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Mendelian Patterns of Inheritance
Chapter 11
Mader: Biology 8th Ed.
Outline
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Blending Inheritance
Monohybrid Cross
– Law of Segregation
Modern Genetics
– Genotype vs. Phenotype
– Punnett Square
Dihybrid Cross
– Law of Independent Assortment
Human Genetic Disorders
Mader: Biology 8th Ed.
Gregor Mendel
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Austrian monk who formulated fundamental
laws of heredity in early 1860s.
– Studied science and mathematics at
University of Vienna.
– Conducted breeding experiments with the
garden pea Pisum sativum.
 Carefully gathered and documented
mathematical data from his
experiments.
Mader: Biology 8th Ed.
Blending Inheritance
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At the time, most breeders believed parents
of contrasting appearance always produce
offspring of intermediate appearance.
Mendel’s experiments helped him formulate
the particulate theory of inheritance.
– Inheritance involves reshuffling of genes
from generation to generation.
Mader: Biology 8th Ed.
Garden Peas
Mader: Biology 8th Ed.
One-Trait Inheritance
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Mendel performed cross-breeding
experiments between true-breeding plants.
– Chose varieties that differed in only one
trait (monohybrid cross)
– Performed reciprocal crosses
 Parental generation = P
 First generation offspring = F1
 Second generation offspring = F2
– Formulated law of segregation
Mader: Biology 8th Ed.
Mendel’s Monohybrid Cross
Mader: Biology 8th Ed.
Mendel’s Monohybrid Cross
Mader: Biology 8th Ed.
Law of Segregation
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Each individual has two factors for each trait.
The factors segregate during gamete
formation.
Each gamete contains only one factor from
each pair of factors.
Fertilization gives each new individual two
factors for each trait.
Mader: Biology 8th Ed.
Modern Genetics View
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Each trait in a pea plant is controlled by two
alleles (alternate forms of a gene).
Dominant allele (capital letter) masks the
expression of the recessive allele (lowercase).
Alleles occur on a homologous pair of
chromosomes at a particular gene locus.
– Homozygous = identical alleles
– Heterozygous = different alleles
Mader: Biology 8th Ed.
Homologous Chromosomes
Mader: Biology 8th Ed.
Genotype Versus Phenotype
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Genotype
– Refers to the alleles an individual receives
at fertilization.
Phenotype
– Refers to the physical appearance of the
individual.
Mader: Biology 8th Ed.
Punnett Square
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All possible kinds of sperm are lined up on
one axis, and all possible kinds of eggs are
lined up on the other axis.
– Every possible combination of alleles is
placed within the squares.
Mader: Biology 8th Ed.
Genetic Inheritance in Humans
Mader: Biology 8th Ed.
Testcross
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A monohybrid testcross is used to determine
if an individual with the dominant phenotype
is homozygous dominant or heterozygous
for a particular trait.
Mader: Biology 8th Ed.
Testcross
Mader: Biology 8th Ed.
Two-Trait Inheritance
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Mendel performed cross using true-breeding
plants differing in two traits.
– Dihybrid Cross
 Observed phenotypes among F2 plants.
 Formulated law of independent
assortment
Mader: Biology 8th Ed.
Law of Independent Assortment
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Each pair of factors segregates
independently of the other pairs.
All possible combinations of factors can
occur in the gametes.
Mader: Biology 8th Ed.
Dihybrid Cross
Mader: Biology 8th Ed.
Dihybrid Cross
Mader: Biology 8th Ed.
Two-Trait Testcross
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A two-trait testcross is used to determine if
an individual is homozygous dominant or
heterozygous for either of the two traits.
Mader: Biology 8th Ed.
Two-Trait Testcross
Mader: Biology 8th Ed.
Human Genetic Disorders
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Autosome - Any chromosome other than a
sex chromosome.
– When a genetic disorder is autosomal
dominant, an individual with AA or Aa has
the disorder.
– When a genetic disorder is autosomal
recessive, only aa individuals have the
disorder.
 Carriers - Individuals unaffected by a
disorder but can have an affected child.
Mader: Biology 8th Ed.
Autosomal Recessive Pedigree
Mader: Biology 8th Ed.
Autosomal Dominant Pedigree
Mader: Biology 8th Ed.
Autosomal Recessive Disorders
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Tay-Sachs Disease
– Progressive deterioration of psychomotor
functions.
Cystic Fibrosis
– Mucus in bronchial tubes and pancreatic
ducts is particularly thick and viscous.
Phenylketonuria
– Lack enzyme for normal metabolism of
phenylalanine.
Mader: Biology 8th Ed.
Autosomal Dominant Disorders
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Neurofibromatosis
– Tan or dark spots develop on skin and
darken. Small, benign tumors may arise
from fibrous nerve coverings.
Huntington Disease
– Neurological disorder leading to
progressive degeneration of brain cells, in
turn causing severe muscle spasms and
personality disorders.
Mader: Biology 8th Ed.
Incomplete Dominance
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Exhibited when the heterozygote has an
intermediate phenotype between that of
either homozygote.
Mader: Biology 8th Ed.
Incomplete Dominance
Mader: Biology 8th Ed.
Multiple Allelic Traits
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When a trait is controlled by multiple alleles,
the gene exists in several allelic forms.
– ABO blood types
Phenotype
Genotype
A
IAIA,IAi
B
IBIB,IBi
AB
IAIB
O
ii
Mader: Biology 8th Ed.
Inheritance of Blood Type
Mader: Biology 8th Ed.
Polygenic Inheritance
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Occurs when a trait is governed by two or
more sets of alleles.
– Each dominant allele has a quantitative
effect on the phenotype, and these effects
are additive.
 Result in continuous variation of
phenotypes.
Mader: Biology 8th Ed.
Height in Human Beings
Mader: Biology 8th Ed.
Terminology
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Pleiotropy - A gene that affects more than
one characteristic of an individual.
 Sickle-cell (incomplete dominance)
Codominance - More than one allele is fully
expressed.
– ABO blood type (multiple allelic traits)
Epistasis - A gene at one locus interferes
with the expression of a gene at a different
locus.
– Human skin color (polygenic inheritance)
Mader: Biology 8th Ed.
Environment and Phenotype
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Himalayan Rabbits - Enzyme coding for
black fur is active only at low temperatures.
– Black fur only occurs on extremities.
Mader: Biology 8th Ed.
Review
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Blending Inheritance
Monohybrid Cross
– Law of Segregation
Modern Genetics
– Genotype vs. Phenotype
– Punnett Square
Dihybrid Cross
– Law of Independent Assortment
Human Genetic Disorders
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.