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Genetic Inheritance According to Gregor Mendel
CHAPTER 9
• Overview of Inheritance
• Mendel's Pea Plants
• Monohybrid Crosses and Segregation
• Definitions of Genetic Terms
• Dihybrid Crosses and Independent Assortment
• Test Crosses
• The Role of Probability
• Pedigrees and Genetic Conditions
• Human Single-Gene Disorders
Heritable Variation and Patterns of Inheritance
• Gregor Mendel
– Was the first
person to
analyze patterns
of inheritance.
– Deduced the
fundamental
principles of
genetics.
In an Abbey Garden
•
Mendel studied garden peas
•
Pea flowers have both male
and female parts
•
The ovary of the carpel
produces eggs by meiosis
•
The anthers of the stamens
produce pollen (sperm
equivalent) by meiosis
•
Pollen grains fly from the
anthers of the same flower or
from another plant to the carpel
and fertilize the eggs in the
ovary
•
Pea flowers are enclosed
within petals such that only
insect pollination can
effectively transfer pollen from
one plant to another
How Mendel Fertilized Pea Flower Eggs and Ensured a Single Pollen Source
Figure 9.4
• He also created true-breeding varieties of plants.
• Mendel then crossed two different true-breeding
varieties.
• Mendel performed many experiments.
– He tracked several characteristics in pea plants
from which he formulated several hypotheses.
True Breeding Varieties Available to Mendel
Figure 9.5
A Single-Factor (Monohybrid) Cross
Figure 9.6a
• Hypotheses From The Monohybrid Cross
– The letters P and p were used to designate flower
color: P = purple, p = white
– There are alternative forms of genes called alleles
– There are two gene forms (alleles) for every
characteristic in the plant
– Some alleles mask or hide the presence of other
alleles; these are dominant alleles symbolized by a
capital letter (eg. P).
– Alleles that can be masked are called recessive
alleles, symbolized by a lowercase letter (eg. p)
Allelic Combinations: Genotype
• Every cell in an organism carries two alleles at a
time for every characteristic
– For a pea plant flower color, a plant could have
cells with PP, Pp, or pp.
• Genotype combinations
– PP is homozygous dominant, yields purple flowers
– pp is homozygous recessive, yields white flowers
– Pp is heterozygous, yields purple flowers
Some Definitions
• Phenotype
– An organism’s physical traits
• Genotype
– An organism’s genetic makeup
• Mendel’s law of segregation
– The two members of an allele pair segregate
(separate) from each other during the production
of gametes.
Following Mendel’s True Breeding Cross
True breeding
varieties are
homozygous
PP
Meiosis
Gametes:
P
Or more simply:
P
P
P
pp
P
p
P
p
Meiosis
p
p
p
p
p
p
Next, determine all the
possible ways the gametes of
one parent can combine with
the gametes of another:
P
Pp
Pp
Fill out a Punnett Square
P
Pp
Pp
Genotype of the
offspring are all Pp
in the ratio:
4 Pp: 0 PP: 0 pp
(Genotypic ratio)
Phenotypic ratio
of the offspring:
4 purple: 0 white
Determining the F2 Offspring
Pp
x
Pp
Genetic Alleles and Homologous Chromosomes
• Homologous chromosomes
– Have genes at specific loci.
– Have alleles of a gene at the same locus.
Mendel’s Law of Independent Assortment
• A dihybrid cross
– Is the mating of parental varieties differing in two
characteristics.
• Two hypotheses for gene assortment in a dihybrid
cross are
– Dependent assortment.
– Independent assortment
• Mendel’s law of independent assortment states that
– Each pair of alleles segregates independently of
the other pairs during gamete formation.
Figure 9.8
Figure 9.23
Genes for Coat Color and Vision Sort Independently into Gametes
Figure 9.9
Using a Testcross to Determine an Unknown Genotype
•
A testcross is a mating between
– An individual of unknown genotype and a homozygous recessive
individual.
The Rules of Probability
• The rule of
multiplication
states that
• The
probability of
a compound
event is the
product of the
separate
probabilities of
the
independent
events.
Human Characteristics That Follow Mendel’s Laws
Figure 9.12
Using a Pedigree to Follow an Autosomal Recessive Gene
•
A family pedigree
– Shows the history of a trait in a family.
– Allows geneticists to analyze human traits.
Human Disorders Controlled by a Single Gene
•
Many human traits
– Show simple inheritance patterns.
– Are controlled by genes on autosomes.
Recessive Disorders
• Most human genetic disorders are recessive.
– Individuals can be carriers of these diseases.
Dominant Disorders
• Some human genetic
disorders are dominant.
– Achondroplasia is a
form of dwarfism.