Mendel’s Pea Plants
- Genetics is the scientific study of heredity.
- Gregor Mendel was an Austrian monk. His work was important to the understanding of
- Mendel carried out his work with ordinary garden peas.
- Mendel knew that:
the male part of each flower produces pollen, (containing sperm).
the female part of the flower produces egg cells.
- During sexual reproduction, sperm and egg cells join in a process called fertilization.
- Fertilization produces a new cell.
- Pea flowers are self-pollinating.
- Sperm cells in pollen fertilize the egg cells in the same flower.
- The seeds that are produced by self-pollination inherit all of their characteristics from
the single plant that bore them.
- Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce
offspring identical to themselves.
- Mendel wanted to produce seeds by joining male and female reproductive cells from
two different plants.
- He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower
with pollen from another plant.
- This process is called cross-pollination.
- Mendel was able to produce seeds that had two different parents.
Punnett square: predicts the results of a genetic cross between individuals of known
Homozygous: pair of identical alleles for a character
Heterozygous: two different alleles for a gene
Phenotype: an organism’s traits
Genotype: an organism’s genetic makeup
Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown
Character: (heritable feature, i.e., fur colour)
Trait: (variant for a character, i.e., brown)
True-bred: (all offspring of same variety)
Hybridization: (crossing of 2 different true-breds)
P generation: (parents)
F1 generation: (first filial generation)
Genes and dominance
- A trait is a specific characteristic that varies from one individual to another.
- Mendel studied seven pea plant traits, each with two contrasting characters.
- He crossed plants with each of the seven contrasting characters and studied their
- Each original pair of plants is the P (parental) generation.
- The offspring are called the F1, or “first filial,” generation.
- The offspring of crosses between parents with different traits are called hybrids.
- The F1 hybrid plants all had the character of only one of the parents.
Genes and dominance
- Mendel's first conclusion was that biological inheritance is determined by factors that
are passed from one generation to the next.
- Today, scientists call the factors that determine traits genes.
- Each of the traits Mendel studied was controlled by one gene that occurred in two
contrasting forms that produced different characters for each trait.
- The different forms of a gene are called alleles.
- Mendel’s second conclusion is called the principle of dominance.
The principle of dominance
The principle of dominance states that some alleles are dominant and others are recessive.
An organism with a dominant allele for a trait will always exhibit that form of the trait.
An organism with the recessive allele for a trait will exhibit that form only when the
dominant allele for that trait is not present.
Leading to the Law of Segregation
Alternative versions of genes (alleles) account for variations in inherited characteristics
For each character, an organism inherits 2 alleles, one from each parent
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s
appearance; the other, the recessive allele, has no noticeable effect on the organism’s
The alleles for each character segregate (separate) during gamete production (meiosis).
Mendel’s Law of Segregation
- Mendel crossed the F1 generation with itself to produce the F2 (second filial)
- The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
- Mendel assumed that a dominant allele had masked the corresponding recessive allele
in the F1 generation.
- The trait controlled by the recessive allele showed up in some of the F2 plants.
- The reappearance of the trait controlled by the recessive allele indicated that at some
point the allele for shortness had been separated, or segregated, from the allele for
- Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated
from each other during the formation of the sex cells, or gametes.
- When each F1 plant flowers and produces gametes, the two alleles segregate from each
other so that each gamete carries only a single copy of each gene.
- Therefore, each F1 plant produces two types of gametes—those with the allele for
tallness, and those with the allele for shortness.
- The gene combinations that might result from a genetic cross can be determined by
drawing a diagram known as a Punnett square.
- Punnett squares can be used to predict and compare the genetic variations that will
result from a cross.
- A capital letter represents the dominant allele for tall.
A lowercase letter represents the recessive allele for short.
In this example,
T = tall
t = short
Gametes produced by each F1 parent are shown along the top and left side.
Possible gene combinations for the F2 offspring appear in the four boxes.
- Organisms that have two identical alleles for a particular trait are said to be
- Organisms that have two different alleles for the same trait are heterozygous.
- Homozygous organisms are true-breeding for a particular trait.
- Heterozygous organisms are hybrid for a particular trait.
- All of the tall plants have the same phenotype, or physical characteristics.
- The tall plants do not have the same genotype, or genetic makeup.
- One third of the tall plants are TT, while two thirds of the tall plants are Tt.
- The plants have different genotypes (TT and Tt), but they have the same phenotype
Probability and Segregation
One fourth (1/4) of the F2 plants have two alleles for tallness (TT).
2/4 or 1/2 have one allele for tall (T), and one for short (t).
One fourth (1/4) of the F2 have two alleles for short (tt).
Because the allele for tallness (T) is dominant over the allele for shortness (t), 3/4 of the
F2 plants should be tall.
The ratio of tall plants (TT or Tt) to short (tt) plants is 3:1.
The predicted ratio showed up in Mendel’s experiments indicating that segregation did
Probabilities predict averages
Probabilities predict the average outcome of a large number of events.
Probability cannot predict the precise outcome of an individual event.
In genetics, the larger the number of offspring, the closer the resulting numbers will get
to expected values.
The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters?
Monohybrid cross vs. dihybrid cross
The two pairs of alleles segregate independently of each other.
Mendel’s Law of Independent Assortment
The Two-Factor Cross: F1
Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY)
with true-breeding plants that produced wrinkled green peas (genotype rryy).
All of the F1 offspring produced round yellow peas (RrYy).
The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r)
and green (y).
The Two-Factor Cross: F2
Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the
alleles would segregate from each other in the F2 generation.
RrYy × RrYy
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
- In Mendel’s experiment, the F2 generation produced the following:
some seeds that were round and yellow
some seeds that were wrinkled and green
some seeds that were round and green
some seeds that were wrinkled and yellow
- The alleles for seed shape segregated independently of those for seed color. This
principle is known as independent assortment.
- Genes that segregate independently do not influence each other's inheritance.
- Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the
- Mendel had discovered the principle of independent assortment.
- The principle of independent assortment states that genes for different traits can
segregate independently during the formation of gametes.
- Independent assortment helps account for the many genetic variations observed in
plants, animals, and other organisms.
Non-single gene genetics
Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex:
Codominance: two alleles affect the phenotype in separate, distinguishable ways.
Ex: Tay-Sachs disease
Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types
Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia
Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression
of a gene at a second locus. Ex: mice coat color
Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic
character Ex: human skin pigmentation and height
The family pedigree
Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell
Dominant disorders: •Huntington’s
Testing: •amniocentesis •chorionic villus sampling (CVS)