THE DIHYBRID CROSS Studying the inheritance of two

Mendel’s peas Character Seed shape Cotyledon colour © 2016 Paul Billiet ODWS Trait Allele

Combinations Genotype RRYY RRYy Rr. YY Rr. Yy RRyy Rryy rr. YY rr. Yy

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 © 2016 Paul Billiet ODWS (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 © 2016 Paul Billiet ODWS

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 3/4 Probability of an F 2 seed being wrinkled = Probability of an F 2 seed being yellow = Probability of an F 2 seed being green = © 2016 Paul Billiet ODWS

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 3/4 Probability of an F 2 seed being wrinkled = 25% or 1/4 Probability of an F 2 seed being yellow = 75% or 3/4 Probability of an F 2 seed being green = 25% or 1/4 © 2016 Paul Billiet ODWS

Therefore ¢ Probability of an F 2 seed being round and yellow = 3/4 x 3/4 = 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 = © 2016 Paul Billiet ODWS

Therefore ¢ Probability of an F 2 seed being round and yellow = 3/4 x 3/4 = 9/16 = 56. 25% ¢ Probability of an F 2 seed being round and green = 3/4 x 1/4 = 3/16 = 18. 75% ¢ Probability of an F 2 seed being wrinkled and yellow = 1/4 x 3/4 = 3/16 = 18. 75% ¢ Probability of an F 2 seed being wrinkled and green = 1/4 x 1/4 =1/16 = 6. 25% © 2016 Paul Billiet ODWS

Predicting how many seeds we could expect to get in a sample 556 x 9/16 round yellow We could expect 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 © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS

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 ¢ © 2016 Paul Billiet ODWS

Mendel & Meiosis ¢ The pairs of chromosomes could orientate in different ways at

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 Genotypes Rr. Yy Phenotypes Round Yellow Proportions 100% © 2016 Paul Billiet ODWS (Selfed)

Dihybrid cross genetic diagram F 1 Genotypes Rr. Yy Phenotypes Round Yellow Proportions 100%

Dihybrid cross genetic diagram Gametes Y y R RY Ry r r. Y ry fertilisation F 2 Genotypes © 2016 Paul Billiet ODWS 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 © 2016

Dihybrid cross proportions Phenotypes Round Yellow Proportions 9/16 or 56. 25% Round Green 3/16

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 using 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). © 2016 Paul Billiet ODWS

Dihybrid test cross Phenotypes Round Yellow Genotypes Rr. Yy Gametes Genotypes Phenotypes Round Yellow Wrinkled Green rryy RY, Ry, r. Y, ry RY ry X Rr. Yy Round Green ry Ry r. Y ry Rryy rr. Yy rryy Wrinkled Yellow Wrinkled Green Proportions 1/4 or 25% © 2016 Paul Billiet ODWS