the study of hereditary variation Genetics the study of hereditary variation © 2016 Paul Billiet ODWS

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). © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

Nature or nurture? Variations may be influenced by what is inherited from the parents (the genotype) Variations may be influenced by the environment the organism encounters as it grows and develops. Genotype (nature) Environment (nurture) Phenotype © 2016 Paul Billiet ODWS

Studying heredity The Neolithic revolution led to folk knowledge Breeding from the animals or plants Gregor Mendel. http://history.nih.gov/exhibits/nirenberg/popup_htm/01_mendel.htm © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS

Mendel’s breeding experiments Taking one character only as an example, seed colour Parents (P) First generation (F1) 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 © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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). © 2016 Paul Billiet ODWS

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 or selfing. © 2016 Paul Billiet ODWS

Selfing Yellow seed producing plants Selfed Yellow seeds Green seeds 6022 Green seeds 2001 About 75% show the dominant trait About 25% show the recessive trait First generation (F1) Second generation (F2) © 2016 Paul Billiet ODWS

Selfing Selfed 33% produce yellow seeds only All green seeds Pure breeding Selfed 66% produce a mixture of yellow & green seeds 33% produce yellow seeds only Yellow seeds 6022 Green seeds 2001 Second generation (F2) Third generation (F3) © 2016 Paul Billiet ODWS

Pure breeding Those plants that only produce one type are called pure breeding (or true breeding). © 2016 Paul Billiet ODWS

The particulate theory of inheritance: Genes 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 nor diluted to give intermediates. The gene is the unit of hereditary information © 2016 Paul Billiet ODWS

Genes and variation A character is controlled by a gene that may come in different types called allelomorphs (meaning “other forms”) or alleles 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. © 2016 Paul Billiet ODWS

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). © 2016 Paul Billiet ODWS

Monohybrid inheritance Consistent patterns revealed in the behaviour of alleles So they are predictable The predictions can be tested by breeding experiments Therefore 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. © 2016 Paul Billiet ODWS

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%) © 2016 Paul Billiet ODWS

Mendel and meiosis Mendel worked this out in 1866 Though he no doubt understood fertilisation, meiosis was not observed until 30 years later. © 2016 Paul Billiet ODWS

Meiosis & Mendel Meiosis 1: Anaphase 1 Maternal and paternal chromosomes segregate (pulled separate on the spindle) They move to opposite poles. © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

In human genetics heterozygotes are called carriers Genotypes Phenotypes YY Homozygous Yellow Pure breeding Yy Heterozygous yy Green In human genetics heterozygotes are called carriers They are carrying a recessive allele without expressing it Many genetic diseases are caused by recessive alleles. © 2016 Paul Billiet ODWS

Genetic diagrams P Phenotypes Yellow seed X Green seed Genotypes YY yy Gametes Y y F1 Yy Yellow Proportions 100% © 2016 Paul Billiet ODWS

Genetic diagrams F1 Genotypes Yy Phenotypes Yellow Proportions 100% Gametes Y y Where there are several possible gametes a Punnett square should be used Selfed F2 Genotypes: Y y YY Yy yy Phenotypes: Yellow Green Proportions: 75% 25% © 2016 Paul Billiet ODWS