Download Dihybrid Crosses

Mendel’s Second Law:
Law of Independent Assortment
Bellwork

Explain Mendel’s first Law of
Inheritance: the Law of Segregation.
Objectives


Explain the Mendel’s second law of
inheritance- the Law of Independent
Assortment
Explain the relationship between the
Law of Independent Assortment and
meiosis
Key terminology



Non-homologous chromosome
Dihybrid Cross
Independent Assortment of
Chromosomes
Monohybrid Crosses

Involves the transmission of a single trait
Next question Mendel Asked:

Would the possession of one particular trait
(e.g. seed shape) influence the segregation
of another trait (e.g. seed color) or would
they segregate independently?
Dihybrid Crosses


Mendel examined simultaneous
inheritance of two traits
The two traits he examined


Seed shape – Round (R, dominant)
versus Wrinkled (r, recessive)
Seed color – Yellow (Y, dominant)
versus green (y, recessive)
Think-Pair-Share
What distinguishes a
monohybrid cross from a
dihybrid cross?
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Developed two hypotheses


Hypothesis 1: Two traits segregate
together
Hypothesis 2: Two traits segregate
independently
Hypothesis 1: Two traits
segregate together



Round (R) always goes with yellow (Y)
Wrinkled (r) always goes with green (y)
Predicted Results from Punnett
Square

3 Round Yellow : 1 wrinkled green
Punnett Square of
Hypothesis 1
Hypothesis 2: Two Traits
Segregate Independently

Based on this hypothesis there are
4 possible gamete combinations



RY, Ry, rY, ry
Cross RrYy X RrYy
Predicted Results:
9 Round, Yellow: 3 wrinkled, Yellow: 3 Round, green: 1 wrinkled, green
Punnett Square of Hypothesis 2
Second Law of Inheritance

Law of Independent Assortment


Why


Each allele pair on non-homologous
chromosomes segregates independently of
other gene pairs during gamete formation
Because non-homologous chromosomes
line up randomly in the center of the cell and
pulled to different poles (N vs S)
How they assort determines the
homologous chromosomes (maternal vs
paternal) the gamete possesses
Think-Pair-Share
Explain the difference between a
homologous chromosome pair
and a non-homologous
chromosome pair.
PROPERTY OF PIMA COUNTY JTED, 2010
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3 Minute Paper
You have three minutes to write
IYOW the Law of Independent
Assortment
Be prepared to share.
PROPERTY OF PIMA COUNTY JTED, 2010
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