5.1: Understanding Inheritance
pg. 202
Selecting breeding is based on the idea that offspring will resemble
their parents. Therefore the must be a way that traits are passed on
from parents to the next generation. Most importantly there must
be away to pass on highly desirable traits, selecting these traits
must be possible.
Early Ideas About Inheritance
Aristotle (384 – 322 BC) was the first to propose a theory of
inheritance, called pangenesis. The egg and sperm are made up of
particles called pangenes, from all parts of the body. When the
sperm and egg are united the pangenes form the parts of the body
they came from. This was based on assumption, not
experimentation.
Antony Von Leeuwenhoek (1632 – 1723) in 1667, using a simple
single lens microscope he discovered living sperm in semen.
Although he could see the sperm, he believed he saw a complete
miniature person in the head of the sperm. It was believed the
father supplied the person and then developed in the mother.
In the 1800’s blending of characteristics became a working theory
of inheritance. The offspring had a blend of each of the parents
characteristics, and was irreversible, lost and would not appear in
future generations.
These theories were based on observation and not experimental
results or scientific evidence. These theories were eventually
disproved.
Figure 5.2: Between 1856 and 1863, Mendel bred, tended, and analyzed more than 28
000 pea plants in his monastery garden
An Austrian Monk, Gregor Mendel, through experimental
research, determined how traits are inherited. He is known as the
Father of Genetics.
Developing a Theory of Inheritance: Gregor Mendel’s
Experiments
Gregor Mendel (1822-1884) studied Botany and Mathematics. He
was able to sort out the mystery of inheritance, by studying the
reproduction of the pea plant. The laws that Mendel discovered are
the foundation of Theory of Inheritance, although that readily
accepted until other evidence in the 1900’s supported his work.
Mendel’s Pea Plants
Pea plants reproduce through sexual reproduction, and are able to
self fertilize and cross fertilize, because they have both male and
female gametes.
Mendel studied pea plants that self fertilized, the outcome was
100% offspring identical to the parent. This occurred from
generation after generation. This type of reproduction was
identified as True Breeding.
True Breeding – are organisms that exhibit the same traits
generation after generation.
Cross Breeding – is the fertilization of a female gamete of a
specific genetic origin with a gamete of a specific genetic origin.
Mendel also controlled his experiments, by selectively fertilizing a
female gamete with a specific male gamete, known as cross pollination.
The Results of Mendel’s True Breeding Crosses
P – Generation: are the organisms initially crossed and are
typically true breeding.
F1 – Generation: are the offspring of a cross of the P generation.
Monohybrid cross: is a cross of two individuals that differ by one
trait.
The characteristics of the pea plants were only expressed in two
distinguishable forms.
Table 5.1: The Seven Traits of Pea Plants Studied by Mendel
Each true breeding plant were used to start his experiments, these
were known as the parental or P generation. A true breeding plant,
with one form of trait was crossed with a true breeding plant with
the other form of trait. The outcome was known as the first filial or
F1 generation. This type of cross was known as a Monohybrid
cross.
Only one trait is monitored during the cross, and hybrid plants are
created, different from the true breeding parents.
Mendel crossed a true breeding yellow pea plant with a true
breeding green pea plant, and the offspring only expressed one
form of the trait, were 100% yellow pea plants. The green colour
disappeared in the F1 generation. This occurred in all of the seven
different traits expressed by peas.
Figure 5.3: After crossing true- breeding plants, only yellow seeds were produced in the
F1 generation. Note that a cross is represented by the symbol X.
The Results of Mendel’s F1 Crosses
Mendel then studied the results from the cross between members
of the F1 generation. The offspring from this cross was called the
second filial or F2 generation. Based on the results Mendel
realized that the green form of seed colour, did not really disappear
in the F1 generation, but instead was not expressed, and reappear
in the F2 generation. The ratio for the cross was 3:1, yellow to
green, known as the Mendelian ratio.
Figure 5.4: For F2 generation plants, the ratio of plants with yellow seeds to plants with
green seeds was approximately 3:1.
The Law of Segregation
Mendel concluded from is experiments and results from the F2
generation that there must be two “factors” for each trait expressed.
These factors are known as alleles (different forms of genes). A
diploid organism will carry two alleles for each gene or trait
expressed.
In the F1 generation, all the pea plants were yellow. Mendel
explained each parent pea plant donated one allele to the offspring,
a yellow allele and a green allele. Therefore yellow must dominate
over green, and yellow trait is expressed.
Mendel then developed the Law of Segregation, explaining that
each parent produced sex gametes through a process of meiosis,
where two alleles segregate, one allele in each gamete. When
fertilization occurs the offspring will have two alleles, one from
each parent. The trait expressed is dependent on whether they
inherited dominant or recessive alleles for the trait.
If the dominant allele is present (one or two), then the dominant
trait is expressed, but if there are two recessive alleles, then the
recessive trait is expressed.
Dominant: is the form of a trait that always appears when an
individual has an allele for it.
Recessive: is the form of a trait that only appears when an
individual has two alleles for it.
Law of Segregation: occurs when traits are determined by pairs of
alleles that segregate during meiosis so that each gamete receives
one allele.
Learning Check, questions 1 – 6, page 205
Genotype and Phenotype
Alleles are represented by using upper case and lower case letters.
The dominant allele is always upper case, using the first letter of
that characteristic. The recessive allele is always lower case, using
the same letter as the dominant allele.
Genotype: is the combination of alleles for any given trait, or the
organism’s entire genetic make up.
Phenotype: are the physical and physiological traits of an
organism.
Homozygous: the organism has two identical alleles for the gene.
Heterozygous: the organism has two different alleles for the gene.
Table 5.2: Genotype and Phenotypes of Pea Plants Seed Colour
Section 5.1 Review, questions 1 – 12, pg. 207