Pisum sativum is model
organism in genetic
A I G E R I M S O LTA BAY E VA
BIOL 105
GENERAL BIOLOGY
FA L L , 2 0 2 1
Objectives
Convenience of use the Garden Pea in genetic research
How Mendel conducted his experiments
 Mendel’s model
The Principle of Segregation
The Principle of Independent Assortment
Alterations in Mendel’s ratios
Garden Peas: The First Model Organism in
Genetics
Gregor Mendel (1822– 1884), a monk who bred pea
plants.
Mendel was the first scientist to effectively apply
quantitative methods to the study of inheritance.
He did not merely describe his observations; he
planned his experiments carefully, recorded the data,
and analyzed the results mathematically.
He produced around 29,000 garden pea plants from
controlled crosses and registered several of their
observable characteristics
Although unappreciated during his lifetime, his work
was rediscovered in 1900.
Mendel’s pea
Pisum sativum, the common pea (also known as the garden
or field pea), is an herbaceous annual in the Fabaceae
(formerly Leguminosae) family, originally from the
Mediterraean basin and Near East.
The garden pea is a self-pollinated diploid (2n=14
chromosomes) with a genome of 4,300 Mb/1C, which is
about 10-fold larger than the model legume Medicago
truncatula, and about 4-fold larger than the soybean
genome.
Practical considerations for use of the
garden pea
Production of hybrid peas by crossing different varieties was
practiced earlier
large number of pure varieties of peas
It is easy to grow.
Its reproductive cycle is short.
It produces large numbers of seeds.
Its matings are easy to control.
Its traits are easily recognizable.
Mendel’s method
Mendel collected pollen from the anthers of a white flower, then
placed that pollen onto the stigma of a purple flower with anthers
removed.
Mendel’s experiment
Seven characters in Mendel’s study of pea plants
Mendel’s experimental design
Mendel usually conducted his experiments in three stages:
1. Mendel allowed plants of a given variety to self-cross for multiple
generations to assure himself that the traits he was studying were
indeed true-breeding
2. Mendel then performed crosses between true-breeding varieties
exhibiting alternative forms of traits. He also performed reciprocal
crosses
3. Finally, Mendel permitted the hybrid offspring produced by these
crosses to self-fertilize for several generations, allowing him to
observe the inheritance of alternative forms of a trait. Most
important, he counted the numbers of offspring exhibiting each trait
in each succeeding generation.
Mendel’s model
Alternative forms of a “factor” account for variations in inherited traits;
Inherited traits pass from parents to offspring as unmodified factors;
Each individual has two sets of factors, one of each pair inherited from the
mother and one from the father;
The paired factors separate during the formation of reproductive cells (the
principle of segregation);
Factors may be expressed or hidden in a given generation, but they are never
lost.
Each factor is passed to the next generation independently from all other factors
(the principle of independent assortment).
Today, scientists use:
What Mendel called “factors.” We now call these factors genes.
The alternative forms of a gene are called alleles.
The term phenotype to refer to the physical appearance of an
organism
The total set of alleles that an individual contains as the individual’s
genotype.
When two haploid gametes containing the same allele fuse during
fertilization, the resulting offspring is said to be homozygous.
When the two haploid gametes contain different alleles, the resulting
offspring is said to be heterozygous.
The 3:1 ratio is actually 1:2:1
The F2 generation exhibits
a 3:1 ratio of both traits
The genotypic ratio “collapses” into the phenotypic ratio
due to the action of the dominant allele making the
heterozygote appear the same as homozygous dominant.
Mendel referred that the factor expressed in the F1
generation (tallness, in our example) is dominant; the
one hidden in the F1 (shortness) is recessive.
Dominant traits mask recessive ones when both are
present in the same individual.
1
2
1
Monohybrid Crosses: The Principle of
Segregation
WHEN HAPLOID GAMETES ARE FORMED, EACH CONTAINS ONLY ONE ALLELE FOR EACH LOCUS
Principle of
Segregation. The
two alleles for a gene
segregate during
gamete formation and
are rejoined at
random, one from
each parent, during
fertilization.
Mendel’s principle of
segregation is related
to the events of
meiosis: The
separation of
homologous
chromosomes during
meiosis results in the
segregation of alleles
in a heterozygote.
Meiosis
Meiosis occurs during gamete formation, producing cells
with half the normal number of chromosomes.
Meiosis produces haploid cells with half the number of
chromosomes.
Meiosis is characterized by the pairing of homologous
chromosomes during prophase I. During this pairing,
homologues may exchange chromosomal material at sites called
chiasmata.
In meiosis I, the homologues separate from each other, reducing
the chromosome number to the haploid state (thus the
reductive division). It is followed by a second division without
replication, during which sister chromatids become separated.
The result of meiosis I and II is four haploid cells.
Genetic Terms
Monohybrid and Dihybrid Crosses
Monohybrid cross: a cross between homozygous parents
with different alleles
Dihybrid cross: parents differ at two loci
Dihybrid Crosses: The Principle
of Independent Assortment
With an understanding of the behavior of single traits, Mendel went
on to ask if different traits behaved independently in hybrids
Consider a cross involving different seed shape alleles
(round, R, and wrinkled, r) and different seed color alleles
(yellow, Y, and green, y).
Traits in a dihybrid cross behave independently
In a dihybrid cross, the alleles of each gene assort
independently. A more precise statement would be stated:
The segregation of different allele pairs is independent.
The F2 generation exhibits four types of progeny in a 9:3:3:1
ratio
Dihybrid Crosses: The Principle of
Independent Assortment
While independent assortment means that
the chromosomes whether dominant or
recessive after gametogenesis goes into any
of the gametes i.e in simple language the
movement of chromosomes is not
affected by movement of other
chromosomes
Genes that are on different chromosomes (like the Y and R genes)
assort independently. The seed color and seed shape genes are
on chromosomes 1 and 7 of the pea genome, respectively, in real
life^11start superscript, 1, end superscript. Genes that are far
apart on the same chromosome also assort independently thanks
to the crossing over, or exchange of homologous chromosome
bits, that occurs early in meiosis I.
Animation Meiosis and independent assortment
© 2019 Cengage. All rights reserved.
Punnett square to analyze Mendel’s
cross
Punnett square: a grid
arrangement that shows the
possible combinations of
alleles
◦ One parent’s alleles are listed
across the top; the alleles of
other parent are listed along the
left side
◦ The squares are filled in with the
resulting F2 combinations
A Dihybrid Cross
Mendelian Inheritance is not always easy to analyze
 In polygenic inheritance, more than one gene can affect a single trait
In reality, few phenotypes result from the action of only one gene.
Instead, most characters reflect multiple additive contributions to the
phenotype by several genes.
For example: hair color, height and skin
color, as well as the non-visible traits such as
blood pressure, intelligence, autism and
longevity, occur on a continuous gradient,
with many variations of quantifiable
increments.
Mendelian Inheritance is not always easy to analyze
 In pleiotropy, a single gene can affect more than one trait
An allele that has more than one effect on phenotype is
said to be pleiotropic.
Pleiotropic effects are difficult to predict, because a. gene
that affects one trait often performs other, unknown
functions
Pleiotropic effects are characteristic of many inherited
disorders in humans, including cystic fibrosis and sickle
cell anemia.
Mendelian Inheritance is not always easy to analyze
 Dominance is not always complete
In incomplete dominance, the phenotype of the
heterozygote is intermediate between the two
homozygotes. For example, in a cross between red- and
white-flowering Japanese four o’clocks,
Codominance
Most genes in a population possess several different
alleles, and often no single allele is dominant; instead,
each allele has its own effect, and the heterozygote
shows some aspect of the phenotype of both
homozygotes.
Example: human blood groups
Incomplete dominance
Mendelian Inheritance is not always easy to analyze
In epistasis, interactions of genes alter genetic ratios
This type of gene interaction, where one gene can
interfere with the expression of another, is called
epistasis.
When gene products act sequentially, as in a biochemical
pathway, an allele expressed as a defective enzyme early
in the pathway blocks the flow of material through the rest
of the pathway. In this case, it is impossible to judge
whether the later steps of the pathway are functioning
properly.
In Mendel’s model is that the products of genes do not
interact.
Mendelian Inheritance is not always easy to analyze
Phenotypes may be affected by the environment
Another assumption, implicit in Mendel’s work,
is that the environment does not affect the
relationship between genotype and phenotype
For example, the alleles of some genes encode heatsensitive products that are affected by differences in
internal body temperature.
The ch allele in Himalayan rabbits and Siamese cats
encodes a heat-sensitive version of the enzyme
tyrosinase, which as you may recall is involved in
albinism.
Coat Color in the Himalayan Rabbit
Summary
Monohybrid Crosses: The Principle of Segregation
Dihybrid Crosses: The Principle of
Independent Assortment
The F1 generation exhibits only one of
two traits with no blending.
Traits in a dihybrid cross behave independently.
The F2 generation exhibits a 3:1 ratio
of both traits.
The 3:1 ratio is actually 1:2:1.
Mendel’s Principle of Segregation explains
monohybrid observations
The Principle of Segregation states that during gamete
formation, the two alleles of a gene separate (segregate).
Parental alleles then randomly come together to form the
diploid zygote.
The Principle of Independent Assortment states
that different traits segregate independently of
one another. The physical basis of independent
assortment is the independent behavior of
different pairs of homologous chromosomes
during meiosis I.
Heredity and Genetics
Modern geneticists correlate transmission of genetic
information with the behavior of chromosomes during
meiosis
◦ Heredity: transmission of genetic info from parent to offspring,
follows predictable patterns
◦ Genetics: the science of heredity
◦ Studying genetic similarities and variation, differences between parents and offspring or
among individuals of a population
Mendelian Inheritance is not always easy to analyze
 Dominance is not always complete
 In polygenic inheritance, more than one gene can affect a single trait
 In pleiotropy, a single gene can affect more than one trait
 Genes may have more than two alleles
 Phenotypes may be affected by the environment
 In epistasis, interactions of genes alter genetic ratios
Next task
1) Read Campbell biology: concepts and connections 7
edition
Chapter “Patterns of inheritance” or “The basic principles of
Heredity”
2) Preparation for Quiz 2
3) TA will be involved in checking the attendance of the class
4) Midterm will be on 28th of Sep
Why is Mendel's work important?
How to use Mendelian model?
For prediction of phenotype
◦ In F1 if you are interested in some phenotype to get all of the similar
◦ In F2 you can calculate how many seeds nee to collect the recessive mutant
◦ When you generating mutant line by breeding, all segregation will be in F2
For conformation
if gene interacts
If gene have more than two allele
If gene have pleotropic effect
If gene effected by environment