Dihybrid Crosses Biology 30 Mrs. S. Pipke-Painchaud

Dihybrid Crosses  the natural progression for Mendel was to study 2 characteristics at the same time.  thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross  ex.. round yellow seeds X wrinkled green seeds

Mendel’s Fourth Law  The Law of Independent Assortment  During gamete formation, segregating pairs of unit factors assort independently of each other.  the two traits are inherited totally independently of each other.  i.e. color is inherited independently of seed shape.  Example:  Homozygous round green X wrinkled homozygous yellow

Solving Dihybrid Crosses  Follow the steps  If you were to cross a Homozygous round green X wrinkled homozygous yellow plant, what would your phenotypic and genotypic ratios be?

Step 1: Make a Key….  G= yellow  g = green  W = round  w = wrinkled

Step 2: Give the genotypes of the parents... Step 2: Give the genotypes of the parents...  Homozygous round green X wrinkled homozygous yellow plant  P1 = WWgg X wwGG

Step 3: Determine the Gametes...  WWgg X wwGG WWgg = Wg, Wg, Wg, Wg wwGG = wG, wG, wG, wG HINT: Foil to get the gametes

Step 4: Fill in the Punnett square... WgWgWgWg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg wGWwGgWwGgWwGgWwGg

Step 5: Answer any questions...  Genotypic ratio  = all WwGg  Phenotypic ratio  = all round yellow G= yellow g = green W = round w = wrinkled

F1 X F1 = WwGg X WwGg  Remember to determine the gametes for your punnett square.

Fill in the Punnett Square WGWgwGwgWGWWGGWWGgWwGGWwGg WgWWGgWWggWwGgWwgg wGWwGGWwGgwwGGwwGg wgWwGgWwggwwGgwwgg

The PATTERN... WGWgwGwg WG WWGG WWGg WwGG WwGg Wg WWGg WWgg WwGg Wwgg wG WwGG WwGg wwGG wwGg Wg WwGg Wwgg wwGg wwgg

What are the ratios?  Genotypic Ratio:  1 WWGG  2 WWGg  1 WWgg  2 WwGG  4 WwGg  2 Wwgg  1 wwGG  2 wwGg  1 wwgg WGWgwGwg WGWWGGWWGgWwGGWwGg WgWWGgWWggWwGgWwgg wGWwGGWwGgwwGGwwGg WgWwGgWwggwwGgwwgg

Phenotypic Ratio:  Phenotypic Ratio:  9 Yellow Round  Dominant Dominant  3 Yellow Wrinkled  Dominant Recessive  3 Green Round  Recessive Dominant  1 Green Wrinkled  Recessive Recessive  Genotypic Ratio:  1 WWGG  2 WWGg  1 WWgg  2 WwGG  4 WwGg  2 Wwgg  1 wwGG  2 wwGg  1 wwgg G= yellow g = green W = round w = wrinkled ** Explain the patterns

The Short Cut:  can only be used if you have  (Big little Big little X Big little Big little)  BbTt x BbTt  Do these fit?  BBTt X BbTt  BbTt x Bb tt

Try this: C= Inflated Podc = constricted pod D = Talld = dwarf  The cross: a plant heterozygous for inflated pod and heterozygous tall is crossed with a another heterozygous inflated pod and heterozygous tall plant. What are the genotypic and phenotypic ratios?  The parents are:  CcDd x CcDd

Example:  Does this fit the short cut?  Yes C= Inflated Pod c = constricted pod D = Tall d = dwarf Genotypic Ratio: Genotypic Ratio: 1 CCDD 2 CCDd 1 CCdd 2 CcDD 4 CcDd 2 Ccdd 1 ccDD 2 ccDd 1 ccdd Phenotypic Ratio: (D) (D) 9 (D) ® 3 ® (D) 3 ® ® 1 Phenotypic Ratio: (D) (D) 9 Inflated Tall (D) ® 3 Inflated Dwarf ® (D) 3 Constricted Tall ® ® 1 Constricted Dwarf

Now try...  Refer to the assigned questions in your duotang.