middle school genetics

MIDDLE SCHOOL GENETICS

SINGLE TRAIT PUNNETT SQUARE

Tt

T

t

T

t

¼

TOOLS TO KNOW

A PUNNET SQUARE IS A TOOL USED TO PREDICT THE POSSIBLE GENOTYPES FOR THE OFFSPRING OF TWO KNOWN PARENTS.

PARENT’S GENES

PA

RE

NT

’S G

EN

ES

TERMS TO KNOW

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

TERMS TO KNOW

HYBRID 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 GRECESSIVE THE TRAIT BEING

DOMINATED OR COVERED UP BY THE DOMINATE TRAIT

REPRESENTED BY A LOWER CASE LETTER

t or g

TERMS TO KNOW

PHENOTYPE 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

HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT SQUARE

THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE B B

b

b

BB X bb


THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE B B

b

b

B B

B B

DROP THE LETTERS ON THE TOP, INTO EACH SQUARE

MOVE EACH LETTER ON THE SIDE, INTO EACH SQUARE

THE ORDER DOES NOT MATTER IN THE BOXES, BUT UPPERCASE FIRST IS A GOOD RULE

b b

b b


B B

b

b

Bb Bb

Bb Bb

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

RESULTS:

PHENOTYPIC: 100% BLACK

4:0 RATIO, BLACK TO BROWN

GENOTYPIC:

100% Bb

4:0 ALL Bb

HOW TO USE A PUNNETT SQUARE

T t

T

t

WHAT ARE THE RESULTS?

PHENOTYPIC:

75% TALL 25% SHORT

3 TO 1 RATIO: TALL TO SHORT

GENOTYPIC:

1TT: 2Tt: 1tt 1:2:1 RATIO

25 %TT, 50% Tt, 25% tt

LET’S LOOK AT ANOTHER PUNNETT SQUARE AND PREDICT THE OUTCOME T T

T t

T t

t t

T IS THE DOMINANT ALLELE FOR TALLNESS

t IS THE RECESSIVE ALLELE FOR SHORTNESS

PRACTICAL APPLICATION OF PUNNETT SQUARES

THE ALLELES OF A PARTICULAR SPECIES OF DOG CAN BE EITHER D (NORMAL HEIGHTH) 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, HOW COULD YOU DETERMINE ITS GENOTYPE?


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?


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

D D

D

D

D

d d

dDD

D

Dd Dd DdDd

Dd Dd

DD

DD

DD

DD


WHAT WOULD BE THE MOST EFFECTIVE CROSS FOR DETERMINING IF THE DOG IS HETEROZYGOUS (Dd) ? CROSSING IT WITH A PUREBRED (DD) WILL NOT HELP.

D

WHAT WOULD THE RESULTS BE IF YOU CROSSED IT WITH ANOTHER HETEROZYGOUS?

d

D

d

D d

d

d

WHAT WOULD THE RESULTS BE IF YOU CROSSED IT WITH A HOMOZYGOUS RECESSIVE (dd)?

DdDD

Dd dd

Dd

Dd

dd

dd


D d

D

d

DdDD

Dd dd

D d

d

d

Dd

Dd

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 A 50% CHANCE OF THE RECESSIVE TRAIT APPEARING.

THIS PROCESS IS CALLED A TEST CROSS. IN A LITTER OF DOGS, IF A RECESSIVE DOG APPEARS, THEN YOU KNOW THAT THE ORIGINAL DOG WAS NOT A PUREBRED.

TAKS FORMATTED ITEMS

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 OFFSPRING WOULD BE EXPECTED TO HAVE BROWN EYES?

ANSWER: 50%


H h

H

h

4

3

1

2

1. WHICH OF THE FOLLOWING HAS THE hh GENOTYPE?

A. 1 & 3B. 2C. 4D. NONE

2. WHICH OF THE FOLLOWING IS A TRUE STATEMENT?

A. INDIVIDUAL 4 IS RECESSIVEB. INDIVIDUALS 1 & 3 ARE HETEROZYGOUSC. INDIVIDUAL 2 IS DOMINANTD. ALL INDIVIDUALS ARE FEMALE

B b

B

b

BB

Bb

Bb

bb


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%

4. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN THE ABOVE CROSS?

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

B B

B

b

BB

Bb

BB

Bb


5. IN THIS CROSS, WHAT IS THE RATIO OF BB TO Bb?

A. 3 : 1B. 4 : 1C. 2 : 2D. 0 : 4

BIBLIOGRAPHY

• Mendel picture provided by wikipedia.com

•Dog and tree pictures provided by Microsoft ClipArt

•Graphics from TEA/ TAKS information booklet Middle School Science Grade 8•Dice graphic from Microsoft Clipart

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