Part 2: Heredity and Mendelian Genetics

Topics: The genetics of inheritance The inheritance of one trait The inheritance of two traits Beyond Mendel’s laws Pt. 1 Pt. 2

The Genetics of Inheritance Traits – specific 
characteristics that are encoded by DNA. Ex. Earlobes, Dimples, 
Curved Fingers, Rolling 
Tongue

Inheritance – The mechanism 
that transmits genetic 
information between 
generations.

Genetics – The branch of 
biology that deals with the 
principles of variation and 
inheritance.

Farmers knew about 
inheritance and variation for thousands of years before these 
principles were scientifically 
proven. People have been selectively 
breeding domesticated animal and plant species for millennia.

The actual mechanisms of 
how these processes 
occurred were unknown.

A few theories arose… The Greek philosophers: Hippocrates, 460-377 B.C.E., 
suggested that the body 
produced “seeds” and that 
these seeds fused to give rise to 
a new individual.

Aristotle, 384-322 B.C.E., 
proposed that male and 
female semen mixed upon 
conception.

In the 1500’s, English 
physician, William Harvey 
theorized that a process 
called epigenesis was how 
inheritance occurred. A 
process based upon the 
development of the embryo 
forming in stages, and 
affected by factors both inside 
and outside the mother.

Dutch Scientist and 
Microscopist, Anton van 
Leeuwenhoek, used a 
microscope in the mid to late 1600’s to examine the gametes of humans and other animals.

Leeuwenhoek later 
proposed that the male sperm 
actually contained a 
“preformed” embryo which 
would later be developed in 
the mother’s uterus.

Preformation 
Theory

Sperm and Egg at Conception

Charles Darwin suggested, in 
the mid 1800’s that children had characteristics that were 
variations of their parents traits, but he was unable to explain the basis of heredity.

The answer to how traits 
were inherited came from 
Darwin’s contemporary 
Gregor Mendel.

“The Father of Modern Genetics” Mendel’s Contributions “The Father of Modern 
Genetics” Gregor Mendel 1822-1884

The Inheritance of One Trait Gregor Mendel, an Austrian 
monk, studied the principles of 
inheritance using the common pea plant, Pisum sativum as 
an experimental model.

Prior to Mendel’s work at the Monastery of St Prior to Mendel’s work at the 
Monastery of St. Thomas in 
Brunn, he attended the 
University of Vienna. While at the University, his 
studies included Botany and Mathematics – training that would later prove to be 
invaluable.

Why did Mendel 
choose the pea plant 
(Pisum sativum) as an 
experimental model for 
his study?

The common pea plant was 
readily available throughout 
Europe. The plant was easy to grow 
and matured quickly.

The position of the sexual 
organs of the plant are entirely 
enclosed in the flower; this allowed Mendel control over 
how the plants reproduced. (i.e. self- or cross-pollination)

The plants exhibited seven 
distinct traits that could be 
easily observed between generations. Each trait had 
only two possible variations.

How did Mendel set up his 
experiment? The first thing he did was to 
set up a purebred plant population for each of the 
traits he wished to study.

This process ensured he was 
working with plants that were 
true-breeding. i.e. the plants produced 
predictable offspring when any 
two plants of the same purebred 
line were bred together.

Mendel prepared a purebred 
stock for each of the seven 
traits he studied. Each of these seven traits 
were easily identified in the 
physical appearance of the 
offspring generation.

The seven traits were: - seed shape - seed colour - flower colour - flower position - pod colour - pod shape - plant height

Before we jump in: - Solving genetics problems involves an understanding of 
PROBABILITY (the measure of 
how likely an event is). Complete the following:

To determine probabiliy: Mathematically To determine probabiliy: # of ways an event can occur = Total # of Outcomes

A Monohybrid Cross Mendel’s first experiment 
involved the crossing of a 
purebred tall plant with a purebred short plant. Mendel called this the P (parental) generation.

From this cross, the offspring 
were referred to as the F1 generation (first filial). These were called hybrids because they resulted from the crossing 
of two different purebred 
plants.

This type of cross is called 
monohybrid because only 
one trait, plant height, was involved.

A monohybrid cross… The P generation 
consists of one 
purebred tall and one purebred 
short plant P generation F1 generation The F1 generation consists of all tall 
plants

The results of the experiment Mendel’s first experiment 
showed that a ratio of 4 tall to 0 
short plants appeared in the F1 generation. That is, all of the 
first filial generation plants were 
tall.

This led Mendel to conclude 
that the trait for tall plants must 
be dominant over the 
recessive trait for short plants.

Dominant & Recessive Traits Dominant Trait – a 
characteristic that is always 
expressed in an individual. 
Only one dominant allele must 
be present for the dominant 
trait to be expressed.

Recessive Trait – a trait that is 
not expressed unless both 
alleles that code for the 
recessive trait are present. In Mendel’s experiments, he 
found that one characteristic 
was always dominant over 
another characteristic.

Mendel’s Principle of Dominance When two different 
purebred individuals are 
crossed, the offspring will 
only express the dominant 
characteristic or trait.

Back to Exp. 1 – Monohybrid Cross Punnett Square Tall Plant T Short 
Plant t

Mendel’s next experiment 
involved the crossing of two 
offspring plants from 
experiment one. He allowed the hybrid tall plants of the F1 generation to undergo self-
pollination.

Exp. 2 – Monohybrid Cross with F1 Offspring Punnett Square Tall Plant T t Tall 
Plant

This produced a second filial 
generation, that Mendel called the 
F2 generation. 75% (3/4) of plants in the F2 generation were tall while 25% (1/4) was short. This ratio of 3:1 has become 
known as the Mendelian Ratio.

Mendel drew four conclusions from his experiments: Each parent in the F1 
generation begins with two hereditary “factors”. Individual factors are either dominant, 
or recessive.

The factors are separated in 
the parent and each parent 
contributes only one factor to 
each offspring.

Each offspring inherits one factor from each parent Each offspring inherits one factor 
from each parent. If the dominant 
factor is inherited by the offspring, it 
will be expressed; even if the 
recessive factor is also present. The recessive factor will only be 
expressed if no dominant factor is 
present.

The Law of Segregation Inherited traits are 
determined by pairs of 
“factors”. These factors 
segregate (separate) when 
the gametes are formed. One 
factor from each pair is 
present in each gamete.

F1 Hybrid Cross (Showing Segregation) F1 generation F1 Hybrid cross Typical 3:1 
Mendelian 
Ratio F2 generation

What are “factors”? We now know that Mendel’s 
factors were genes. A gene can occur in one of two alternate 
forms called alleles.

When two alleles are present, 
a dominant allele is expressed 
and the recessive allele is not. 
Each of these alleles will pass 
on to the next generation 
where they may or may not be 
expressed.

The arrangement of alleles. Alleles can be identified by 
using a conventional two-letter 
code system. i.e. T (tall) or t (short)

The dominant trait name 
usually determines the letters of the alphabet to be used for the code. For example the two-letter code for a tall plant would be TT, with both alleles in the code being upper case for a dominant trait. The code 
for a short plant must be tt.

If the code read Tt, it should 
be understood that this 
individual would be hybrid tall – because of the presence of the 
lower case recessive allele.

The arrangement in which 
both alleles are represented by 
the same-case letter is called Homozygous. To clarify, one must indicate 
the organism as being either Homozygous dominant (TT), or 
as being Homozygous 
recessive (tt)

The arrangement in which 
both alleles are represented by 
two different-case letters is called Heterozygous.

The following terms will help you 
read about and describe heredity: Genotype refers to the alleles that an 
organism contains for a particular 
trait.

A tall-stem pea plant could have two different genotypes, TT and Tt A tall-stem pea plant could have two 
different genotypes, TT and Tt. A 
short-stem pea plant can have only 
one genotype, tt, since the allele for 
tall stems (T) is dominant over the 
allele for short stems (t).

Phenotype refers to the 
observable traits of an 
individual.

Since a pea plant can be tall or 
short, there are only two 
possible phenotypes for this 
characteristic. The tall phenotype may have 
two different genotypes, TT or 
Tt.

Sample Problem 1 Consider a cross between a pea plant that is 
heterozygous for round seeds and a pea plant that has 
wrinkled seeds. Determine the genotypes of the 
possible offspring.

Sample Problem 2 A plant that is homozygous for purple flowers 
is crossed with a plant that has white flowers. 
If the purple condition is dominant over the 
white condition, what are the genotypes and 
phenotypes of the F1 generation?

Sample Problem 3 Determine the genotypes of the parents if the following 
offspring are produced.