Mendel's First Law of Genetics (Law
of Segregation)
• Mendel was able to demonstrate that traits
were passed from each parent to their
offspring through the inheritance of genes.
• Mendel's experimental organism was a
common garden pea (Pisum sativum), which
has a flower that lends itself to selfpollination.
Mendel's work showed:
1. Each parent contributes one factor of each
trait shown in offspring.
2. The two members of each pair of factors
segregate from each other during gamete
formation.
3. The blending theory of inheritance was
discounted.
4. Males and females contribute equally to the
traits in their offspring.
5. Acquired traits are not inherited.
Results from Mendel's Experiments
Parental
Cross
F2
F1
Phenotypic
Phenotype
Ratio
F2
Ratio
Round x
Wrinkled
Seed
Round
5474
Round:1850
Wrinkled
2.96:1
Yellow x
Green
Seeds
Yellow
6022
Yellow:2001
Green
3.01:1
Red x
White
Flowers
Red
705 Red:224
White
3.15:1
Tall x
Dwarf
Plants
Tall
l787 Tall:227
Dwarf
2.84:1
What is seen in the F1 generation?
We always see only one of the two parental
phenotypes in this generation.
The F1 possesses the information needed to
produce both parental phenotypes in the
following generation.
The F2 generation always produced a 3:1 ratio
where the dominant trait is present three times
as often as the recessive trait.
Mendel coined two terms to describe the
relationship of the two phenotypes based on the
F1 and F2 phenotypes.
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Mendel's Conclusions
1. The hereditary determinants are of a
particulate nature. These determinants are
called genes.
2. Each parent has a gene pair in each cell for
each trait studied. The F1 from a cross of
two pure lines contains one allele for the
dominant phenotype and one for the
recessive phenotype. These two alleles
comprise the gene pair.
3. One member of the gene pair segregates
into a gamete, thus each gamete only
carries one member of the gene pair.
4. Gametes unite at random and irrespective of
the other gene pairs involved.
Using symbols we can depict the cross of tall
and short pea plants in the following manner:
The F2 generation was created by selfing the F1
plants. This can be depicted graphically in a
Punnett square.
D
DD
Union of Gametes D
(Tall)
At Random
Dd
d
(Tall)
d
Dd
(Tall)
Punnett
Square
dd
(Short)
Genotypic ratio of F2: 1 DD : 2 Dd : 1 dd
Phenotypic ratio of F2: 3 tall : 1 dwarf
Mendel's First Law - the law of segregation;
during gamete formation each member of the
allelic pair separates from the other member to
form the genetic constitution of the gamete
Mendel performed one other cross to confirm
the hypothesis of segregation --- the backcross.
Backcross - the cross of an F1 hybrid to one of
the homozygous parents;
Mendel crossed it to a pure line, homozygote
dwarf plant.
Backcross: Dd x dd
Female
Gametes
D
d
Male
Gametes
d
DD
(Tall)
dd
(Short)
Backcross One or (BC1) Phenotypes: 1 Tall :
1 Dwarf
BC1 Genotypes: 1 Dd : 1 dd