AP Biology 1.4 Essential Life Process Information. A 1:2:1 phenotypic ratio is the hallmark of what type of cross?
|AP Biology||Essential Life Process Information|
|Change in Genetic Makeup||Phenotypic variation and fitness|
|Test Prep||AP Biology|
Let’s use a Punnett Square to figure it out. [A punnett square]
Honestly, we use a Punnett Square to figure everything out.
Although…they’re really not the best for advanced calculus…[Boy attempting to use a punnett square to solve a calculus problem]
Okay, so…keep in mind that incomplete dominance means that a phenotype with a mixed pairing
of genes will display neither the dominant nor recessive trait, but its own unique trait. [Mendel and Punnett stood together discussing the punnett square]
All right…how’s A lookin’?
“Monohybrid cross with simple dominance”.
Well…that cross yields a 3 to 1 ratio.
In other words, any phenotype containing a dominant “A” gene will display that trait. [Mendel crossing out Punnett and replacing it with Mendel]
Which is what makes it “simple dominance,” and that’s not what we’re looking for.
Option C…a Dihybrid cross…examines the phenotypic outcomes of 2 traits, not 1, which
is what a monohybrid cross does.
This yields 16 combinations, not 4…the sum of 1 to 2 to 1.
Way too many combos for our liking, so that’s not the answer.
How about D, a test cross? [A punnett square with question marks]
A test cross can be used to determine the genotype of a plant or animal. [baby deers in a field]
Think of it as working backwards to see what an organism’s Punnett Square would look
Very useful…and fun, if you’re bored on a train…but it’s not the kind of cross
The correct answer is B, a monohybrid cross with incomplete dominance. [A monohybrid cross punnett square]
Think about the example of human hair. [A woman brushing her hair]
In humans, "S" is for straight hair, while "C" is for curly hair. [A woman with curly hair and a woman with straight her in a hairdressers store]
However, a mixed "SC" will have wavy hair.
Anyway, this is what makes it an example of incomplete dominance, as opposed to simple
So, this type of cross yields a 1 to 2 to 1 phenotypic ratio.
Two out of four possible offspring of a monohybrid cross with incomplete dominance will have [B answer circled in green]
a mixed phenotype.
Let’s give it up for Punnett Squares.
Have you ever had more fun in your life? [Punnett telling Mendel to forget about the naming of Punnett Squares]