1. MIDDLE SCHOOL GENETICSTt t t
MIDDLE SCHOOL GENETICS
SINGLE TRAIT PUNNETT SQUARE

2. STUDENT EXPECTATION
STUDENTS WILL MAKE PREDICTIONS ABOUT POSSIBLE OUTCOMES OF VARIOUS GENETIC COMBINATIONS OF INHERITED CHARACTERISTICS T t TT Tt T T

3. Punnett Squares
PARENT’S GENES
A Punnett Square is a tool used to predict the possible genotypes for the offspring of two known parents.
PARENT’S GENES

4. T,t TT, tt Tt, Gg Key Terms ALLELES
Different forms of a trait that a gene may have
T,t
HOMOZYGOUS
An organism with two alleles that are the same
TT, tt
HETEROZYGOUS
An organism with two different alleles for a trait
Tt, Gg

5. Tt, Gg T OR G t or g TERMS TO KNOW HYBRID DOMINANT RECESSIVE
Same as heterozygous
Tt, Gg
DOMINANT
A trait that dominates or covers up the other form of the trait
Represented by an uppercase letter
T OR G
RECESSIVE
The trait being dominated of covered up by the dominate trait
Represented by a lower case letter
t or g

6. TERMS TO KNOW PHENOTYPE TALL, SHORT, GREEN, WRINKLED GENOTYPE
The physical appearance of an organism
(WHAT IT LOOKS LIKE)
TALL, SHORT, GREEN, WRINKLED
GENOTYPE
The gene order of an organism
(WHAT ITS GENES LOOK LIKE)
TT, GG, Tt, gg
Gg, tt
RATIO
The relationship in numbers between two or more things
3:1, 2:2, 1:2:1

7. MONOHYBRID (ONE TRAIT) PUNNETT SQUARE
The parents’ alleles go on the outside of the square B B b
BB X bb b

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

9. HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT SQUARE
WHAT DO THE RESULTS SHOW?
RESULTS: B B
PHENOTYPIC: 100% BLACK
4:0 RATIO, BLACK TO BROWN
IF B IS THE DOMINANT ALLELE FOR BLACK b Bb Bb
AND b IS THE RECESSIVE ALLELE FOR BROWN
GENOTYPIC:
100% Bb
4:0 ALL Bb b Bb Bb
THEN WE CAN MAKE PREDICTIONS ABOUT THE OUTCOMES

10. HOW TO USE A PUNNETT SQUARE
WHAT ARE THE RESULTS?
Let’s Predict T t
Phenotype?
____% tall
____% short
___ TO ___ RATIO: TALL TO SHORT
T is the dominant allele for TALLNESS T T T T t t T t t t
Genotype?:
__TT: __Tt: __tt __:__:__ RATIO
___ %TT, ___%Tt, ___% tt
t is the recessive allele for SHORTNESS

11. 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?

12. 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?

13. 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 DD DD D D DD DD D D D D D DD DD d Dd Dd d d Dd Dd Dd Dd

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

15. PRACTICAL APPLICATION OF PUNNETT SQUARESD 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.

16. Quiz
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?

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

18. Quiz
3. If B is the allele for black fur and b is the allele for white fur, what percent would be black?
25%
50%
100%
75%
B b B BB Bb b Bb bb
4. What fraction is homozygous dominant in the above cross?
1/2
1/4
1/3
3/4

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