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Lecture

BIOLOGY 1A03 Lecture Notes - Thomas Hunt Morgan, Blending Inheritance, Mendelian Inheritance


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Lovaye Kajiura

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Biology Chapter 13: Mendel and the Gene
Mendel’s Experiments with a Single Trait
Genetics is the branch of biology that focuses on the inheritance of traits
Heredity is the transmission of traits from parents to offspring
A trait is any characteristic of an individual, ranging from overall height to the primary
structure of a protein (category)
Mendel set out to address the most fundamental of all issues concerning heredity
o What are the basic patterns in the transmission of traits from parents to
offspring?
At that time, two hypotheses had been formulated
o The first was blending inheritance, which claimed that the traits observed in a
mother and father blend together to form the traits observed in their offspring
As a result, the offspring’s traits are intermediate between the mother’s
and father’s traits
o The second was called the inheritance of acquired characters, which claimed
that traits present in parents are modified, through use and passed on to their
offspring in the modified form
The idea is that the genetic determinants present in an individual are
modified through use and passed on to offspring in the modified form
A model organism for genetic studies is a species where individuals are small, short-
lived, inexpensive to care for, able to produce large numbers of offspring and easy to
manipulate experimentally
Mendel chose to study the pea plant Pismum sativum
Under normal conditions, garden peas pollinate themselves rather than requiring
pollen from other pea plants for fertilization to occur
Self-fertilization (selfing) takes place when pollen from one flower falls on the female
reproductive organ of that same flower
Selfing is common because pollen from other plants rarely reaches the flower since its
petals form a compartment that encloses the male and female reproductive organs
However, Mendel circumvented this arrangement by removing the male reproductive
organs from a flower before any pollen formed
Later, he could transfer pollen from another flower to that flower’s female
reproductive organs with a brush
This type of mating is referred to as cross-pollination and using this technique, Mendel
could control the matings of his model organism
Traits Mendel studied
o Mendel conducted his experiments on varieties of peas that differed in seven
traits: seed shape, seed colour, pod shape, pod colour, flower colour, flower and
pod position and stem length
o Phenotype is the observable features of an individual (round seed)
o In the pea populations Mendel studied, two distinct phenotypes existed for all
seven traits

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o Mendel began his work by obtaining individuals from pure lines
o A pure line consist of individuals that produce offspring identical to themselves
when they are self-pollinated or crossed with another member of the pure-line
population
o This result was important because he knew what the offspring from the matings
between pure-lines would look like
o He could then compare these results with outcomes of crosses between
individuals from different pure-lines
o Hybrids are offspring from matings between true-breeding parents that differ in
one or more traits
Inheritance of a Single Trait
o Mendel began his single-trait crosses by crossing individuals from round and
wrinkled seeded pure lines
o Adults used in an initial experimental cross represent the parental generation
o Their progeny are called the F1 generation (first filial)
o Subsequent generations from the F1 parents are called the F2, F3 generation etc.
o In his first set of crosses, Mendel took pollen from round seeded plants and
placed it on the female reproductive organs of plants from the wrinkle seeded
line
All the seeds produced by progeny from this cross were round
The traits did not blend together to form an intermediate phenotype,
instead the round seeded form appeared intact
This result was in stark contrast to the predictions of the blending-
inheritance hypothesis
Also, the genetic determinant for wrinkled seeds seemed to have
disappeared
o Mendel performed a second set of crosses, this time with pollen taken from an
individual germinated with a wrinkle seeded pea
o These crosses represent a reciprocal cross, a set of matings where the mother’s
phenotype in the first cross is the father’s phenotype in the second cross and
vice versa
In this case, the results of the reciprocal crosses were identical
All of the F1 progeny in the second cross had round seeds, just like the
first cross
This second cross established that it does not matter whether the
genetic determinants for seed shape are located in the male or female
parent
Dominant and Recessive Traits
o Mendel planted the F1 seeds and allowed the individuals to self-pollinate when
they matured
o He collected the resulting F2 generation and observed that some seeds were
round, while others were wrinkled
o The wrinkled shape reappeared in the F2 generation after disappearing
completely in the F1 generation

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o Mendel invented some important terms to describe the result
o He designated the genetic determinant for wrinkled shape as recessive
o In contrast, he referred to the genetic determinant for round seeds as dominant
Important to note that dominance has nothing to do with fitness of
individuals
o In genetics, the term dominance and recessiveness identify only which
phenotype is observed in individuals carrying two different genetic
determinants
o Mendel also noticed that the round and wrinkled seeds of the F2 generation
were present in a ratio of 3:1
The Nature and Behaviour of Hereditary Determinants
o To explain the patterns that he observed, Mendel proposed a competing
hypothesis called particulate inheritance
o He maintained that hereditary determinants for traits do not blend together or
acquire new or modified characteristics through use
o They maintain their integrity from generation to generation
o Instead of blending together, they act like discrete entities or particles
o Gene is now used to indicate the hereditary determinant for a trait
o Mendel also proposed that each individual has two versions of each gene, called
alleles
o Different alleles are responsible for the variation in the traits that Mendel
studied
o The alleles that are found in a particular individual are called its genotype
o An individual’s genotype has a profound effect on its phenotype, its physical
traits
o Mendel proposed that some alleles are dominant and others are recessive
o The Principle of Segregation
To explain the 3:1 ratio of phenotypes in F2 individuals, Mendel reasoned
that the two alleles of each gene must segregate into different gamete
cells during the formation of eggs and sperm in the parents
As a result, each gamete contains one allele of each gene, an idea called
the principle of segregation
Individuals with two copies of the same allele are said to be homozygous
Individuals with two different alleles for the same gene are said to be
heterozygous
Mendel’s Claims
o Peas have two alleles of each gene (true for many other organisms)
o Alleles do not blend together (hereditary determinants maintain their integrity
from generation to generation)
o Each gamete contains one allele of each gene (due to principle of segregation)
o Males and females contribute equally to the genotype of their offspring
o Some alleles are dominant to other alleles
Mendel’s Experiment with Two Traits
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