Presentation on theme: "Genetic Variation Chapter 10 and 11 in the course textbook especially pages 187-197, 227-228, 250-255."— Presentation transcript:
1 Genetic VariationChapter 10 and 11 in the course textbook especially pages , ,
2 Genetic Inheritance & Variation No 2 organisms in a sexually reproducing species are the same (except “clones” or monozygotic twins)Genetic variation is essential for evolution and change to occurThere are 2 main processes that generate variation:MutationRecombination
3 Mutation and Recombination Mutation is a change in the genetic informationRecombination is a different arrangement of the same genetic materialThe cat sat on the mat (1)The bat sat on the hat – mutation (2)The cat sat on the hat – recombination of 1 and 2
4 The main properties of DNA The genetic material must be able to:Store informationReplicate (when cells divide)Express information (as proteins)Mutate at a low frequency (less than 1 in a million)DNA is a molecule that is very well suited to doing all 4 of these
5 Mutation Can occur in any cell at any time, cause may be: Internal (e.g. mistakes during replication of DNA)External (e.g. radiation, chemicals)Most mutations have no effect (neutral)A few mutations are harmfulA very few mutations are beneficialOnly harmful and beneficial mutations are acted on by natural selectionMutations may be non-coding (not in part of gene that codes for protein - have no effect, or affect gene expression) or coding…….
6 Effects of coding mutations Synonymous: the cat ate the ratMissense: the fat ate the ratNonsense: the cat ate theFrameshift: the cax tat eth era tSynonymous has no effect on protein, nonsense makes a smaller protein, missense/frameshift make incorrect protein
8 Mutation during DNA replication Replication of DNA is not perfectly accurate, but there are several ways to correct the mistakesACGTACGTAACGTG...TGCATGCATTGAACGGTDNA polymerase makes about 1 mistake per 105 bp.DNA polymerase has a “proof-reading” activity to correct itsown mistakes (99%).After DNA replication there is a “mismatch repair” system tocorrect remaining mistakes (99.9%).This leaves an overall error rate of about 1 base in 1010.
9 Error correction in DNA replication Overall error rate is about per divisionAbout 1 mistake per cell per division in humans
10 Mutation due to environmental factors Mutations may be caused by chemicals or radiationChemicals (“mutagens”) may disrupt hydrogen bonds between bases, by modifying them or getting between themRadiation (including ultra-violet and radioactive emissions) can damage structure of basesThese agents may be natural or man-made
11 Mendel’s experimentsGregor Mendel (a 19th century Czech monk) worked out the basic laws of genetic inheritance by breeding pea plantsHe chose simple characteristics that are determined by single genes (monogenic)Many characters such as height, IQ, disease susceptibility are determined by several genes (polygenic)
12 Mendel’s first cross P1 (parental) generation: wrinkled seeds crossed with smooth seedsF1 generation: all smooth seeds. Crossedwith itself………...F2 generation: smooth and wrinkledin ratio 3:1
13 Mendel’s genetic hypothesis Genes come in pairs. Each of the parents has2 copies of this gene. The “A” form gives smoothseeds, the “a” form gives wrinkled.AAaaParents produce gametes (eggs, sperm, pollen)which have 1 copy of the gene.AaAaFertilisation produces the F1 generation, all smoothbecause the “A” form is dominant over “a”;“a” is recessiveAaEach F1 plant produces equal numbers of A and agametes which fertilise at random to produce the F2plants. 1/4 of them are AA (smooth), 1/2 are Aa(smooth) and 1/4 are aa (wrinkled).
14 Cross with two genes AABB aabb ab AaBb AB Ab aB ab AB Ab aB ab AB AB 4 types of gametesin equal numbers9/16 yellow/smooth3/16 green/smooth3/16 yellow/wrinkled1/16 green/wrinkled
15 Summary of Mendel’s experiments Genes in an organism come in pairsSome forms (“alleles”) of a gene are dominant over other alleles which are recessiveOne (at random) of each pair of genes goes into a gamete (segregation)Gametes meet randomly and fertiliseThe numbers and types of offspring in a cross are determined by the above lawsSeparate genes behave independently of each other (later, exceptions to this rule were found)
16 Genes and chromosomesGenes can have several different forms due to mutations in DNA sequence. These forms are called alleles. Property of having different forms is called polymorphismNormal human body cells (“somatic” cells) are diploid: 23 pairs of chromosomes:Numbers 1-22 (autosomes)X and Y (sex chromosomes)XX in females, XY in malesGametes (eggs, sperm, pollen) are haploid, i.e. they have a single copy of each chromosome
17 Phenotype, Genotype, Alleles The phenotype of an organism is its observable propertiesThe genotype is the set of alleles it has for all of its genes (5,000 in bacteria; 35,000 in humans)New alleles are created by mutation and their effect the phenotype may be dominant or recessive
18 Modes of inheritanceDominant alleles affect the phenotype when present in 1 copy (heterozygous), e.g. Huntington’s diseaseRecessive alleles affect the phenotype only when present in 2 copies (homozygous), e.g. cystic fibrosisCan tell whether dominant or recessive by studying Mode of Inheritance in families
19 Autosomal dominant inheritance Person with trait in each generationMales and females equally likelyto show traitWhere 1 parent is heterozygous,about 50% of offspring show traitExample: Huntington’s disease
20 Autosomal recessive inheritance Trait may “skip” generationsMales and females equally likely to show traitHeterozygotes (“carriers”) do not show traitAbout 25% of offspring of 2 carriers will show traitExample: cystic fibrosis
21 X-linked recessive inheritance Carrier (heterozygous,unaffected) mothers pass the traitto about 50% of sonsTrait is never transmittedfrom father to sonIn the population, trait will be much more common in malesthan females. Example: muscular dystrophy