Outline
Mendelian Patterns of Inheritance
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Chapter 11
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Mader: Biology 8th Ed.
Blending Inheritance
Monohybrid Cross
– Law of Segregation
Modern Genetics
– Genotype vs. Phenotype
– Punnett Square
Dihybrid Cross
– Law of Independent Assortment
Human Genetic Disorders
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Fig. 11.1
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.
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Garden Peas
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.
Mader: Biology 8th Ed.
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Fig. 11.2
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 = F
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Second generation offspring = F2
Formulated law of segregation
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Mendel’s Monohybrid Cross
Mendel’s Monohybrid Cross
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
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Law of Segregation
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Modern Genetics View
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.
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Mader: Biology 8th Ed.
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
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Homologous Chromosomes
Genotype Versus Phenotype
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Mader: Biology 8th Ed.
Genotype
– Refers to the alleles an individual
receives at fertilization.
Phenotype
– Refers to the physical appearance of the
individual.
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Punnett Square
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Genetic Inheritance in Humans
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.
Mader: Biology 8th Ed.
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Testcross
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Testcross
A monohybrid testcross is used to
determine if an individual with the dominant
phenotype is homozygous dominant or
heterozygous for a particular trait.
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Mader: Biology 8th Ed.
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Two-Trait Inheritance
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Law of Independent Assortment
Mendel performed cross using truebreeding plants differing in two traits.
– Dihybrid Cross
! Observed phenotypes among F plants.
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Each pair of factors segregates
independently of the other pairs.
All possible combinations of factors can
occur in the gametes.
Formulated law of independent
assortment
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
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Dihybrid Cross
Dihybrid Cross
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Mader: Biology 8th Ed.
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Two-Trait Testcross
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.
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Mader: Biology 8th Ed.
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Autosomal Recessive Pedigree
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.
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Mader: Biology 8th Ed.
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Autosomal Dominant Pedigree
Autosomal Recessive Disorders
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Mader: Biology 8th Ed.
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.
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Autosomal Dominant Disorders
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Incomplete Dominance
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.
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Mader: Biology 8th Ed.
Exhibited when the heterozygote has an
intermediate phenotype between that of
either homozygote.
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Incomplete Dominance
Multiple Allelic Traits
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Mader: Biology 8th Ed.
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
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Inheritance of Blood Type
Polygenic Inheritance
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Mader: Biology 8th Ed.
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.
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Height in Human Beings
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)
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Review
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.
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Mader: Biology 8th Ed.
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.
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Mader: Biology 8th Ed.
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