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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• 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
1. Monohybrid cross - cross involving a single traite.g. flower colour
2. 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 segregation1. Alternative versions of genes account for variations in
inherited characteristics 2. 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 appearance
4. 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
Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Wrinkled seeds
RR x rr
R
R
rr
Rr
RrRr
Rr
Genotype: Rr
Phenotype: Round
GenotypicRatio: All alike
PhenotypicRatio: 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
rR
RR
rrRr
Rr
Genotype: RR, Rr, rr
Phenotype: Round & wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
F1 Monohybrid Cross Review
• Heterozygous x heterozygous• Offspring:
25% Homozygous dominant RR50% Heterozygous Rr25% 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!
1. Between blue flower, BB and yellow, yy2. 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
• 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
36
Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1
Phenotypic ratio 9:3:3:1
copyright cmassengale
DIHYBRID CROSS
• 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
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypicratio
RY Ry rY ry
RY
Ry
rY
ry
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 eyesBb = brown eyesbb = blue eyes
CC = curly hairCc = curly haircc = straight hair
bC b___
bc
TEST CROSS
Possible results:
44
bC b___
bc bbCc bbCc
C bC b___
bc bbCc bbccor
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 LAWLAW PARENT CROSS OFFSPRING
DOMINANCE / True-breeding TT x tt
tall x short100% 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