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Patterns of Inheritance
Chapter 12
Early Ideas of Heredity
Before the 20th century, 2 concepts were the
basis for ideas about heredity:
-heredity occurs within species
-traits are transmitted directly from parent
to offspring
This led to the belief that inheritance is a
matter of blending traits from the parents.
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Early Ideas of Heredity
Botanists in the 18th and 19th centuries
produced hybrid plants.
When the hybrids were crossed with each
other, some of the offspring resembled the
original strains, rather than the hybrid
strains.
This evidence contradicted the idea that
traits are directly passed from parent to
offspring.
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Early Ideas of Heredity
Gregor Mendel
-chose to study pea plants because:
1. other research showed that pea hybrids
could be produced
2. many pea varieties were available
3. peas are small plants and easy to grow
4. peas can self-fertilize or be crossfertilized
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Early Ideas of Heredity
Mendel’s experimental method:
1. produce true-breeding strains for each
trait he was studying
2. cross-fertilize true-breeding strains having
alternate forms of a trait
-perform reciprocal crosses as well
3. allow the hybrid offspring to self-fertilize
and count the number of offspring showing
each form of the trait
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Monohybrid Crosses
F2 plants exhibited both forms of the trait in
a very specific pattern:
¾ plants with the dominant form
¼ plant with the recessive form
The dominant to recessive ratio was 3 : 1.
Mendel discovered the ratio is actually:
1 true-breeding dominant plant
2 not-true-breeding dominant plants
1 true-breeding recessive plant
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Monohybrid Crosses
gene: information for a trait passed from
parent to offspring
alleles: alternate forms of a gene
homozygous: having 2 of the same allele
heterozygous: having 2 different alleles
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Monohybrid Crosses
genotype: total set of alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
phenotype: outward appearance of an
individual
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Monohybrid Crosses
Principle of Segregation
Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization.
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Monohybrid Crosses
Some human traits are controlled by a single
gene.
-some of these exhibit dominant
inheritance
-some of these exhibit recessive
inheritance
Pedigree analysis is used to track
inheritance patterns in families.
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Testcross
Testcross: a cross used to determine the
genotype of an individual with dominant
phenotype
-cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
-the phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
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Extensions to Mendel
Mendel’s model of inheritance assumes that:
-each trait is controlled by a single gene
-each gene has only 2 alleles
-there is a clear dominant-recessive
relationship between the alleles
Most genes do not meet these criteria.
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Extensions to Mendel
Polygenic inheritance occurs when
multiple genes are involved in controlling
the phenotype of a trait.
The phenotype is an accumulation of
contributions by multiple genes.
These traits show continuous variation
and are referred to as quantitative traits.
For example – human height
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Extensions to Mendel
Pleiotropy refers to an allele which has
more than one effect on the phenotype.
This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia.
In these diseases, multiple symptoms can
be traced back to one defective allele.
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Extensions to Mendel
Incomplete dominance: the heterozygote
is intermediate in phenotype between the
2 homozygotes.
Codominance: the heterozygote shows
some aspect of the phenotypes of both
homozygotes.
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Extensions to Mendel
The human ABO blood group system
demonstrates:
-multiple alleles: there are 3 alleles of the I
gene (IA, IB, and i)
-codominance: IA and IB are dominant to i
but codominant to each other
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Extensions to Mendel
The expression of some genes can be
influenced by the environment.
for example: coat color in Himalayan rabbits
and Siamese cats
-an allele produces an enzyme that allows
pigment production only at temperatures
below 30oC
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Extensions to Mendel
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Extensions to Mendel
The products of some genes interact with
each other and influence the phenotype of
the individual.
Epistasis: one gene can interfere with the
expression of another gene
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