1. Mendelian Genetics
Siti Sarah Jumali
Level 3 Room 14
Ext 2123
Adapted from Rashidah Iberahim’s
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 man
Background
• Parent has a small farm in
Austria
• Receive agricultural training
in school
• Studied in University of
Vienna
• 1st 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 F1
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 F1
generation from the cross of two pure lines and
reappears in the F2 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
• F1 generation – first filial generation
(son/daughter)
• F2 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, F1 gen, F2 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 Cross
F1 Phenotype
F2 Phenotypic
Ratio
F2 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
R
R
_____________
MENDEL’S SECOND LAW
Law of Independent Assortment
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 truebreeding 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 (F1 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 F1 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
TEST CROSS cont’d
Additional readings
• Refer to this site and this
or this
Slides for grab
• www.mediafire.com
• Log in using sarahjumali@ns.uitm.edu.my
• Password: microbiology