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Mendelian
Genetics
(I) Foundations of
Genetics
(A)
 Traits
Heredity
(characteristics) are passed down
from the parent to the offspring.
 Chromosomes of each gamete (sperm and
ova) contain the traits of the parent.
 The branch of science that is concerned
with the ways in which hereditary
information is transmitted to offspring is
known as Genetics.
(B)
Gregor Mendel
1. An Austrian monk who performed a series of
experiments with sweet peas (1856-1868).
2. He crossed different pea plants displaying
opposite traits, like tallness vs. shortness.
3. From this he wanted to see how these traits were
inherited by the offspring plants.
4. Based on his findings, Mendel proposed that
certain traits were inherited as a result of the
transmission of hereditary factors.
5. Mendel’s hereditary factors, called genes , are
located on the chromosomes.
6. Two genes (one from each parent)
called alleles determine the trait of the
offspring.
7. In the early 1900’s, T.H. Morgan
carried out breeding experiments
(crosses) with the fruit fly, Drosophila
melanogaster that supported Mendel’s
findings.
Background Information
1.
2.
Genes = alleles (found on chromosomes)
Capital letter = dominance
Lower-case letter = recessive
Ex: T = tallness which is dominant
t = shortness which is recessive
Homozygous
 Pure
 Both
alleles are the same
 TT = homozygous tall
 tt = homozygous short
Heterozygous
 Also
known as Hybrid
 Both alleles are different
 Tt = heterozygous or hybrid
(II) Major Concepts
Proposed by
Mendel
1. Principle of Dominance
 Also
referred to as Law of Dominance.
 States that the dominant allele will mask
(cover) the recessive allele if the pair is
hybrid.
 Tt = tall, since T is dominant for tallness.
 The only time that the recessive allele is
shown is when its homozygous recessive.
 tt = short
2. Principle of Segregation
 States
that the alleles of an allelic pair
will separate and then recombine to
form a new trait.
(In other words…the gene from each parent
will combine to produce the traits in the
offspring.)
 Ex: Punnett square
Drawing Punnet Squares
Ex: Tt x tt
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5
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7
Punnet Square
Genotype vs. Phenotype
 Genotype-
genetic makeup (is it
homozygous or heterozygous).
 Phenotype- the appearance of the
offspring due to the genotype.

Y = the dominant allele for yellow
y = the recessive allele for green

GENOTYPES
1. Homozygous Dominant (YY)
2. Heterozygous (Yy)
3. Homozygous Recessive (yy)

RESULTING PHENOTYPE
1. Yellow
2. Yellow
3. Green
(III)
P1
(parents)
Generations

F1
(offspring)

F2
(grandchildren)
(IV) Intermediate Inheritance
 The
hybrid offspring are phenotypically
different (look different) than their
homozygous parents.
 Two types:
1. Codominance
2. Incomplete Dominance
Codominance
Both alleles are
dominant.
 Mixture of the 2 traits.
 F1 generation = roan
cattle. Cattle that is
roan will have both
white hairs and red
hairs on it (mixture).

R = allele for red cattle
W = allele for white cattle
red x white ---> roan
RR x WW ---> 100% RW
(roan)
Incomplete Dominance
Think pink roses.
 Both alleles are
dominant.
 Blending of traits.
 F1 generation = pink
roses. The roses will beR = allele for red flowers
pink due to a blending W = allele for white flowers
of traits.
red x white ---> pink

RR x WW ---> 100% RW

In codominance and incomplete
dominance, the F2 generation is always
1:2:1
(V) Multiple Alleles
 More
than two alleles for a given
trait may be present within each
cell.
 Ex: blood types
Blood Types
I = dominant
i = recessive
http://www.borg.com/~lubehawk/multalle.htm
GENOTYPES
I AI A
IAi
RESULTING PHENOTYPES
Type A
Type A
I BI B
IBi
Type B
Type B
IAIB
Type AB
ii
Type O