MENDELIAN GENETICS Mendel’s work Monohybrid inheritance and principal of segregation Dihybrid inheritance and the principal of independent assortment Test cross

INTRODUCTION Explaining the mechanism of inheritance The mechanism relates to the numbers of characteristic of inheritance The simple characteristic leads to the simpler crossing over mechanism and ration This is followed by excluding the mutation effects that will be discussed later (chapter 4)

GREGOR MENDEL Study in University of Vienna His parents has a small farm in Austria

GREGOR MENDEL (cont) Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until the turn of the 20th century

GREGOR MENDEL (cont) Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents Called the “Father of Genetics"

MENDEL’S PEA PLANT TRAITS

GREGOR MENDEL (cont) Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA

GENETIC TERMINOLOGIES Character – heritable feature that varies among individuals Trait – each variant for the character True-breeding – Plants homozygous for a characteristic are true-breeding (Self-pollinate) Hybridization – mating or crossing over of two true-breeding varieties P generation – parental generation/parent F1 generation – first filial generation (son) F2 generation – second filial generation

GENETIC TERMINOLOGIES (cont) Allele- alternate version of a gene Homozygote – pair of identical alleles for a character Heterozygote – two different alleles for a character (Bb) Dominate allele – expressed in the heterozygote Recessive allele – not expressed in the heterozygote Homozygous dominant- BB Homozygous recessive - bb Genotype – genetic makeup Phenotype – appearance of an organism

TYPES OF GENETIC CROSS Monohybrid cross - cross involving a single trait e.g. flower colour Dihybrid cross - cross involving two traits e.g. flower colour & plant height

PUNNET SQUARE Diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup. 3 steps / generation = P gen, F1 gen, F2 gen Heterozygous allele - ? Homozygous allele - ? Phenotype - ? Genotype - ?

PUNNET SQUARE (cont) Can be used for monohybrid and also dihybrid cross.

LAW OF INHERITANCE Gregor Mendel introduce 2 laws Law of Segregation Law of Independent Assortment

LAW OF SEGREGATION Inherit only ONE characteristic @ Monohybrid Producing 3:1 of phenotypic inheritance Mendel use a large group of sample size to explain this law Leads to a development of a model known as Mendel’s Model

MENDEL MODEL Four concepts in law of segregation Alternative versions of genes account for variations in inherited characteristics For each character, an organism inherit two alleles, one from each parent If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has NO noticeable effect in the organism’s appearance The two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes

1 . ALTERNATIVE VERSIONS OF GENES ACCOUNT FOR VARIATIONS IN INHERITED CHARACTERISTICS Have 2 choices of alleles Existing in two version Depending on the phenotypic or characteristic derive in the genetic make up Eg. Purple flower and white flower

2. FOR EACH CHARACTER, AN ORGANISM INHERIT TWO ALLELES, ONE FROM EACH PARENT Each somatic cell in a diploid organism has two sets of chromosome Genetic locus represent twice in diploid cell, once in homolog of a specific pair of chromosome

3. IF THE TWO ALLELES AT A LOCUS DIFFER, THEN ONE, THE DOMINANT ALLELE, DETERMINES THE ORGANISM’S APPEARANCE; THE OTHER, THE RECESSIVE ALLELE, HAS NO NOTICEABLE EFFECT IN THE ORGANISM’S APPEARANCE The plant have more purple colour due to its dominant allele, vice versa

4. THE TWO ALLELES FOR A HERITABLE CHARACTER SEGREGATE (SEPARATE) DURING GAMETE FORMATION AND END UP IN DIFFERENT GAMETES An egg or sperm gets only one of the two alleles that are present in the somatic cell of the organism making the gamete The correspond depending on the types of reproduction between meiosis and mitosis Further discussion after test cross

Example of MONOHYBRID CROSS

P1 Monohybrid Cross r r Rr Rr R R Rr Rr Genotype: Rr Phenotype: Round Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr r r Genotype: Rr Phenotype: Round Genotypic Ratio: All alike Phenotypic Ratio: All alike Rr Rr R R Rr Rr

P1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F1 generation Genotypic & Phenotypic ratio is ALL ALIKE

F1 Monohybrid Cross R r RR Rr R r Rr rr Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr R r Genotype: RR, Rr, rr Phenotype: Round & wrinkled G.Ratio: 1:2:1 P.Ratio: 3:1 RR Rr R r Rr rr

F1 Monohybrid Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1

HOW DOES THE PEAS LOOK LIKE?

Genotypic Ratio & Phenotypic Ratio

TEST YOURSELF! Between blue flower, BB and yellow, yy Between small leaf, ff and big leaf, Ff

LAW OF INDEPENDENT ASSORTMENT TWO characteristics at the same time @ Dihybrid cross Eg. Leaf colour and leaf size Using both dominant and recessive alleles in each of the characteristics.

INDEPENDENT ASSORTMENT in CHROMOSOME ndependent Assortment: Mendelian theory that, as meiosis ends, genes on pairs of homologus chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs on other chromosomes.

Mendel performed dihybrid crosses in plants that were true-breeding for TWO traits. E.g a plant with green pod colour and yellow seed, cross-pollinated with a plant that had yellow pod colour and green seeds. Green pod colour = GG Yellow seed colour = YY Yellow pod colour = gg Green seed colour = yy The resulting F1 generation were all heterozygous for green pod colour and yellow seeds (GgYy)

DIHYBRID CROSS Involves two pairs of contrasting traits

copyright cmassengale DIHYBRID CROSS Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 Phenotypic ratio 9:3:3:1 copyright cmassengale

DIHYBRID CROSS RrYy x RrYy RY Ry rY ry RY Ry rY ry Traits: Seed shape & Seed colour Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations

DIHYBRID CROSS RY Ry rY ry RY Ry rY ry

DIHYBRID CROSS RY Ry rY ry Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy

HYPOTHESIS/CONCLUSION The alleles of seed colour and seed shape sort into gametes independently of each other. Phenotypic ratio for IA = 9:3:3:1

TEST CROSS To determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous

TEST CROSS (cont) In some sources, the ‘test cross’ is defined as being a type of backcross between the recessive homozygote and F1 generation. F1 progeny are mated back to one of their parents (or to individual with a genotype identical to the parent) Backcross is often used synonymously with testcross.

TEST CROSS A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bbC__ x bbcc BB = brown eyes Bb = brown eyes bb = blue eyes CC = curly hair Cc = curly hair cc = straight hair bC b___ bc

copyright cmassengale TEST CROSS Possible results: bC b___ bc bbCc C bC b___ bc bbCc bbcc or c copyright cmassengale

If the plant being tested is homozygous If the plant being tested is heterozygous

G?W? X ggww (G=yellow; g=green; W=round; w=wrinkled) What will the expected phenotypic ratios be for the above testcross?

SUMMARY of MENDEL’S LAW PARENT CROSS OFFSPRING DOMINANCE / True-breeding TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods