Genetics the study of hereditary variation © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

Genotype and phenotype Genotype is the letter or term used to describe the allele of an individual gene or pair of genes Phenotype – is how the gene (or pair) shows itself, how it appears.

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 ODWSODWS

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 © 2007 Paul Billiet ODWSODWS

Studying heredity The Neolithic revolution Breeding from the animals or plants Gregor Mendel © 2007 Paul Billiet ODWSODWS

Father of Genetics  Monk and teacher.  Experimented with purebred tall and short peas.  Discovered some of the basic laws of heredity.  Studied seven purebred traits in peas.  Called the stronger hereditary factor dominant.  Called the weaker hereditary factor recessive.  Presentation to the Science Society in1866 went unnoticed.  He died in 1884 with his work still unnoticed.  His work rediscovered in  Known as the “Father of Genetics”.

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 © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

Mendel’s Peas  In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.)  The flower is the reproductive organ and the male and female are both in the same flower.  He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred.

Mendel’s breeding experiments Taking one character only as an example, seed colour Parents (P) First generation (F 1 ) 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 © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

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 © 2007 Paul Billiet ODWSODWS

Selfing Yellow seed producing plants Selfed Yellow seeds 6022 Green seeds 2001 About 75% show the dominant trait About 25% show the recessive trait First generation (F 1 ) Second generation (F 2 ) © 2007 Paul Billiet ODWSODWS

Selfing All green seeds Pure breeding Selfed 66% produce a mixture of yellow & green seeds 33% produce yellow seeds only Pure breeding Yellow seeds 6022 Green seeds 2001 Second generation (F 2 ) Third generation (F 3 ) © 2007 Paul Billiet ODWSODWS

Mendel’s Results Mendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall. When these tall offspring were crossed the result was a ratio of 3 tall to 1 short.

Mendel’s Peas

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 ODWSODWS

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

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 or diluted to give intermediates The gene is the unit of hereditary information © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

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 ODWSODWS

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 ODWSODWS

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 ODWSODWS

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 ODWSODWS

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

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 ODWSODWS

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 ODWSODWS

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 GenotypesPhenotypes YYHomozygousYellowPure breeding YyHeterozygousYellow yyHomozygousGreenPure breeding © 2007 Paul Billiet ODWSODWS

PPhenotypesYellow seedXGreen seed GenotypesYYyy GametesYYyy F1F1 PhenotypesYy GenotypesYellow Proportions100% Genetic diagrams © 2007 Paul Billiet ODWSODWS

F1F1 PhenotypesYy GenotypesYellow Proportions100% GametesYy Genetic diagrams Where there are several possible gametes a Punnett square should be used F2F2 Genotypes:Yy YYYYy y yy Phenotypes:YellowGreen Proportions:75%25% Selfed © 2007 Paul Billiet ODWSODWS

Gene diagram – Flower colour Genotype of alleles- R = red flower r = yellow flower All genes occur in pairs – so 2 alleles affect a characteristic – possible combinations are; genotype RR Rrrr PhenotypeRED RED YELLOW

Gene diagram – Flower colour Malefemale RRrr parent gamete R R r r Offspring genotype Rr Phenotype All red

Gene diagram – Flower colour Malefemale Rr parent gamete R r R r Offspring genotype RR Rr rr Phenotype Red yellow red red 3 red : 1 yellow

Gene diagram – Flower colour Malefemale RrRrrr parent gamete R r r r Offspring genotype RrRr RrRrrr Phenotype Red yellow yellow red Red 50% yellow 50%

Punnett Square Another method of showing crosses

Gamete genotypes are inserted Parent genotypes are inserted B=black b=white Bb male black bb white female Bb b b What are the crosses Bb bb Bbbb 2 white and 2 black offspring 50:50 chance with these parents

Examples on inherited traits

(D)Wet ear wax vs. Dry ear wax (r)

(D)Free Earlobe vs. Attached (r)

Six-digits (D)

Cleft chin

Dimples

Freckles

Webbed toes

Hallux vs. Morton’s Toe

TEST CROSS How to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes

Selfing or test cross? Plants can be selfed Homozygous plants (AA) are pure breeding Heterozygotes (Aa) will give 75% dominant (AA and Aa) and 25% recessive (aa) phenotypes in their offspring Animals are not usually hermaphroditic so they cannot be selfed Animals require a test cross to be carried out (though these days a gene probe is more likely to be used). © 2007 Paul Billiet ODWSODWS

Coat colour in mice TraitsAllelesGenotypesPhenotypes GreyGGGGrey WhitegGgGrey ggWhite Grey mice could have one of two different genotypes, GG or Gg. If they are crossed with a white mouse (gg) these genotypes will give two different results © 2007 Paul Billiet ODWSODWS

Homozygous Grey Phenotypes x White GenotypesGGgg GametesGGgg gg GGg G OffspringGreyWhite 100%0% © 2007 Paul Billiet ODWSODWS

Heterozygous Grey Phenotypes x White GenotypesGggg GametesGggg gg GGg ggg OffspringGreyWhite 50% © 2007 Paul Billiet ODWSODWS

Question 2 black rats are mated, they have 13 offspring. All of which are black, what are the genotypes of the parents The male is re-mated with a white female, the offspring are 6 black and 7 white What were the genotypes of the original parents Rats have 2 coat colours – black B, white b. Both BB BB BUT one could be Bb Female BB Male Bb