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NOTES –
Genetics:
Independent
Assortment &
Dihybrid
Crosses
Are traits inherited together?


Mendel wondered if the inheritance of one
trait had anything to do with the
inheritance of another
Dihybrid Cross (Two-Factor Cross) –
a cross where two traits are examined at
the same time

(EX. Tongue Rolling & PTC Tasting)
What did Mendel do?

Mendel looked at seed shape and seed
color




Shape:
Color:
R = round
Y = yellow
r = wrinkled
y = green
P1 = true-breeding round, yellow seed
plant with true breeding wrinkled, green
seed plant
P1 = RRYY x rryy
The F1 Generation
The F1 Generation



Each true-breeding round, yellow seed plant
produces gametes with one round allele and one
yellow allele (RY)
Each true-breeding wrinkled, green seed plant
produces gametes with one wrinkled allele and
one green allele (ry)
100% of the offspring are heterozygous for both
traits (RrYy) and produce round yellow seeds
The F2 Generation


When the F1 generation is allowed to crosspollinate (RrYy X RrYy), would round and
yellow stay together or would they separate?
What about wrinkled and green?
When Mendel crossed his F1 plants, he got
round, yellow seed plants and green, wrinkled
seed plants but he also got round, green seed
plants and wrinkled, yellow seed plants
The F2 Generation
The F2 Generation



Each heterozygous
round, yellow seed plant
can produce four
different allele
combinations in its
gametes (RY, Ry, rY,
ry)
The phenotypic ratio of the offspring was 9:3:3:1
Conclusion: The inheritance of one trait does not
influence the inheritance of another
The Principle of Independent
Assortment


Genes for different traits are inherited
independently from one another
EX. During pea plant gamete formation,
the gene that determines seed color
assorts independently from the gene that
determines seed shape

You can get round with yellow OR round with
green OR wrinkled with yellow OR wrinkled
with green