1 Dihybrid Cross - Weebly · Dihybrid Cross n Hybrid is another way of saying heterozygous. A...

Life Science: MolecularDihybrid Crosses

Second Law of Heredity

n By observing the inheritance patterns for two different traits Mendel proposed the second law of genetics the principle of independent assortment.

Independent Assortment

Genes for different traits can segregate independently during the formation of gametes.

n Each dominant trait is represented by a capital letter, ‘R’ for round, ‘Y’ for yellow.

n The recessive traits are represented by lower case letters, ‘r’ for wrinkled, ‘y’ for green, etc.

n When writing a genotype for two traits we use 4 letters, (e.g. RRYY)

RRYYRY

RY

Two-Factor Cross

RY

RY

Two-Factor Cross: F1

Punnett Square:n A double homozygous

dominant male produces one type of gamete (RY).

RRYY

RY RY RY RY

rryy

Two-Factor Cross: F1

Punnett Square:n A double homozygous

recessive female produces one type of gamete (ry).

RRYY rryy

ry ry ry ry

RY RY RY RY

Two-Factor Cross: F1

Punnett Square:n All of the F1 individuals are

heterozygous for both traits.

ry

ry

ry

ry

RRYY rryy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RY RY RY RY

Dihybrid Cross

n Hybrid is another way of saying heterozygous. A hybrid plant has one of each allele.

n A plant that is heterozygous for two different traits (e.g. RrYy) is called a double-heterozygote, or dihybrid.

Two-Factor Cross: F2

RrYy

RY Ry rY ry

RrYyPunnett Square:n A double heterozygous

male produces four types of gametes (RY, Ry, rY, ry).

Two-Factor Cross: F2

RY Ry rY ry

RY Ry rY ry

RrYy RrYyPunnett Square:n A double heterozygous

male produces four types of gametes (RY, Ry, rY, ry).

Two-Factor Cross: F2

Punnett Square:n The dominant and

recessive traits appear in different combinations in the F2 generation.

RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

RrYy RrYy

Phenotypic Ratios

Pheno = Physical

n 9/16 Round & Yellow

RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

RrYy RrYy

RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

Phenotypic Ratios

Pheno = Physical

n 9/16 Round & Yellown 3/16 Round & Green

RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

RrYy RrYy

Phenotypic Ratios

Pheno = Physical

n 9/16 Round & Yellown 3/16 Round & Greenn 3/16 Wrinkled & Yellow RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

RrYy RrYy

Phenotypic Ratios

Pheno = Physical

n 9/16 Round & Yellown 3/16 Round & Greenn 3/16 Wrinkled & Yellown 1/16 Wrinkled & Green

RY

Ry

rY

ry

RRYY RRYy rRYY rRyY

RRYy RRyy rRYy rRyy

RrYY RryY rrYY rryY

RrYy Rryy rrYy rryy

RY Ry rY ry

RrYy RrYy

Phenotypic Ratios

n Mendel’s experimental results were very close to the 9:3:3:1 ratio that Punnett squares predict.

9 Round (dominant) and Yellow (dominant)

3 Round (dominant) and Green (recessive)

3 Wrinkled (recessive) and Yellow (dominant)

1 Wrinkled (recessive) and Green (recessive)