AP Biology 1.4 Essential Life Process Information

AP Biology 1.4 Essential Life Process Information. A 1:2:1 phenotypic ratio is the hallmark of what type of cross?

APAP Biology
AP BiologyEssential Life Process Information
Change in Genetic MakeupPhenotypic variation and fitness
LanguageEnglish Language
Test PrepAP Biology

Transcript

00:19

Let’s use a Punnett Square to figure it out. [A punnett square]

00:24

Honestly, we use a Punnett Square to figure everything out.

00:26

Although…they’re really not the best for advanced calculus…[Boy attempting to use a punnett square to solve a calculus problem]

00:29

Okay, so…keep in mind that incomplete dominance means that a phenotype with a mixed pairing

00:34

of genes will display neither the dominant nor recessive trait, but its own unique trait. [Mendel and Punnett stood together discussing the punnett square]

00:41

All right…how’s A lookin’?

00:43

“Monohybrid cross with simple dominance”.

00:45

Well…that cross yields a 3 to 1 ratio.

00:48

In other words, any phenotype containing a dominant “A” gene will display that trait. [Mendel crossing out Punnett and replacing it with Mendel]

00:53

Which is what makes it “simple dominance,” and that’s not what we’re looking for.

00:57

Option C…a Dihybrid cross…examines the phenotypic outcomes of 2 traits, not 1, which

01:02

is what a monohybrid cross does.

01:04

This yields 16 combinations, not 4…the sum of 1 to 2 to 1.

01:09

Way too many combos for our liking, so that’s not the answer.

01:12

Buh-bye, C.

01:13

How about D, a test cross? [A punnett square with question marks]

01:14

A test cross can be used to determine the genotype of a plant or animal. [baby deers in a field]

01:19

Think of it as working backwards to see what an organism’s Punnett Square would look

01:23

like.

01:24

Very useful…and fun, if you’re bored on a train…but it’s not the kind of cross

01:27

we’re after.

01:28

The correct answer is B, a monohybrid cross with incomplete dominance. [A monohybrid cross punnett square]

01:33

Think about the example of human hair. [A woman brushing her hair]

01:35

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]

01:39

However, a mixed "SC" will have wavy hair.

01:43

Anyway, this is what makes it an example of incomplete dominance, as opposed to simple

01:48

dominance…

01:49

So, this type of cross yields a 1 to 2 to 1 phenotypic ratio.

01:52

Two out of four possible offspring of a monohybrid cross with incomplete dominance will have [B answer circled in green]

01:56

a mixed phenotype.

01:57

Let’s give it up for Punnett Squares.

01:59

Have you ever had more fun in your life? [Punnett telling Mendel to forget about the naming of Punnett Squares]