• Mendel also investigated the inheritance of

two pairs

of contrasting characters in the pea plant.

• This is referred to as the

dihybrid

cross.

Pure breeding round seeds yellow cotyledons X pure breeding wrinkled seeds green cotyledons F1 progeny all round yellow peas

• The F1 progeny were

selfed

: • F2 progeny Round Yellow

315

round green

108

wrinkled yellow

101

wrinkled green

32

The ratio between the numbers of different seeds reduces to:

9.8 : 3.38 : 3.15 : 1.0

Or about

9 : 3 : 3 : 1

• Mendel also noticed that two new combinations not represented in the parents appeared in the progeny • Round or wrinkled seeds can turn up with either green or yellow cotyledons.

• The two pairs of factors were

inherited independently.

• Either one of the contrasting characters could be passed on to the offspring • Therefore the heterozygote plant (F1 – round and yellow) must produce

four

types of gamete in equal numbers

RY

F1 phenotype

Ry RrYy

Produces 4 different gametes

rY ry

• Use this information to draw the initial cross and the F1 cross on your worksheet.

• How many different phenotypes are there and what will they be?

Answers

• CROSS 1 • Parents

genotype gametes Round yellow X wrinkled green RRYY rryy RY ry

RY RY ry RrYy RrYy ry RrYy RrYy

• F1 phenotype – all round and yellow • F1 genotype – all RrYy (heterozygous for both characters)

F1 generation crossed

• F1 phenotype – round yellow X

round yellow

• Genotype - RrYy X RrYy • Gametes RY Ry rY ry

RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

• F2 generation Phenotypes – Round and yellow – 9 Round and green – 3 Wrinkled and yellow - 3 Wrinkled and green - 1

The Dihybrid Test Cross

• As with monohybrids the appearance of the heterozygous dominant offspring will be identical to that of the homozygous dominant organism.

• How could they be distinguished?

• If the organism in question is heterozygous dominant and is crossed with a homozygous recessive individual then the resulting progeny would show all possible combinations of characters in the ratio • 1 : 1 : 1 : 1

• This is described as a test cross in which the genotype of the F1 offspring can be determined.

• Perform a genetic cross to show the possible genotypes resulting from a test cross for a homozygous dominant individual (RRYY) and a heterozygote (RrYy)