t T Tt t

MIDDLE SCHOOL GENETICS SINGLE TRAIT PUNNETT SQUARE

¼

T

STUDENT EXPECTATION

STUDENTS WILL MAKE PREDICTIONS ABOUT POSSIBLE OUTCOMES OF

T

VARIOUS GENETIC COMBINATIONS OF INHERITED CHARACTERISTICS

T T TT t Tt

Punnett Squares

PARENT’S GENES

A Punnett Square is a tool used to predict the possible genotypes for the offspring of two known parents.

Key Terms

ALLELES HOMOZYGOUS

Different forms of a trait that a gene may have

T,t

An organism with two alleles that are the same

TT, tt

HETEROZYGOUS

An organism with two different alleles for a trait

Tt, Gg

HYBRID DOMINANT RECESSIVE

TERMS TO KNOW

Same as heterozygous

Tt, Gg

A trait that dominates or covers up the other form of the trait Represented by an uppercase letter The trait being dominated of covered up by the dominate trait

T

OR

G

Represented by a lower case letter

t

or

g

PHENOTYPE GENOTYPE RATIO

TERMS TO KNOW

The physical appearance of an organism (WHAT IT LOOKS LIKE) The gene order of an organism (WHAT ITS GENES LOOK LIKE)

TALL, SHORT, GREEN, WRINKLED TT, GG, Tt, gg Gg, tt

The relationship in numbers between two or more things

3:1, 2:2, 1:2:1

MONOHYBRID (ONE TRAIT) PUNNETT SQUARE The parents’ alleles go on the outside of the square

b

BB X bb

B B b

A MONOHYBRID (ONE TRAIT) PUNNETT SQUARE The parents’ alleles go on the outside of the square

b B

B b

B

B b Drop the letters on the top, into each square

b

B b B b Move each letter on the side, into each square

HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT SQUARE WHAT DO THE RESULTS SHOW?

IF B IS THE DOMINANT ALLELE FOR BLACK AND b IS THE RECESSIVE ALLELE FOR BROWN THEN WE CAN MAKE PREDICTIONS ABOUT THE OUTCOMES b b B Bb Bb Bb Bb B RESULTS: PHENOTYPIC: 100% BLACK 4:0 RATIO, BLACK TO BROWN GENOTYPIC: 100% Bb 4:0 ALL Bb

Let’s Predict T is the dominant allele for TALLNESS HOW TO USE A PUNNETT SQUARE T T T T t T t WHAT ARE THE RESULTS?

Phenotype? ____% tall ____% short ___ TO ___ RATIO: TALL TO SHORT t is the recessive allele for SHORTNESS t T t t t Genotype?: __TT: __Tt: __tt __:__:__ RATIO ___ %TT, ___%Tt, ___% tt

APPLICATION OF PUNNETT SQUARES

•

The alleles of a particular species of dog can be either “D” (normal height) or “d” (dwarf).

•

The heterozygous (Dd) and homozygous dominant (DD) form of this dog look the same (Tall). If you found a stray dog of this breed, could you tell its genotype?

PRACTICAL APPLICATION OF PUNNETT SQUARES

COULD A DOG BE CROSSED WITH ANOTHER DOG TO DETERMINE IF HE WAS PUREBRED FOR TALLNESS?

WHAT GENOTYPE SHOULD THE DOG HAVE THAT IS BEING USED FOR THE CROSS?

APPLICATION OF PUNNETT SQUARES If the dog is purebred (DD), it doesn’t matter what you cross it with, the offspring will always look like the dominant!

D D D DD DD D DD DD D D DD D DD d D Dd D Dd d Dd Dd d Dd Dd

APPLICATION OF PUNNETT SQUARES d D What would be the most effective cross for determining if the dog is heterozygous (Dd)? D DD d Dd Dd dd What would the results be if you crossed it with another heterozygous? What would be the results if your crossed it with a homozygous recessive (dd)? D d d Dd dd d Dd dd

D

PRACTICAL APPLICATION OF PUNNETT SQUARES

D d D d DD Dd d Dd dd d Dd dd d Dd dd The heterozygous cross would only give you a 25% chance of the recessive trait appearing. The most effective cross was using the homozygous recessive. This would give you a 50% chance of the recessive trait appearing. This process is called a test cross. In a litter of dogs, if a recessive god appears, then you know that then one of the parents was not a purebred.

In

Drosophila

Melanogaster (Fruit Flies), red eye color (R) is dominant over brown eye color (r). If the flies in the picture were crossed, what percent of their off spring would have brown eyes? Quiz

Quiz 1. Which of the following has the hh genotype A. 1 & 3 B. 2 C. 4 D. NONE H h 2. Which of the following is a true statement? A. Individual 4 is recessive B. Individual 1 & 3 are heterozygous C. Individual 2 is dominant D. All individuals are females H h 4 3 1 2

Quiz 3. If B is the allele for black fur and b is the allele for white fur, what percent would be black? A. 25% B. 50% C. 100% D. 75% B b B b BB Bb Bb bb 4. What fraction is homozygous dominant in the above cross?

A. 1/2 B. 1/4 C. 1/3 D. 3/4

Quiz 5 . In this cross, what is the ratio of BB to Bb? A. 3 : 1 B. 4 : 1 C. 2 : 2 D. 0 : 4 B b B B BB Bb BB Bb