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Inheritance of one trait
Gregor Mendel
Pages 123 -134
• Gregor Mendel (1822–1884), an Austrian
monk,
• studied garden pea plants as means to
explain the inheritance of characteristics.
• He conducted a series of experiments on plants over an
eight-year period (1853–1861). Following his experiments,
Mendel published a paper outlining his conclusions.
• Mendel conducted his research using the common pea
plant (Pisum sativum),
.
These plants were easy to cultivate and had several
traits that could be studied
The seven traits that Mendel studied
How did Mendel set up his
experiment?
• The first thing he needed to do was to
obtain purebred plants for the trait he
wanted to study. A purebred organism is
descended from ancestors of a distinct
type, or breed. Purebred organisms in a
given species or variety all share similar
traits.
Mendel’s First Experiment:
A Monohybrid Cross
parent generation, P generation.
Offspring filial generation, F1
generation.
monohybrid cross,
A dominant trait is a characteristic
that is always expressed,
A recessive trait is a characteristic
that is latent (present but inactive)
The trait for tall pea plants was
dominant over the trait for short
plants.
All the pea plants grew tall if they
possessed the dominant trait for
size — tall.
The results …
Mendel conducted this experiment many times
using true-breeding plants for each of the seven
traits he had chosen to study. He obtained the
same results every time: one trait was dominant
over the other. Mendel concluded that heredity
was definitely not just a blending of traits. He
also concluded that when plants with two
contrasting traits are crossed, one trait is always
dominant over the other. This led him to
formulate the principle of dominance: when
individuals with contrasting traits are crossed,
the offspring will express only the dominant trait.
Law of Segregation
• The next experiment Mendel conducted
involved breeding the F1 generation. He
allowed the hybrid tall plants of the F1
generation to self-pollinate. This produced
the second filial generation, or F2
generation.
Mendelian ratio
• Three out of four plants in the F2 generation
were tall while one was short. Mendel repeated
this experiment many times and examined all
seven traits. He obtained the same results time
after time. The F2 generation resembled one
parent from the P generation 75% of the time
and the other parent from the P generation 25%
of the time. This ratio of 3 : 1 is known as the
Mendelian ratio.
Based on his observations of traits
in the F1 and F2 generations, Mendel drew the
following conclusions:
• Each parent in his F1 generation starts with two
hereditary “factors.” One factor is dominant and
the other is recessive.
• The factors separate in the parent. Only one
factor from each parent is contributed to the
offspring.
• Each offspring inherits one factor from each
parent. If the dominant factor is present it will be
expressed even if the recessive factor is also
present.
• The recessive factor will be expressed if only
recessive factors are present.
The law of segregation
• The law of segregation states that
inherited traits are determined by pairs of
“factors.” These factors segregate
(separate) in the gametes, with one in
each gamete.
Genes & alleles
• We know today that Mendel’s “factors”
were genes, the part of the chromosome
that governs the expression of a particular
trait.
• A gene can occur in alternate forms called
alleles.
Using letters to represent the different
alleles, a purebred tall pea plant will
have two uppercase letters, TT.
Homozygous
(the two alleles are the
same).
Each parent contributes 1
allele to the offspring.
Heterozygous
(the two alleles are
different.)
Ratio of 3 dominant
traits to 1 recessive
trait.
The dominant trait
can be either
homozygous or
heterozygous.
The recessive trait is
always homozygous.
Punnett Squares
A Punnett square
is used to calculate the
probability of inheriting a
particular trait.
The genotype is the
genetic make-up of an
organism.
The phenotype is the
appearance of the trait in
an organism.
Review of Terms:
•
•
•
•
•
•
•
•
Allele
Gene
Purebred
Homozygous
Heterozygous
Punnett square
Phenotype
Genotype
•
•
•
•
Dominant
Recessive
Hybrid
Filial
Test Cross
• In order to determine the genotype of an
individual, you must perform a test cross.
A test cross involves crossing an individual
of unknown genotype with a homozygous
recessive individual. The offspring will
exhibit certain phenotypes that will allow
you to determine if your unknown is either
homozygous dominant or heterozygous.
Example
• A woman has a father who died of
Huntington’s disease. What is the
probability that she will develop the
symptoms of the disease?
(Hint: Huntington’ disease is caused by a
recessive allele.)
GRASP
• Given: Huntington’s is recessive
Father died of Huntington’s
therefore father must carry 2
recessive genes.
Required: Probability daughter will develop
Huntington’s disease.
• Analysis:
let “h” = Huntington’s allele
let “H” = normal allele.
Father’s genotype = “hh” father will give one allele
to daughter.
• Substitute: If mother is “normal” then probability is:
H
H
h
Hh
Hh
h
Hh
Hh
mother
father
• Solve: daughter will NOT develop Huntington’s but will carry the
Huntington’s disease allele.
• If mother is a carrier of the Huntington’s allele
(Hh) then the probability is
h
h
father
H
h
Hh
hh
Hh
hh
mother
50:50 chance
that the daughter
will inherit a
recessive gene
from both parents
and therefore she
would develop
Huntington’s
disease.