Genetics

1. Genetics the study of hereditary variation © 2007 Paul Billiet ODWS

2. Phenotype • Organisms have characteristic appearances • These appearances may vary from one individual to another • The characteristics shown by an organism is called its phenotype • (From the Greek phainein = to show and typos = type) © 2007 Paul Billiet ODWS

3. Variations Variations between organisms may be: • interspecific – variations between different species (e.g. tigers have stripes and leopards have spots) • intraspecific – variations within a species (e.g. blood type or height in humans) It is intraspecific variations that concern us here. © 2007 Paul Billiet ODWS

4. Environment (nurture) Genotype (nature) Phenotype Nature or nurture? • Variations may be influenced by what is inherited from the parents (thegenotype) • Variations may be influenced by the environment the organism encounters as it grows and develops. © 2007 Paul Billiet ODWS

5. Studying heredity • The Neolithic revolution • Breeding from the animals or plants • Gregor Mendel http://history.nih.gov/exhibits/nirenberg/popup_htm/01_mendel.htm © 2007 Paul Billiet ODWS

6. Pea plants for genetics • Peas have many recognisable characteristics (e.g. seed shape) • They are easy to cultivate • Their life cycle is reasonably short so results can be obtained quickly • Peas produce a large number of offspring (seeds), which makes results easier to verify http://www.ppdl.purdue.edu/PPDL/images/pisum-sativum.jpg © 2007 Paul Billiet ODWS

7. Pea plants for genetics • Peas have hermaphroditic flowers • Self fertilisation is possible. • The male parts can be pulled out to emasculate the flowers, preventing self fertilisation jeantosti.com/fleurs4/pois.htm © 2007 Paul Billiet ODWS

8. Female sex cells from a yellow-seeded plant Male sex cells in pollen from a green-seeded plant Cross fertilised (crossed) All seeds produced turned out yellow Mendel’s breeding experiments Taking one character only as an example, seed colour • Parents (P) • First generation (F1) © 2007 Paul Billiet ODWS

9. The reciprocal cross • Mendel tried the cross the other way round • Green seed female plant x yellow seed male plant • The same results were obtained © 2007 Paul Billiet ODWS

10. Dominant and recessive traits • The green seed coloured trait had disappeared but it reappeared in later generations as though it were hidden • Traits that disappear and reappear (e.g. green seed colour in peas) are called recessive • Those that hide them are called dominant traits (e.g. yellow seed colour in peas) © 2007 Paul Billiet ODWS

11. Selfing • Mendel produced a second generation of plants using the first generation. • He brushed the male pollen grains onto the female parts of the same flower • This is called self pollination and it leads to self fertilisation orselfing © 2007 Paul Billiet ODWS

12. Yellow seed producing plants Selfed Yellow seeds 6022 Green seeds 2001 About 75% show the dominant trait About 25% show the recessive trait Selfing First generation (F1) Second generation (F2) © 2007 Paul Billiet ODWS

13. Yellow seeds 6022 Green seeds 2001 Second generation (F2) Selfed Selfed Third generation (F3) 33% produce yellow seeds only Pure breeding 66% produce a mixture of yellow & green seeds All green seeds Pure breeding Selfing © 2007 Paul Billiet ODWS

14. Pure breeding Those plants that only produce one type are called pure breeding (or true breeding) © 2007 Paul Billiet ODWS

15. Mendel concluded from this and other similar experiments, that characters are controlled by factors (later called genes) These genes like separate particles, passed on from generation to generation They are not changed or diluted to give intermediates The gene is the unit of hereditary information The particulate theory of inheritance: Genes © 2007 Paul Billiet ODWS

16. Genes and variation • A character is controlled by a gene that may come in different types called allelomorphs (meaning “other forms”) or alleles • These different alleles produce the different traits in a character In the above example • Pea seed colour is controlled by the seed colour gene • There are two alleles of this gene (dialleleic), the yellow allele and the green allele. • The yellow allele is dominant and the green allele is recessive. About 30% of human genes are thought to be diallelelic © 2007 Paul Billiet ODWS

17. Symbols for genes Dominant alleles are given CAPITAL CASE LETTERS Recessive alleles are given small case letters Use letters that look different when written as small case and capital case (e.g. avoid C, O, P, S, U) © 2007 Paul Billiet ODWS

18. Monohybrid inheritance • The behaviour of the alleles controlling different traits revealed patterns in the way they are inherited • These patters always seemed to be the same so they are considered as a scientific law • If we only consider the inheritance of one character (e.g. seed colour) and ignore all the others (such as flower colour, seed shape etc) this is called monohybrid inheritance © 2007 Paul Billiet ODWS

19. The Law of Segregation • Organisms seem to possess two genes for each character (they are diploid). This pair of genes segregate (separate) when gametes are made (they are haploid) • Pairs of genes are reformed when the gametes fuse and they recombine in definite proportions (e.g. 75% to 25%) © 2007 Paul Billiet ODWS

20. Mendel and meiosis • Mendel worked this out in 1866 • Though he no doubt understood fertilisation, meiosis was not observed until 30 years later © 2007 Paul Billiet ODWS

21. Meiosis & Mendel Meiosis 1: Anaphase 1 • Maternal and paternal chromosomes segregate (pulled separate on the spindle) • They move to opposite poles © 2007 Paul Billiet ODWS

22. The Law of Segregation revisited • Organisms seem to possess two genes for each character (they are diploid). This pair of genes segregate (separate) when gametes are made (they are haploid) = meiosis • Pairs of genes are reformed when the gametes fuse and they recombine in definite proportions (e.g. 75% to 25%) = fertilisation © 2007 Paul Billiet ODWS

23. Combinations of genes • The combination of alleles in an individual is called the genotype • If the two alleles are the same it is homozygous • If the two alleles are different it is heterozygous © 2007 Paul Billiet ODWS

24. In human genetics heterozygotes who have a dominant and a recessive allele are called carriers They are carrying a recessive allele without expressing it Many genetic diseases are caused by recessive alleles © 2007 Paul Billiet ODWS

25. Genetic diagrams © 2007 Paul Billiet ODWS

26. Genetic diagrams Where there are several possible gametes a Punnett square should be used Selfed © 2007 Paul Billiet ODWS