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History
of
Genetics
Biology
Genetics
• How characteristics are transmitted from
parents to offspring
Gregor Mendel
Gregor Mendel
(1822-1884)
• worked with Pisum satium
(pea plant)
Mendel studied
7 characteristics
• Flower color
• Flower position
along stem
• Seed color
• Seed shape
• Pod shape
• Pod color
• Stem length
Mendel’s Experiments
Mendel’s First Cross
• Crossed “purebred” purple
with “purebred” white
+ P (parent) generation
- F1 – purple
• Crossed F1 hybrids
+ produced F2 generation
- 3 purple : 1 white
Mendel’s Ideas of Inheritance
• Alternative versions of genes (alleles) account for
variations in inherited characters.
+ each gene resides at a particular location of a
chromosome and can exist in variable forms (two different
forms for flower color)
P
Pp
p
Mendel’s Ideas of Inheritance
• For each characteristic, an organism inherits two alleles,
one from each parent.
+ an organism is either homozygous (two of the same
alleles) or heterozygous (two different alleles)
Homozygous
Heterozygous
Mendel’s Ideas of Inheritance
• If the two alleles differ, the dominant allele is fully
expressed and the recessive allele has no noticeable
affect on appearance.
+ heterozygotes express the dominant trait
RR
rr
Rr
Mendel’s Law of Segregation
• The two alleles for each character segregate during
gamete production.
+ ovum/sperm get only one of the two alleles (haploid)
P generation
Phenotype: Purple/White
Genotype: PP/pp
F1 generation
Phenotype: Purple
Genotype: Pp
F2 generation?
Genotype/Phenotype can
be predicted using a
Punnet Square
Mendel’s Law of Independent
Assortment
Each pair of alleles segregates into gametes independently.
WH
wh
Wh
wH
Mendel’s Law of Independent
Assortment
Parental Genotypes
Parental Gametes?
GGWW
ggww
GW
gw
GgWw
F1 Genotypes?
F1 Gametes?
GW
Gw
gW
gw
Beyond Mendel...
Different Types of
Inheritance:
Complete Dominance
Mendel’s Traits
Incomplete Dominance
Snapdragon Color
Incomplete Dominance
One allele does not dominant the other in the
heterozygous condition. Both alleles are expressed
but a new phenotype appears that is intermediate.
RR
Rr
rr
Codominance
Both alleles in the Heterozygous condition
are dominant.
CC
Cc
cc
Codominance & Multiple
Alleles