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Dihybrid Crosses Predicting TWO traits at once! Mendel’s Law of Independent Assortment What is the Law of Independent Assortment? Genes for different traits can segregate independently during the formation of gametes. What does that really mean? The inheritance of one gene does not influence the inheritance of another gene. Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “blue eye” gene – the two aren’t connected! We said… Just because you got your dad’s “tall” gene doesn’t mean you will also necessarily get his “brown eye” gene – the two aren’t connected! But what if we want to know the chances that you DO get both genes? Or what if we want to know the chances of not one particular gene, but TWO particular genes? Would a regular Punnett Square work? In order to predict TWO traits at once, you will need to do a... Four possible combinations of genes on the top and side (parental gametes) Sixteen total possible gene combinations (offspring) Now lets try our own example…. People like to eat green, round peas – not wrinkled and not yellow! So a farmer would want to know, if he was breeding plants, what the chances are that he would get the ideal crop (and make the most money!). R = round r = wrinkled Y = yellow y = green Both plants he is starting with are RrYy So we need to make a GIANT Punnett Square to cross RrYy x RrYy…and then figure out the “chances” that the farmer will get round, green offspring! When you are predicting only one trait, each parent has two possible genes to give. When you are predicting two traits, each parent has FOUR possible genes to give! Parent 1: Parent 2: So Parent 1 could give the following four combinations of genes to their offspring… And Parent 2 could give the following four combinations of genes to their offspring… Parent 2: Parent 1: Parent 1: Round, Yellow Round, Yellow Round, Yellow Round, Yellow Round, green Round, Yellow Round, green Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow Round, Yellow Round, green Parent 2: Round, Yellow wrinkled, Yellow wrinkled, green Parent 1: NOW LET’S MAKE SOME PREDICTIONS! Round, Yellow Round, Yellow Round, Yellow Round, Yellow Round, green Round, Yellow Round, green Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow Round, Yellow Round, green Parent 2: Round, Yellow wrinkled, Yellow wrinkled, green How many of the farmer’s plants will produce the desired Round, green peas? How many will produce the yellow peas, which no one will want to buy? So would it be wise for the farmer to cross these two RrYy plants? Round, Yellow Round, Yellow Round, Yellow Round, Yellow Round, Yellow Round, green Round, Yellow Round, green Round, Yellow Round, Yellow wrinkled, Yellow wrinkled, Yellow Round, Yellow Round, green wrinkled, Yellow wrinkled, green Ready for another example?!? Dog-faced Man This husband and wife want the ultimate circus freak kid – one with elephant skin and a hairy face! If they have a baby, what are the chances that they’ll have the ideal circus freak? And what are the chances that they will just have a normal kid?!?! Dog-faced Man Normal Face = F Hairy Face = f Normal Skin = Q Elephant Skin = q Dad must be ff for face. Genetic tests show he is Qq for skin. Mom must be qq for skin. Her dad was also “dog-faced”, so we know that she must be Ff for face. So Mom is Ffqq And Dad is ffQq Which means that Mom and Dad each can give FOUR possible combination of the two genes…. Mom could give Fq or Fq or fq or. fq Dad could give fQ or fQ or fq or. fq If they each can give FOUR possible combination of genes, how many boxes is our Punnett Square going to have? Mom = Ffqq Dad = ffQq Fq Fq fq fq fQ FfQq FfQq ffQq ffQq Ffqq Ffqq ffqq ffqq FfQq fQ FfQq ffQq ffQq Ffqq Ffqq ffqq ffqq fq fq Mom = Ffqq Fq Fq fq fq FfQq ffQq ffQq normal skin Normal face, normal skin Hairy face, normal skin Hairy face, normal skin Normal face, elephant skin Normal face, elephant skin Hairy face, elephant skin Hairy face, elephant skin Normal face, normal skin Normal face, normal skin Hairy face, normal skin Hairy face, normal skin Ffqq Ffqq ffqq ffqq Normal face, elephant skin Normal face, elephant skin Hairy face, elephant skin Hairy face, elephant skin FfQq fQ Normal face, Dad = ffQq NOW LET’S MAKE Ffqq Ffqq ffqq ffqq fq SOME FfQq FfQq ffQq ffQq PREDICTIONS! fQ fq How many of the circus freaks children will be “ideal” circus freaks themselves, with elephant skin and a hairy face? How many of their kids will be perfectly normal? How many will have just one circus freak trait? How many will carry a recessive gene that they may pass on to their children? fQ fq fQ fq Fq Fq fq fq FfQq FfQq ffQq ffQq Normal face, normal skin Normal face, normal skin Hairy face, normal skin Hairy face, normal skin Ffqq Ffqq ffqq ffqq Normal face, elephant skin Normal face, elephant skin Hairy face, elephant skin Hairy face, elephant skin FfQq FfQq ffQq ffQq Normal face, normal skin Normal face, normal skin Hairy face, normal skin Hairy face, normal skin Ffqq Ffqq ffqq ffqq Normal face, elephant skin Normal face, elephant skin Hairy face, elephant skin Hairy face, elephant skin
