1. Mendelian Genetics Adapted from Rashidah Iberahim’s Siti Sarah Jumali Level 3 Room 14 Ext 2123

MENDELIAN GENETICS 1.Mendel’s work 2.Monohybrid inheritance and principle of segregation 3.Dihybrid inheritance and the principle of independent assortment 4.Test cross

An intro on Mendelian Genetics Explains on the mechanism of inheritance The mechanism relates to the numbers or characteristics of inheritance Simple characteristics lead to simpler crossing over mechanism and ration This laws excludes mutation effects that will be discussed later

GREGOR MENDEL The manBackground Parent has a small farm in Austria Receive agricultural training in school Studied in University of Vienna 1 st outstanding and well known experiment was on green peas which leads to Mendel’s laws

MENDELIAN GENETICS DEFINITION Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual Allelic pair - the combination of two alleles which comprise the gene pair Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote Genotype - the specific allelic combination for a certain gene or set of genes

Definition cont’d Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F 1 generation from the cross of two pure lines Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F 1 generation from the cross of two pure lines and reappears in the F 2 generation

TERMINOLOGIES Character- heritable features that vary between individuals Trait- each variant for the character Hybridization – mating or crossing of true breeding varieties Hybrid – the result of hybridization P generation – parental generation/ parent F 1 generation – first filial generation (son/daughter) F 2 generation – second filial generation

TERMINOLOGIES cont’d Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa) Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair

PUNNET SQUARE Diagram to predict the allele composition of offspring from a cross between individuals of known genetic make up Involve 3 steps/ generation = P gen, F 1 gen, F 2 gen Heterozygous – one dominant and one recessive allele, Pp Homozygous – both either dominant or recessive, PP or pp Phenotype – appearance or observable trait Genotype – genetic make up

Punnett Square

LAWS OF INHERITANCE Gregor Mendel introduced 2 laws 1.Law of Segregation 2.Law of Independent Assortment

MENDEL’S FIRST LAW Law of Segregation

LAW OF SEGREGATION Inherit only 1 characteristic Produce 3:1 ratio for phenotypic inheritance He concluded it from his large sample size experiment to explain this law Lead to the development of model known as Mendel’s model

MENDELIAN MODEL OF INHERITANCE 4 concepts in Law of segregation Alternative versions of genes account for variations in inherited characters For each character, an organism inherits two alleles, one from each parent If the two alleles at a locus differ, then the dominant allele determines the organism’s appearance; the recessive allele becomes unnoticeable in the organisms’ appearance During gamete formation, the 2 alleles of heritable character separate (segregate), and end up in different gametes

Alternative versions of genes in account for variations in inherited characters There are 2 choices of alleles Exist in 2 versions Depending on the phenotype or characteristics derived in the genetic make up Eg purple flower and white flower

For each character, an organism inherits two alleles, one from each parent Each somatic cell in a diploid organism has 2 sets of chromosome Genetic locus represent twice in diploid cell, once in homolog of a specific pair of chromosome

If the two alleles at a locus differ, then the dominant allele determines the organism’s appearance; the recessive allele becomes unnoticeable in the organisms’ appearance Eg The plant has more purple color because the purple allele is dominant and vice versa

Results from Mendel's Experiments Parental CrossF 1 Phenotype F 2 Phenotypic Ratio F 2 Ratio Round x Wrinkled Seed Round 5474 Round: 1850 Wrinkled 2.96:1 Yellow x Green Seeds Yellow 6022 Yellow: 2001 Green 3.01:1 Red x White Flowers Red 705 Red: 224 White 3.15:1 Tall x Dwarf Plants Tall l787 Tall: 227 Dwarf 2.84:1

The 2 alleles of heritable character separate (segregate), 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 outcome will depend on the types of reproduction between meiosis and mitosis Will be discussed after test cross

Purple and white cross

Questions ???

Discuss The possible products of red bean RR and green bean rr combination The possible products of red bean Rr and green bean rr combination The possible products of red bean RR and red bean Rr combination

Example of Final Exam question round, R, 5474 wrinkled, r, 1850 using the letters R and r for the seed texture alleles, diagram Mendel’s crosses showing the genotype of the plants in each generation

3: 1 ratio R vs r RR vs rr, Rr vs rr, Rr vs Rr RR

_____________ Law of Independent Assortment MENDEL’S SECOND LAW Second Part

LAW OF INDEPENDENT ASSORTMENT Two characters at the same time Called Dihybrids; e.g seed color and seed shape Still have dominant and recessive alleles in each of the characteristics

LAW OF INDEPENDENT ASSORTMENT cont’d This principle states that alleles for a trait separate when gametes are formed These allele pairs are then randomly unite at fertilization Mendel concluded this theory through Monohybrid crosses There were cross-pollination experiments with the pea plants differ in one trait, e.g pod color

Cross in plants that were true- breeding for 2 traits Mendel performed dihybrid crosses in plants that were true-breeding for 2 traits E.g, a plant that had green pod color and yellow seed color was cross-pollinated woth a plant that had yellow pod color and green seeds In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The offspring produced (F 1 generation) were all heterozygous for green pod color and yellow seeds (GgYy)

INDEPENDENT ASSORTMENT IN CHROMOSOMES

Using this Punnet Square..

HYPOTHESIS/ CONCLUSION The phenotypes enable to predict the genotype possibilities It was observed that there are yellow and green seeds with some wrinkled and some round in shape This event refers to the independent sorting of the alleles into gametes

Phenotypic ratio for independent assortment is 3:1 BUT.. Genotypic ration is 9:3:3:1

TEST CROSS Used to determine whether the dominant trait seen is either homozygous or heterozygous Involve breeding the individual tested with another individual that expresses only recessive version for the same trait If all offspring displays the dominant phenotype, the individual would be homozygous dominant; if the offspring display dominant and recessive phenotypes, then the individual is heterozygous dominant

TEST CROSS cont’d In some sources, “test cross” is referred to as backcross between the recessive homozygote and F 1 generation If the individual tested produces any recessive offspring ( except in cases of incomplete penetrance), its genotype is heterozygous. If all of the offspring are phenotypically dominant, then its genotype is homozygous.

TEST CROSS cont’d

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