Inheritance
Genetics in the 1860’s
Gregor Mendel
Never saw or knew
about chromosomes
Deduced the
principles of genetic
inheritance using
garden peas
Genetics in the 1860’s
The pea flower is
easy to manipulate
and control
fertilization
experiments
Genetics in the 1860’s
Could control selffertilization (produce
‘true breeders’)
Cross-fertilization
between two plants that
have a variation in a
trait (flower color)
Collect seeds and see
the traits (genes) of the
next generation
Mendelian Genetics
Each trait usually had two forms…can you
guess why?
Mendelian Genetics
How do alleles differ
from genes?
In-Class Activity
Why were the
flowers all purple in
F1 generation?
Why did the white
flower re-appear in
the F2 generation?
Homologous
Chromosomes
Dominance and
recessiveness
Some Mendel Terms
Homozygous: when the alleles inherited
on both homologous chromosomes are
the same
Mom = purple flower (P)
 Dad = purple flower (P)
 Could also be white flower

Heterozygous: when the alleles inherited
are different
Mom = purple flower (P)
 Dad = white flower (p)

In-class Activity
Make a prediction as to what offspring
will be produced from the following
matings:
White flower with White flower
 Heterozygous with Homozygous recessive
 Purple flower with White flower

Testcross
Who can tell what the
genotype is of a
dominant trait?


Black coat could be BB
or Bb
Testcross is used to
identify unknown
dominants
Principle of Independent
Assortment
Each pair of alleles segregates
independent of the other pairs during
gamete formation
Deduce from a dihybrid cross
Dihybrid Cross
Independent Assortment
Independent Assortment
Pedigrees and Family
Trees
Follow traits from one
generation to the next
How could geneticists
use this information?
In-class Activity
If Jonathon and
Elizabeth had another
child (whew!), what is
the probability that the
child will be deaf?
Recessive Disorders
Dominant Disorders
Family Pedigrees
Variations in Mendel’s
Principles
Some patterns of genetic inheritance
can’t be explained by probabilities of
dominant and recessive traits
Especially traits that exist in more that
two forms (hair color, skin color, eye
color, etc.)
Incomplete Dominance
In heterozygous
individuals, no
dominance is shown
Incomplete Dominance
Human hypercholesterolemia
Codominance & Multiple
Alleles
There are more than
just two blood types

A, B, AB, O
Three alleles exist
(IA, IB, i)
Codominance & Multiple
Alleles
Pleiotropy
Polygenic Inheritance
More than one gene affects the phenotype of
a trait
Skin/eye color is the product of many genes
Epistasis
Nature vs Nurture
Hydrangea plants
vary color but have
same alleles
Differences occur
because of soil
acidity
From Mendel to Chromosome
Mitosis (1875); meiosis (1890) processes
By 1900’s
Association of chromosome behavior with
Mendelian genetics
 Chromosomes – genes are both paired
 Homologues separate (Independent
Assortment)

Walter Sutton, Theodor Boveri & others
(1902)

Chromosomal Theory of Inheritance
Chromosomes & Genes
T. H. Morgan and
his fruit flies
Male Color Blindness
Hemophilia in the Royal
Family of Russia
Chromosomes & Linked
Genes
Genetic Recombination
Undergraduate of Morgan named Sturtevant
Reasoned the greater the recombination, the
farther apart