Dihybrid Crosses A cross that deals with two

Dihybrid Crosses • A cross that deals with two different genes or traits • Example: eye color and hair color

Mendel did not only study the genetic inheritance of a single trait, he also studied the inheritance of two separate traits with a single cross Instead of a punnett square containing 1 trait with two alleles, it now has 2 traits with 4 alleles.

Mendel’s law of Independent Assortment was created from his study of dihybrid crosses; this law states that genes assort independently; one gene doesn’t influence the inheritance of another. ◦ All factors have an equal possibility of being donated to the offspring. ◦Aa. Bb

A yellow round, seed, genotype Yy. Rr, produces the following gametes ◦ YR, Yr, y. R or yr Y = yellow y = green R = round r = wrinkled

Dihybrid Cross To calcuate the offspring of a cross between Yy. Rr and Yy. Rr we use a Punnett Square:

Gamete s YR Yr y. R yr Fill in the chart with the offspring genotypes, keeping the “y’s” and the”r’s”

YR Yr y. R yr YYRR YYRr Yy. RR Yy. Rr YR Yr y. R yr Notice Dominant alleles first, then Recessive.

YR Yr y. R yr YYRR YYRr Yy. RR Yy. Rr YR Yr YYRr YYrr Yy. Rr Yyrr Yy. RR Yy. Rr yy. RR yy. Rr y. R Yy. Rr yr Yyrr yy. Rr yyrr

Dihybrid Crosses: A dihybrid cross between two heterozygous parents will always produce a 9: 3: 3: 1 phenotypic ratio. In the example: ◦ ◦ 9 Yellow Round 3 Yellow wrinkled 3 green Round 1 green wrinkled

Dihybrid Crosses A generic ratio for any dihybrid heterozygous cross: ◦ 9 Dominant ◦ 3 Dominant Recessive ◦ 3 Recessive Dominant ◦ 1 Recessive

True Breeding A parent is said to be TRUE BREEDING if it produces only one gamete for a specific trait. It is homozygous. ◦ YYRR is true breeding for yellow, round seeds ◦ yyrr is true breeding for green wrinkled seeds A cross between two true breeding parents will always produce only one possible offspring (why? )

True Breeding YYrr x yy. RR Yy. Rr YYRR x yy. RR Yy. RR

Example: A homozygous individual for tongue rolling and widows peak (both dominant) mates with an individual who can not roll their tongue and does not have a widows peak. Calculate the phenotypic & genotypic ratio of the offspring of both the F 1 and the F 2 generations. ◦ Step #1 is to assign allele’s to the traits

What are the genotypes for the intial parents? TTHH What x tthh gametes can each parent make? Phenotypic F 1? and Genotypic ratios in the

How do we get the F 2? Parent Next cross: slide = Punnett square

Gamete s

Gamete s TH Th t. H th

Gamete s TH Th t. H th TH TTHh Tt. HH Tt. Hh Th TTHh TThh Tt. Hh Tthh t. H Tt. Hh tt. HH tt. Hh th Tt. Hh Tthh tt. Hh tthh

Phenotypes – for each possible genotype, write the phenotype Genotypes TTHH TTHh Tt. HH TThh Tthh tt. HH tt. Hh tthh ___ / 16 ___ / 16 ___ / 16 Rolling, widows peak Rolling, widows pea Rolling, straight Non-rolling, widows peak Non-rolling, straight Prove the 9: 3: 3: 1 Ratio