Mendel Discovered that
Inheritance Follows
Rules of Chance
OBJECTIVE:
Explain Mendel’s Principles of Segregation
And Independent Assortment
•Mendel crossed
purple with white
and got all purple
•Was the factor for
white flowers now
lost forever?
•Mendel crossed
the F1 generation
and the answer
was NO
 F1 GENERATION
WERE ALL PURPLE
F2 GENERATION 
WERE ¾ PURPLE
AND ¼ WHITE
RESULTS:
 The factor for white flowers did NOT
disappear in the F1 plants
 So what happened?
 The F1 generation only displayed purple
flowers because purple is dominant
 The F2 generation displayed white
flowers in addition to the purple because
the recessive gene showed up.
Mendel’s 4 Hypotheses:
1. There are alternative forms of genes called
alleles
Gene : Flower :: Allele : Purple or White
Gene : Eye color :: Allele : Blue or Brown
2. For each inherited gene, an organism has
two alleles (one from each parent)
 Homozygous: alleles are the same (PP or pp)
 Heterozygous: alleles are different (Pp)
Mendel’s Hypotheses
continued….
3.
4.
Homozygous dominant (PP) Purple
Homozygous recessive (pp) White
Heterozygous dominant (Pp) Purple, but the
individual carries the recessive allele
The two alleles for a gene segregate
(separate) during the formation of gametes
(sex cells).
Each gamete has one allele.
When sperm and egg fuse, so do the alleles.
This is how you get traits from both mom and dad
Law of Segregation~
• every individual has two alleles of each gene
• the two alleles for each trait separate when
gametes are made.
• each gamete receives one of these alleles.
Law of Segregation
Mendel’s Law of Segregation
• First, Mendel crossed yellow and green peas.
•Green is dominant and yellow is recessive.
RESULT: All of the offspring were green.
• Next, he crossed two of the green offspring.
RESULT: Yellow peas reappeared!
CONCLUSION:
Alleles can separate from each other during
meiosis.
Mendel called this the Law of Segregation.
Dihybrid cross
• A cross in which the parents differ from each
other in two traits.
• Question to be answered:
In a dihybrid cross, will the two traits stay
together in the next generation or will they
be inherited independently of each other?
First Generation
Second Generation
DIHYBRID CROSS:
results in 9:3:3:1 phenotypic ratio
What would be the result of a Dihybrid Cross?
• Example: shape and color of pea.
• Mendel crossed round, yellow peas with green,
wrinkled peas.
• All F1 peas were round and yellow.
• He allowed the F1 plants to self fertilize
• The resulting F2 plants exhibited four different
phenotypes.
Law of Independent Assortment
• Genes for different traits are inherited
independently of each other.
• For example:
When a pea plant with the genotype RrYy
produces gametes, two things happen:
1. the alleles R and r will separate from each other
law of segregation
2. they will separate from the alleles Y and y
law of independent assortment
Principle of Independent Assortment
• This law states that allele pairs separate
independently during the formation of gametes.
– During gamete formation in an F2 cross, a
particular allele for one character can be paired with
either allele of another character.
• The alleles for different genes are sorted into
gametes independently of one another.
– Therefore, traits are transmitted to offspring
independently of one another.