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.
MENDELIAN GENETICS 1. Mendel’s work 2. Monohybrid inheritance and principal of segregation 3. Dihybrid inheritance and the principal of independent assortment 4. 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• Mendels 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"
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 CROSS1. Monohybrid cross - cross involving a single trait e.g. flower colour2. 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 INHERITANCEGregor 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 MODELFour concepts in law of segregation1. Alternative versions of genes account for variations in inherited characteristics2. For each character, an organism inherit two alleles, one from each parent3. 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 appearance4. The two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes
1 . ALTERNATIVE VERSIONS OF GENESACCOUNT FOR VARIATIONS ININHERITED 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, ANORGANISM 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 LOCUSDIFFER, THEN ONE, THE DOMINANTALLELE, DETERMINES THE ORGANISM’SAPPEARANCE; THE OTHER, THERECESSIVE ALLELE, HAS NO NOTICEABLEEFFECT IN THE ORGANISM’SAPPEARANCE• 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
P1 Monohybrid CrossTrait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Wrinkled seeds RR x rr r r Genotype: Rr Phenotype: Round R Rr Rr Genotypic Ratio: All alike R Rr Rr Phenotypic Ratio: All alike
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• 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 & R RR Rr wrinkled G.Ratio: 1:2:1 r Rr rr P.Ratio: 3:1
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
• 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
DIHYBRID CROSS RY Ry rY ry Round/Yellow: 9RY RRYY RRYy RrYY RrYy Round/green: 3Ry RRYy RRyy RrYy Rryy wrinkled/Yellow: 3rY RrYY RrYy rrYY rrYy wrinkled/green: 1 9:3:3:1 phenotypicry RrYy Rryy rrYy rryy ratio
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 CROSSA mating between an individual of unknown genotypeand a homozygous recessive individual.• Example: bbC__ x bbccBB = brown eyesBb = brown eyesbb = blue eyes bC b___CC = curly hair bcCc = curly haircc = straight hair
TEST CROSS Possible results: bC b___ C bC b___ cbc bbCc bbCc or bc bbCc bbcc copyright cmassengale 44
If the plant being tested is If the plant being tested is homozygous 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 LAWLAW PARENT CROSS OFFSPRINGDOMINANCE / True- TT x tt 100% Ttbreeding tall x short tall Tt x Tt 75% tallSEGREGATION tall x tall 25% shortINDEPENDENT RrGg x RrGg 9/16 round seeds & green pods 3/16 round seeds & yellow podsASSORTMENT round & green x 3/16 wrinkled seeds & green pods round & green 1/16 wrinkled seeds & yellow pods