Punnett Square Notes

What is Genetics? Genetics is the scientific study of heredity

What is a Trait? A trait is a specific characteristic that varies from one individual to another. Examples: Brown hair, blue eyes, tall, curly

What is an Allele? Alleles are the different possibilities for a given trait. Every trait has at least two alleles (one from the mother and one from the father) Example: Eye color – Brown, blue, green, hazel Examples of Alleles: A = Brown Eyes a = Blue Eyes B = Green Eyes b = Hazel Eyes

What are Genes? Genes are the sequence of DNA that codes for a protein and thus determines a trait.

Gregor Mendel Father of Genetics 1st important studies of heredity Identified specific traits in the garden pea and studied them from one generation to another

R r Dominant vs. Recessive Dominant - Masks the other trait; the trait that shows if present Represented by a capital letter Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present Represented by a lower case letter R r

Dominant & Recessive Practice TT - Represent offspring with straight hair Tt - Represent offspring with straight hair tt - Represents offspring with curly hair T – straight hair t - curly hair

Genotype vs. Phenotype Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has. Example: Tt, ss, GG, Ww Phenotype – The physical characteristics of an organism; The way an organism looks Example: Curly hair, straight hair, blue eyes, tall, green

Homozygous vs. Heterozygous Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt) Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR rr Rr

Intro to Punnett Squares

Punnett Squares Punnett Square – Diagram showing the gene combinations that might result from a genetic cross Used to calculate the probability of inheriting a particular trait Probability – The chance that a given event will occur

Punnett Square Parent Offspring Parent

Before we go further lets review how to set up a Punnett Square… We begin by constructing a grid of two perpendicular lines.

B B b Next, put the genotype of one parent across the top and the other along the left side. For this example lets consider a genotype of BB crossed with bb. B B Notice only one letter goes above each box It does not matter which parent’s genotype goes on either side. b

Next, fill in the boxes by copying the column and row head-letters down and across into the empty spaces. B B b B b B b b B b B b

W w W W W W w w W w w w Parents in this cross are heterozygous (Ww). Usually write the capital letter first W W W W w Lets say: W- dominant white w- recessive violet w W w w w Parents in this cross are heterozygous (Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have violet flowers? ANSWER: 25% (homozygous recessive)

You Try It Now! Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)

TT x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T T t

TT x tt Step Two: Complete the Punnett Square T T t Tt

TT x tt Tt t Genotype: 4 - Tt Phenotype: 100% Tall Step Three: Write the genotype and phenotype T T t Genotype: 4 - Tt Phenotype: 100% Tall Tt Remember: Each box is 25%

Red hair (R) is dominant over blond hair (r) Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond. R r r Rr rr What percentage of the offspring will have red hair? 50%

T t Tt tt t Let’s try some more… In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a short pea plant. T t What are the percentage of phenotypes? Tt tt t 50% tall 50% short

Rr R R r 0%  Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes. R R Rr What is the possibility of a red eye off springs? r 0%  Discuss with students other possible answers

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?

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?

PRACTICAL APPLICATION OF PUNNETT SQUARES D D IF THE DOG IS PUREBRED (DD), IT DOESN’T MATTER WHAT YOU CROSS IT WITH, THE OFFSPRING WILL ALWAYS LOOK LIKE THE DOMINANT. DD DD D D DD DD D D D D D DD DD d Dd Dd d d Dd Dd Dd Dd

PRACTICAL APPLICATION OF PUNNETT SQUARES WHAT WOULD BE THE MOST EFFECTIVE CROSS FOR DETERMINING IF THE DOG IS HETEROZYGOUS (Dd) ? CROSSING IT WITH A PUREBRED (DD) WILL NOT HELP. WHAT WOULD THE RESULTS BE IF YOU CROSSED IT WITH ANOTHER HETEROZYGOUS? WHAT WOULD THE RESULTS BE IF YOU CROSSED IT WITH A HOMOZYGOUS RECESSIVE (dd)? D d D d D DD Dd d Dd dd d d Dd dd Dd dd

PRACTICAL APPLICATION OF PUNNETT SQUARES D 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 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 ANSWER: 50% 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%

TAKS FORMATTED ITEMS H h H 4 1 h 3 2 WHICH OF THE FOLLOWING HAS THE hh GENOTYPE? 1 & 3 2 4 NONE H 4 1 h 3 2 2. WHICH OF THE FOLLOWING IS A TRUE STATEMENT? INDIVIDUAL 4 IS RECESSIVE INDIVIDUALS 1 & 3 ARE HETEROZYGOUS INDIVIDUAL 2 IS DOMINANT ALL INDIVIDUALS ARE FEMALE

TAKS FORMATTED ITEMS B b B BB Bb b Bb bb 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 BB Bb b Bb bb 4. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN THE ABOVE CROSS? 1/2 1/4 1/3 3/4

TAKS FORMATTED ITEMS B B B BB BB b Bb Bb 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

https://prezi.com/obu9xols11_j/easter-egg-genetics/

Potters’ Hair Colors Solve the two questions below and use Punnett Square to demonstrate how you arrived at your answers. Question 1: Harry has dark hair like his father, but his mom had red hair. Using the genotypes of rr (red hair), Rr (dark/brown hair), RR (dark/brown hair), what possible genotypes does each of the Potters have? Question 2: Harry marries Ginny who has red hair. What are possible genotypes of their children’s hair colors?

https://youtu.be/Mehz7tCxjSE

Incomplete Dominance Incomplete Dominance - Situation in which one allele is not completely dominant over another. Example – Red and white flowers are crossed and pink flowers are produced.

1) Incomplete dominance Let’s assume that dragons show incomplete dominance for fire breathing. The F allele provides lots of fire and the F’ allele gives no fire. If a dragon that has very strong fire is crossed with a dragon that has moderate fire, what will their offspring be like? Under what conditions can a baby dragon be born that never has fire? Justify your answer with Punnett Squares.

Codominance Codominance - Situation in which both alleles of a gene contribute to the phenotype of the organism. Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan). +

2) Codominance Let’s say that the color of merpeople’s tail is controlled by a codominant gene and the alleles are blue (B) and green (G). Show a cross between two merpeople who have bluish-green tails (BG). Give the offspring phenotypes with percentages.

Multiple Alleles Multiple Alleles- Three or more alleles of the same gene. Even though three or more alleles exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.

Examples of Multiple Alleles Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.

Examples of Multiple Alleles Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types Hair Color – Too many alleles exist to count There are over 20 different shades of hair color.