the dihybrid cross
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THE DIHYBRID CROSS. Studying the inheritance of two characters simultaneously SBI3U. Independent Assortment. Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?. Mendel performed dihybrid crosses to find out. - PowerPoint PPT PresentationTRANSCRIPT
THE DIHYBRID CROSS
Studying the inheritance of two characters simultaneously
SBI3U
Independent Independent AssortmentAssortment
Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?
Mendel performed dihybrid crosses to find out.
Mendel’s conclusion: Different characters are inherited independently.
Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another?
The alternative and incorrect hypothesis: dependent inheritance.
Because it’s the only way to explain the pattern of inheritance.
4
The Punnett Square
Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.
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The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment
The alignment of one pair of homologs is independent of any other.
Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.
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Peas & Humans
Consider a cross between parents heterozygous for both deafness and albinism.
This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.
Mendel’s peas
Character Trait Allele
Seed shape Round R
Wrinkled r
Cotyledon colour Yellow Y
Green y
Combinations
Genotype Phenotype
RRYY Round Yellow
RRYy Round Yellow
RrYY Round Yellow
RrYy Round Yellow
RRyy Round Green
Rryy Round Green
rrYY Wrinkled Yellow
rrYy Wrinkled Yellow
rryy Wrinkled Green
Is the inheritance of one character
affected by the inheritance of another?
P Phenotypes Round Yellow
x Wrinkled Green
(Pure Bred)
F1 Phenotypes All Round Yellow
(Selfed)
F2 Phenotypes Seed colour
Yellow Green TOTAL RATIO
Seed Round 315 108 423
shape Wrinkled 101 32 133
TOTAL 416 140 556
RATIO
Is the inheritance of one character
affected by the inheritance of another?
P Phenotypes Round Yellow
x Wrinkled Green
(Pure Bred)
F1 Phenotypes All Round Yellow
(Selfed)
F2 Phenotypes Seed colour
Yellow Green TOTAL RATIO
Seed Round 315 108 423 3.18
shape Wrinkled 101 32 133 1
TOTAL 416 140 556
RATIO 2.97 1
A dihybrid cross can be treated as two
separate monohybrid crosses
The expected probability of each type of seed can be calculated:
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled =
Probability of an F2 seed being yellow =
Probability of an F2 seed being green =
A dihybrid cross can be treated as two
separate monohybrid crosses
The expected probability of each type of seed can be calculated:
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled = 25% or ¼
Probability of an F2 seed being yellow = 75% or ¾
Probability of an F2 seed being green = 25% or ¼
Therefore
Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25%
Probability of an F2 seed being round and green =
Probability of an F2 seed being wrinkled and yellow =
Probability of an F2 seed being wrinkled and green =
Therefore
Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25%
Probability of an F2 seed being round and green = ¾ x ¼ = 3/16 = 18.75%
Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75%
Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16 = 6.25%
Predicting how many seeds we could expect to
get in a sample We could expect
What Mendel observed
556 x 9/16 round yellow 313 315
556 x 3/16 round green 108
556 x 3/16 wrinkled yellow
101
556 x 1/16 wrinkled green 32
Predicting how many seeds we could expect
to get in a sample We could expect
What Mendel observed
556 x 9/16 round yellow 313 315
556 x 3/16 round green 104 108
556 x 3/16 wrinkled yellow
104 101
556 x 1/16 wrinkled green 35 32
THE LAW OF INDEPENDENT ASSORTMENT
It appears that the inheritance of seed shape has no influence over the inheritance of seed colour
The two characters are inherited INDEPENDENTLY
The pairs of alleles that control these two characters assort themselves independently
Mendel & Meiosis
The pairs of chromosomes could orientate in different ways at Anaphase 1
P Phenotypes
Round Yellow seed
X Wrinkled Green seed
(Pure bred)
Genotypes
RRYY rryy
meiosis meiosis
Gametes RY ry
fertilisation
F
1
Phenotypes
RrYy (Selfed)
Genotypes
Round Yellow
Proportions
100%
Dihybrid cross genetic diagram
F
1
Phenotypes
RrYy (Selfed)
Genotypes
Round Yellow
Proportions
100%
meiosis
Gametes Y y
R RY Ry
r rY ry
Dihybrid cross genetic diagram
Gametes Y y
R RY Ry
r rY ry
fertilisation
F2 Genotypes RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
Dihybrid cross genetic diagram
Dihybrid cross proportions
Phenotypes ProportionsRound Yellow 9/16 or 56.25%
Round Green
Wrinkled Yellow
Wrinkled Green
Dihybrid cross proportions
Phenotypes ProportionsRound Yellow 9/16 or 56.25%
Round Green 3/16 or 18.75%
Wrinkled Yellow
3/16 or 18.75%
Wrinkled Green
1/16 or 6.25%
Dihybrid test cross
In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross
This is done with a homozygous recessive (rr) individual
The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy)
Dihybrid test cross
Phenotypes
Round Yellow X Wrinkled Green
Genotypes RrYy rryy
Gametes RY, Ry, rY, ry ry
Genotypes RY Ry rY ry
ry RrYy Rryy rrYy rryy
Phenotypes
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Proportions
1/4 or 25%
1/4 or 25%
1/4 or 25%
1/4 or 25%