Definition Crosses looking at two traits simultaneously Mendels law of segregation states that Mendels law of independent assortment states that

An example: Parents Wrinkled peas = r Round peas = R Green peas = y Yellow peas = Y Cross 2 pure breeding parents. One is homozygous dominant for both traits, the other homozygous recessive for both. The parental genotypes are:

RRYY (round yellow) x rryy (wrinkled green) For parent RRYY what are all the possible combinations of alleles in the gametes? For parent rryy what are all the possible combinations of alleles in the gametes?

RRYY (round yellow) x rryy (wrinkled green) The Punnett grid needs to be 4x 4 since each parent produces 4 combinations of gametes All offspring are: gametesRY ry

RrYy is called a dihybrid What are all the possible combinations of alleles in the gametes of a dihybrid? RrYy

Create a Punnett grid To show a cross between two dihybrids, RrYr (round yellow) and RrYr (round yellow):

Another example The phenotypes of two independent traits show a 9:3:3:1 ratio in the F 2 generation. In this example, coat color is indicated by B (brown, dominant) or b (white) while tail length is indicated by S (short, dominant) or s (long). When parents are homozygous for each trait ('SSbb and ssBB), their children in the F 1 generation are heterozygous at both loci and only show the dominant phenotypes. If the children mate with each other, in the F 2 generation all combination of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box)

THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously

Mendel’s peas CharacterTraitAllele Seed shapeRoundR Wrinkledr Cotyledon colourYellowY Greeny

Combinations GenotypePhenotype RRYYRound Yellow RRYyRound Yellow RrYYRound Yellow RrYyRound Yellow RRyyRound Green RryyRound Green rrYYWrinkled Yellow rrYyWrinkled Yellow rryyWrinkled Green

Is the inheritance of one character affected by the inheritance of another? P PhenotypesRound Yellow xWrinkled Green (Pure Bred) F 1 PhenotypesAll Round Yellow (Selfed) F 2 Phenotypes Seedcolour YellowGreenTOTALRATIO SeedRound shapeWrinkled TOTAL RATIO

Is the inheritance of one character affected by the inheritance of another? P PhenotypesRound Yellow xWrinkled Green (Pure Bred) F 1 PhenotypesAll Round Yellow (Selfed) F 2 Phenotypes Seedcolour YellowGreenTOTALRATIO SeedRound shapeWrinkled TOTAL RATIO2.971 © 2007 Paul Billiet ODWSODWS

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 What Mendel observed 556 x 9/16 round yellow x 3/16 round green x 3/16 wrinkled yellow x 1/16 wrinkled green32

Predicting how many seeds we could expect to get in a sample We could expect What Mendel observed 556 x 9/16 round yellow x 3/16 round green x 3/16 wrinkled yellow x 1/16 wrinkled green3532

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

The pairs of chromosomes could orientate in different ways at metaphase 1 – random assortment

Gamete production is able to generate a wide variety due to: Crossing over during prophase I Random orientation during metaphase I

PPhenotypesRound Yellow seed XWrinkled Green seed (Pure bred) GenotypesRRYYrryy meiosis GametesRYry fertilisation F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% Dihybrid cross genetic diagram

F1F1 PhenotypesRrYy(Selfed) GenotypesRound Yellow Proportions100% meiosis GametesYy RRYRy rrYry Dihybrid cross genetic diagram

GametesYy RRYRy rrYry fertilisation F2F2 GenotypesRYRyrYry RYRRYYRRYyRrYYRrYy RyRRYyRRyyRrYyRryy rYRrYYRrYyrrYYrrYy ryRrYyRryyrrYyrryy Dihybrid cross genetic diagram

Dihybrid cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green Wrinkled Yellow Wrinkled Green

Dihybrid cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green3/16 or18.75% Wrinkled Yellow3/16 or18.75% Wrinkled Green1/16 or6.25%

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 PhenotypesRound YellowXWrinkled Green GenotypesRrYyrryy GametesRY, Ry, rY, ryry GenotypesRYRyrYry RrYyRryyrrYyrryy PhenotypesRound Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions1/4 or 25%