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Genetic Variation Chapter 10 and 11 in the course textbook especially pages 187-197, 227-228, 250-255.

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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 Variation Chapter 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 occur There are 2 main processes that generate variation: –Mutation –Recombination

3 Mutation and Recombination Mutation is a change in the genetic information Recombination is a different arrangement of the same genetic material The 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 information –Replicate (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 harmful A very few mutations are beneficial Only harmful and beneficial mutations are acted on by natural selection Mutations 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 rat Missense:the fat ate the rat Nonsense:the cat ate the Frameshift:the cax tat eth era t Synonymous 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 mistakes ACGTACGTAACGTG... TGCATGCATTGAACGGT DNA polymerase makes about 1 mistake per 10 5 bp. DNA polymerase has a proof-reading activity to correct its own mistakes (99%). After DNA replication there is a mismatch repair system to correct remaining mistakes (99.9%). This leaves an overall error rate of about 1 base in

9 Error correction in DNA replication Overall error rate is about per division About 1 mistake per cell per division in humans

10 Mutation due to environmental factors Mutations may be caused by chemicals or radiation Chemicals (mutagens) may disrupt hydrogen bonds between bases, by modifying them or getting between them Radiation (including ultra-violet and radioactive emissions) can damage structure of bases These agents may be natural or man-made

11 Mendels experiments Gregor Mendel (a 19th century Czech monk) worked out the basic laws of genetic inheritance by breeding pea plants He 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 Mendels first cross P1 (parental) generation: wrinkled seeds crossed with smooth seeds F1 generation: all smooth seeds. Crossed with itself………... F2 generation: smooth and wrinkled in ratio 3:1

13 Mendels genetic hypothesis Aa AAaa A a A a A a Genes come in pairs. Each of the parents has 2 copies of this gene. The A form gives smooth seeds, the a form gives wrinkled. Parents produce gametes (eggs, sperm, pollen) which have 1 copy of the gene. Fertilisation produces the F1 generation, all smooth because the A form is dominant over a; a is recessive Each F1 plant produces equal numbers of A and a gametes which fertilise at random to produce the F2 plants. 1/4 of them are AA (smooth), 1/2 are Aa (smooth) and 1/4 are aa (wrinkled).

14 Cross with two genes AB ab AABBaabb AaBb ABabaBAb 4 types of gametes in equal numbers AB Ab aB ab AB Ab aB ab 9/16 yellow/smooth 3/16 green/smooth 3/16 yellow/wrinkled 1/16 green/wrinkled

15 Summary of Mendels experiments Genes in an organism come in pairs Some forms (alleles) of a gene are dominant over other alleles which are recessive One (at random) of each pair of genes goes into a gamete (segregation) Gametes meet randomly and fertilise The numbers and types of offspring in a cross are determined by the above laws Separate genes behave independently of each other (later, exceptions to this rule were found)

16 Genes and chromosomes Genes can have several different forms due to mutations in DNA sequence. These forms are called alleles. Property of having different forms is called polymorphism Normal 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 males Gametes (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 properties The 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 inheritance Dominant alleles affect the phenotype when present in 1 copy (heterozygous), e.g. Huntingtons disease Recessive alleles affect the phenotype only when present in 2 copies (homozygous), e.g. cystic fibrosis Can tell whether dominant or recessive by studying Mode of Inheritance in families

19 Autosomal dominant inheritance Person with trait in each generation Males and females equally likely to show trait Where 1 parent is heterozygous, about 50% of offspring show trait Example: Huntingtons disease

20 Autosomal recessive inheritance Trait may skip generations Males and females equally likely to show trait Heterozygotes (carriers) do not show trait About 25% of offspring of 2 carriers will show trait Example: cystic fibrosis

21 X-linked recessive inheritance Carrier (heterozygous, unaffected) mothers pass the trait to about 50% of sons Trait is never transmitted from father to son In the population, trait will be much more common in males than females. Example: muscular dystrophy

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