Mendelisme
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
What is Inheritance?
• Passing on genetic information from parents
to offspring
• Humans have 23 pairs of chromosomes
(46 total)
– 1 of each pair is from one parent and the other
is from the other parent
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosomes
• Long strands of DNA that carry the genetic
information on building and sustaining a
living human being.
• Each chromosome contains many genes
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
What is a Gene?
• A segment of a chromosome that containing
the code for a single protein (enzyme)
- The enzyme causes a chemical reaction that
allows a trait to be expressed.
ie. Production of a pigment
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Alleles
• Alleles are alternate forms of a gene
– Homozygous individuals have two identical
alleles for a trait
– Heterozygous individuals have two different
alleles for a trait
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Gregor Mendel (1822 - 1884)
Mendel was a monk who was
trained in both biology and
mathematics.
He used his knowledge of
biology to do test crosses on
garden peas and count the
progeny.
He used his knowledge of
mathematics to manipulate his
data and derive his “postulates”
for inheritance.
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
In an Abbey Garden
• Mendel studied
garden peas
– These plant are
easily manipulated
– These plants can
self-fertilize
Stamen
Carpel
Figure 9.5
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• Mendel
carried out
some crossfertilizations
1 Removed
stamens from
purple flower
White
Stamens
Carpel
Parents
(P)
2 Transferred pollen
from stamens of
white flower to
Purple carpel of purple
flower
3 Pollinated carpel
matured into pod
4 Planted seeds
from pod
Offspring
(F1)
Figure 9.6
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s started with traits with homozygous
alleles He called these true breeding plants
Mendel’s plants showed simple dominance
Presence of the dominant allele meant that the
trait would be expressed whether homozygous or
heterozygous
For the recessive form of the trait, expression
only occurs in the homozygous form
Dominant alleles are shown as uppercase
letters
Recessive alleles are shown as lower case
letters
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Principles of Segregation
• Mendel performed many experiments
• He tracked several characteristics in pea
plants from which he formulated several
hypotheses
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Dominant
Flower color
Purple
Recessive
Axial
Terminal
Seed color
Yellow
Green
Round
Recessive
Pod shape
Inflated
Constricted
Pod Color
Green
Yellow
Stem length
Tall
White
Flower position
Seed shape
Dominant
Dwarf
Wrinkled
Figure 9.7
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Monohybrid
Crosses
• A monohybrid
cross is a cross
between parent
plants that differ
in only one
characteristic
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
All plants have
purple flowers
F1 Generation
Fertilization
among F1 plants
(F1  F1)
F2 Generation
3/
of plants
have purple
flowers
4
(a) Mendel’s crosses tracking one
characteristic (flower color)
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1/
of plants
have white
flowers
4
Figure 9.8a
• Mendel developed four hypotheses
from the monohybrid cross
– There are alternative forms of genes,
now called alleles
– For each characteristic, each organism
has two genes
– Gametes carry only one allele for each
inherited characteristic
– Alleles can be dominant or recessive
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
An explanation
of Mendel’s
results,
including a
Punnett
square
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Wild type traits are those most
often seen in nature
Selective breeding brings out selected alleles for a trait
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
• Phenotype
– An organism’s physical traits
– How does it look?
• Genotype
– An organism’s genetic makeup
– What alleles are present?
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s principle of segregation
• Pairs of alleles segregate (separate) during
gamete formation; the fusion of gametes at
fertilization creates allele pairs again
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Principle of Independent
Assortment
• Two hypotheses for gene assortment in a
dihybrid cross
– Dependent assortment
– Independent assortment
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Using a Testcross to Determine
an Unknown Genotype
• A testcross is a
mating between
– An individual of
unknown genotype
and
– A homozygous
recessive
individual
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Family Pedigrees
• Mendel’s principles
apply to the
inheritance of many
human traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Figure 9.14
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
• A family pedigree
– Shows the history of a trait in a family
– Allows researchers to analyze human traits
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Human Disorders Controlled by a
Single Gene
• Many human traits
– Show simple inheritance patterns
– Are controlled by genes on autosomes
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Table 9.1
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Recessive Disorders
• Most human
genetic disorders
are recessive
• Individuals can
be carriers of
these diseases
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Dominant Disorders
• Some human genetic
disorders are dominant
– Achondroplasia is a form
of dwarfism
Figure 9.17
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings