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GENETICS
CH. 12
(and 10.1)
Origins of Genetics:
A. Gregor Mendel: Father of Modern
Genetics
1. Austrian monk
2. Spent 14 yrs. Working in the
monastery, taught high school, and was
also in charge of the garden.
3. Changed biology forever with his
work.
Mendel’s Breeding
Experiments
1) Bred different varieties of the garden pea in an
attempt to understand heredity.
2) Useful Features of Peas:
 Several traits show different forms
(alternating) Ex. Tall/Short, Yellow/Green
 Self-pollinating & Cross-pollinating
 Easy to grow
 Produce many offspring
 True-breeding – if allowed to self-pollinate
they would produce offspring identical to
themselves.
The Origin of Genetics
• Mendel cross-pollinated two different plants with
different characteristics and then studies the
results (cross-bred).
• Trait – specific characteristics that varies from one
individual to another.
• By studying two contrasting characteristic plants
and their offspring he created hybrids
(heterozygous).
• Biological inheritance is determined by factors
that are passed from one generation to the next.
• Gene- chemical factor that determines traits
(segments of DNA that code for proteins!)
Genetic Principles:
• Alleles – different forms of a gene. Ex.
Height – Tall (T) & Short (t)
• Discovered the principle of dominance –
some alleles are dominant and others are
recessive.
Rules:
 The dominant allele for a particular trait
will ALWAYS exhibit that form of the trait.
 Recessive alleles will only be shown
when the dominant allele is NOT present.
RESULTS:
• Crossing 2 parents
that were true bred
(either fully dominant
or fully recessive)
created offspring that
all showed the
dominant trait.
Example: flowers
PP (purple) x pp
(white)
P
P
p
Pp
Pp
p
Pp
Pp
RESULTS:
• Second generation
crossing Pp x Pp
created a 3:1 ratio of
purple to white
flower plants
• Purple – 75%
• White – 25%
• PP – 25%, Pp – 50%,
pp – 25%
P
p
P
p
PP
Purple
Pp
purple
Pp
purple
Pp
white
Mendel’s Theory:
1) Hypothesis: Different versions of a gene
are called alleles. An individual usually has
2 alleles for a gene, each inherited from a
different parent.
2) Individuals with the same two alleles for a
gene are homozygous.
3) Individuals with the two different alleles
are heterozygous for that trait.
GENOTYPE &
PHENOTYPE
4) Genotype & Phenotype: The set of alleles
that an individual has is called its
GENOTYPE (G = GENETIC)
The physical expression of a trait or
outward appearance of the genotype is
called the PHENOTYPE (P = PHYSICAL)
Ex. Bb – genotype (B-brown, b-blue)
Brown - phenotype
Laws of Heredity:
• Law of Segregation: The different alleles
for a trait separate when gametes are
formed.
• Law of Independent Assortment: The genes
for different traits can segregate
independently during the formation of
gametes.
• Both laws account for the many genetic
variations of organisms.
Laws of Heredity
Punnett Squares
Punnett Squares: Show the different gene
combinations that might result from a genetic
cross. Used to predict and compare the genetic
variations that will result from a cross.
• Probability can be used to predict the outcomes
of a cross (% and ratios).
• Monohybrid cross – compares the outcome of 1
set of alleles or 1 trait.
• Dihybrid Cross – compares the outcome of 2 sets
of alleles 2 traits.