humans choose individual organisms with desirable traits and interbreed them to increase the allele frequency of those desired traits in the gene pool
Selective breeding steps 1 Determine the desired trait 2 Interbreed parents who show the desired trait 3 Select the offspring with the best form of the trait and interbreed these offspring 4 Continue this process until the population reliably reproduces the desired trait
Problems with selective breeding Decreases the frequency of all other alleles for this trait Reduces the genetic variation within the gene pool Gene linkage means that selecting for one allele may result in the selection of a number of other traits
Created by: emmawalton05
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