Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Gregor Mendel
He pioneered the
study of genetics
in the 1850s with
crossing
experiments on
garden peas Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Pea characteristics
studied by Mendel
Mendel chose for study
only characteristics that
occurred as two distinct
traits, such as flower
color: purple or white,
with no intermediates
Mendel’s
technique for crosspollinating
flowers from
different pea
varieties
Smooth x Wrinkled
Mendel crossed two pea
varieties with different seed
shapes: smooth and wrinkled.
While the F1 generation had
all smooth seeds, the wrinkled
trait reappeared in 25% of the
F2 generation. He concluded that individuals
must carry two versions of the
hereditary “factor” controlling
each trait.
Cummings M.R. (2006)
Meiosis
Campbell & Reece (2002)
This diagram represents only one pair of homologous chromosomes.
Meiosis I is unusual because the two homologs from each pair are
segregated into different daughter cells. (This is not so in mitosis.)
Principle of segregation, Punnett square
Can you spot an error in part (b) near the top?
From Campbell and Reece (2001)
Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Chromosomes in a plant cell
Pairs of Homologous Chromosomes
In most organisms,
chromosomes exist in pairs
of two homologs, one
derived from the paternal
parent and the other from
the maternal parent. Except
for the sex chromosomes,
homologs have similar
versions (alleles) of the
same genes.
Error: Lac/lac refers to the
gene for lactase, not lactose.
Separation of Homologs During Meiosis I
Campbell & Reece (2002)
This diagram tracks just one pair of homologous
chromosomes. The first meiotic division separates the
homologs of each pair, thus reducing the chromosome
number from 2n per diploid cell to n per gamete. Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Incomplete Dominance of Flower
Color in Snapdragon
When red and white
snapdragons are crossed,
the F1 generation has pink
flowers. In peas, the
equivalent cross had a
different result.
Cummings M.R. (2006)
Principle of segregation, Punnett square
Campbell and Reece (2001)
In snapdragon, the F1 and
half of the F2 of a red x white
cross have pink flowers.
These plants have only one
functional allele, R, of a gene
encoding an enzyme required
for making the red flower
pigment. Two copies of R are
necessary to make enough
pigment for red flowers. In peas, half the normal
dosage of an analogous gene
(P) is enough to make
adequate amounts of the
purple flower pigment, so that
PP and Pp genotypes produce
the same phenotype.
Codominant Expression of Blood Antigens
Cummings M.R. (2006)
The major human blood antigens (A and B) are encoded by a
gene located on chromosome 9 that exists in three alleles.
Allele IA encodes antigen A, allele IB encodes antigen B, and
allele IO encodes no antigen.
Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Brachydactyly Type BDA2 (OMIM 112600)
Photos and
radiograph
show hands
or feet of the
individuals
marked with
capital letters
in the
following
pedigree. Brachydactyly Type BDA2 (OMIM 112600)
Pedigrees of two human families with brachydactyly type BDA2. Circle
represent females, squares males. Filled symbols indicate disease phenotype.
The pedigrees indicate that brachydactyly is a dominant autosomal disorder.
Capital letters mark the individuals photographed in the preceding slide. Albinism (OMIM 203100)
Albinism is caused by the
lack of pigment in skin, hair,
and the retina of the eye.
Typically, it is a recessive
autosomal disorder, caused
by mutations in the gene for
tyrosinase, an enzyme
involved in the synthesis of
the black pigment melanin. Cummings M.R. (2006)
Segregation of Albinism
In humans as in pea
plants, homologous
alleles for albinism
segregate during
gamete formation.
As a recessive
autosomal disorder,
it shows up in 25%
of the offspring of a
heterozygous pair,
even though both
parents are
phenotypically
normal. Cummings M.R. (2006)
Red-green Color Blindness
Red-green color blindness is an X-linked recessive trait. In this pedigree,
Xn = normal X chromosome, Xp = X chromosome causing phenotype
Hemophilia and History
Cummings M.R. (2006)
Hemophilia (faulty blood clotting) is an X-linked recessive disorder.
Queen Victoria of England was its most prominent carrier. Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Marfan Syndrome Reveals Pleiotropic Gene
Cummings M.R. (2006)
Marfan syndrome is a dominant autosomal disorder of
the gene for fibrillin, a key protein in fibrous
connective tissue. Affected people are tall but tend to
have various abnormalities in their connective tissues,
including dilated blood vessels prone to rupture.
Antagonistic Pleiotropy of Mutant ß-Globin Alleles
Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Polygenic
Inheritance of Skin
Pigmentation
According to the hypothesis
shown here, three separate
genes affect the amount of
skin pigment independently.
Each dominant allele is
thought to contribute one
“unit” of pigmentation in an
additive way. The Punnett
square shows the seven
classes of skin pigmentation
that would result. Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Eugenics Movement
Cummings M.R. (2006)
To advocate governmental control of human procreation,
eugenicists set up exhibits and gave lectures at public events.
Pleiotropy, Polygeny & Environment
Many human genes affect multiple traits (pleiotropy), while many human traits
are affected by several genes (polygeny). In addition to genes, environmental
factors contribute to human traits and may influence natural selection.
Genetic Analysis of Peas and Humans
•  Patterns of Inheritance were recognized early
•  Experiments With Peas Reveal Basic Genetic
Principles
•  Mendel’s Ideas Were Better Understood After the
Discovery of Chromosomes
•  Molecular Biology Explains Dominance and
Recessiveness •  Pedigrees Reveal “Mendelian” Traits in Humans
•  A Single Gene May Affect Multiple Phenotypic Traits
•  A Single Trait May Be controlled by Multiple Genes
•  The Eugenics Movement Marfan heart
Cummings M.R. (2006)
Mendel’s rules of “segregation” and “independent
assortment” explain the results of dihybrid crosses.