THE DIHYBRID CROSS Studying the inheritance of two

THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously SBI 3 U

Independent Assortment 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.

The Punnett Square Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment. 4

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. 5

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. 6

Mendel’s peas Character Cotyledon colour Allele Round R Wrinkled Seed shape Trait r Yellow Y Green y

Combinations Genotype RRYY RRYy Rr. YY Rr. Yy RRyy Rryy rr. YY rr. Yy rryy Phenotype Round Yellow Round Green Wrinkled Yellow Wrinkled Green

Is the inheritance of one character affected by the inheritance of another? P Phenotypes Round Yellow F 1 Phenotypes F 2 Phenotypes x Wrinkled Green All Round Yellow (Selfed) Seed colour Yellow Green TOTAL Seed Round 315 108 423 shape Wrinkled 101 32 133 TOTAL 416 140 556 RATIO (Pure Bred) RATIO

Is the inheritance of one character affected by the inheritance of another? P Phenotypes Round Yellow F 1 Phenotypes F 2 Phenotypes x Wrinkled Green All Round Yellow (Pure Bred) (Selfed) 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 F 2 seed being round = 75% or ¾ Probability of an F 2 seed being wrinkled = Probability of an F 2 seed being yellow = Probability of an F 2 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 F 2 seed being round = 75% or ¾ Probability of an F 2 seed being wrinkled = 25% or ¼ Probability of an F 2 seed being yellow = 75% or ¾ Probability of an F 2 seed being green = 25% or ¼

Therefore ¢ Probability of an F 2 seed being round and yellow = ¾ x ¾ = 9/16 = 56. 25% ¢ Probability of an F 2 seed being round and green = ¢ Probability of an F 2 seed being wrinkled and yellow = ¢ Probability of an F 2 seed being wrinkled and green =

Therefore ¢ Probability of an F 2 seed being round and yellow = ¾ x ¾ = 9/16 = 56. 25% ¢ Probability of an F 2 seed being round and green = ¾ x ¼ = 3/16 = 18. 75% ¢ Probability of an F 2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18. 75% ¢ Probability of an F 2 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 556 x 9/16 round yellow What Mendel observed 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

Dihybrid cross genetic diagram P Phenotypes Round Yellow seed Genotypes RRYY rryy meiosis RY ry Gametes X Wrinkled Green seed (Pure bred) fertilisation F 1 Phenotypes Rr. Yy Genotypes Round Yellow Proportions 100% (Selfed)

Dihybrid cross genetic diagram F 1 Phenotypes Rr. Yy Genotypes Round Yellow Proportions (Selfed) 100% meiosis Gametes Y y R RY Ry r r. Y ry

Dihybrid cross genetic diagram Gametes Y y R RY Ry r r. Y ry fertilisation F 2 Genotypes RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy

Dihybrid cross proportions Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions 9/16 or 56. 25%

Dihybrid cross proportions Phenotypes Round Yellow Proportions 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 Genotypes Round Yellow Rr. Yy Gametes RY, Ry, r. Y, ry Genotypes Proportions Wrinkled Green rryy ry RY ry Phenotypes X Ry r. Y ry Rr. Yy Rryy rr. Yy rryy Round Wrinkled Yellow Green 1/4 or 25%