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Exploring Mendelian
Genetics
Law of Independent Assortment
• Does the segregation of one pair of alleles
affect the segregation of another pair of
alleles?
– Mendel performed an experiment to follow
two different genes from generation to
generation in a Two-Factor Cross = Dihybrid
Cross
Dihybrid Cross: F1 Generation
• Mendel crossed true-breeding plants that
produced only round yellow peas (RRYY)
with plants that produced wrinkled green
peas (rryy). All F1 offspring from this cross
had round yellow peas (RrYy)
• This cross produced hybrid plants for the
next cross…
Dihybrid Cross
Cross a Round, Yellow peas plant (RRYY) with
plants that produce Wrinkled, Green peas (rryy).
RRYY x rryy
R = round seed
RY
All offspring
produced have
Round, Yellow
peas.
ry
RrYy
Round,
Yellow
RY
RY
RY
RrYy
RrYy
RrYy
Round,
Yellow
Round,
Yellow
Round,
Yellow
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
ry
RrYy
r = wrinkled seed
Y = yellow seed
y = green seed
Dihybrid Cross: F2 Generation
– Mendel crossed two F1 plants (RrYy)
heterozygous for both seed shape and color
– Using a Punnett square, we can see that
alleles for seed shape segregate
independently of those for seed color = Law of
Independent Assortment – the inheritance of
one character has no effect on the inheritance
of another.
Dihybrid Cross
Cross two plants that are heterozygous for round,
yellow seeds (RrYy).
RrYy x RrYy
R = round seed
RY
Phenotype Ratio -
rY
ry
RRYy
RrYY
RrYy
Round,
Yellow
Round,
Yellow
Round,
Yellow
RRyy
RrYy
Rryy
Round,
Green
Round,
Yellow
Round,
Green
rY RrYY
RrYy
rrYY
rrYy
Round,
Yellow
Wrinkled,
Yellow
Wrinkled,
Yellow
RrYy
Rryy
rrYy
rryy
Round,
Green
Wrinkled,
Yellow
RY RRYY
Round,
Yellow
9 Round, Yellow
3 Round, Green
Ry
RRYy
Round,
Yellow
Ry
3 Wrinkled, Yellow
1 Wrinkled, Green
9 : 3 : 3 : 1 Ratio
Round,
Yellow
ry
Round,
Yellow
Wrinkled,
Green
r = wrinkled seed
Y = yellow seed
y = green seed
Dihybrid Cross
Genotype Ratio –
1 RRYY : 2 RRYy :
2 RrYY : ….Ahhh!
Punnett square
shows a 9:3:3:1
phenotypic ratio
–
9 yellow round : 3
yellow wrinkled : 3
green round: 1 green
wrinkled
Blind dog
Blind dog
Phenotypes
Genotypes
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
(a) Possible phenotypes of Labrador retrievers
Mating of double
heterozygotes
(black coat, normal vision)
BbNn
BbNn
Blind
Phenotypic
ratio of
offspring
9 black coat,
normal vision
(b) A Labrador dihybrid cross
3 black coat,
blind (PRA)
Blind
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
Summary of Mendel’s Principles
– Inheritance of biological information is determined by
genes and there are different versions (alleles)
– Each adult organism has two copies of each gene –
one from each parent (could then be heterozygous or
homozygous)
– Some forms of a gene may be dominant or recessive
– Alleles for different genes usually segregate
independently of one another (law of segregation/law
of independent assortment)