Punnett Squares

Be ready to answer!  Which is an example of heterozygous alleles?  Tt  MM  mm

Be ready to answer!  Which is an example of heterozygous alleles?  Tt  MM  mm

Review  Which is an example of homozygous dominant alleles?  Tt  MM  mm

Review  Which is an example of homozygous dominant alleles?  Tt  MM  mm

Review  Which is an example of a purebred?  Tt  MM  mm

Review  Which is an example of a purebred?  Tt  MM  mm

Review  Which is an example of a phenotype?  Tt  tt  Can roll tongue  None of these are examples of phenotypes

Review  Which is an example of a phenotype?  Tt  tt  Can roll tongue  None of these are examples of phenotypes

{ Probability and Punnett Squares

 The likelihood that a specific event will occur  If you flip a coin 100 times, you should get about 50 heads, 50 tails  Probability

 Fertilization: endless possibilities depending on which sperm fertilizes the egg  Figuring out the probability of specific traits can be HARD! But one tool makes it easier… Probability in Genetics

 The value of studying genetics is in understanding how we can predict the likelihood of inheriting particular traits. This can help plant and animal breeders in developing varieties that have more desirable qualities. It can also help people explain and predict patterns of inheritance in family lines.  One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett Read Only

PUNNETT SQUARES  charts showing the possible combinations of alleles in a cross  show the probability of getting a certain type of offspring

Example with flies:

Monohybrid Cross: a cross that involves only one trait Phenotypes:Genotypes:

Each square shows the chance of getting a baby with that genotype If there are 4 squares, what percent is each square worth?

THE PARENTS GENOTYPES  DAD = Tt (heterozygous)  MOM = Tt (heterozygous) PHENOTYPES  DAD = Tall  MOM = Tall

One parent’s genes go across the top (one per box) The other parent’s genes go down the side (one per box)

Each letter goes into each box below it Each letter goes into each box next to it

Now we can predict the possibility that the parents will have a baby with that kind of phenotype

What is the possibility (%) that the parents will have a tall baby? TT = tall 25% Tt = tall 50% So 75% chance

What is the possibility that the parents will have a short baby? tt = short 25% So 25% chance

THE OFFSPRING  TT (homozygous dominant)  Tt (heterozygous)  tt (homozygous recessive) GENOTYPES 1TT:2Tt:1tt (1:2:1)—ratio 25% TT, 50% Tt, 25% tt

THE OFFSPRING  TT (tall)  Tt (tall)  tt (short) PHENOTYPES 3 Tall :1 Short (3:1) 75% Tall, 25% Short

Cross a homozygous dominant with a recessive (for height where T is dominant and tall). What are my genotypes?

Cross a homozygous dominant with a recessive (for height where T is dominant and tall). What are my genotypes? TT and tt

Cross a homozygous dominant with a recessive (for height where T is dominant and tall). Create a Punnett square.

Find the percent chance of each genotype and each phenotype

Find the percent chance of each genotype and each phenotype -Tt: 100% -Tall: 100%

Cross a heterozygote with a homozygous recessive (for height where T is dominant and tall). What are the genotypes to cross? Cross a heterozygote with a homozygous recessive (for height where T is dominant and tall). What are the genotypes to cross?

Cross a heterozygote with a homozygous recessive (for height where T is dominant and tall). What are the genotypes to cross? -Tt and tt

Find the percent chance of each genotype and each phenotype Find the percent chance of each genotype and each phenotype

Find the percent chance of each genotype and each phenotype -Tt:50% -tt:50% -tall:50% -short:50% What is the ratio of dominant: recessive? Find the percent chance of each genotype and each phenotype -Tt:50% -tt:50% -tall:50% -short:50% What is the ratio of dominant: recessive?

Find the percent chance of each genotype and each phenotype -Tt:50% -tt:50% -tall:50% -short:50% What is the ratio of dominant: recessive? 2:2 Find the percent chance of each genotype and each phenotype -Tt:50% -tt:50% -tall:50% -short:50% What is the ratio of dominant: recessive? 2:2

 Red is dominant to white. Using the letter R to represent red flower color, make a cross between:  homozygous dominant x heterozygous  Find the percent chance of each genotype and each phenotype  Write the ratio of dominant:recessive genotypes Punnett Squares: Practice

 Red is dominant to white. Using the letter R to represent red flower color, make a cross between:  homozygous dominant x heterozygous—RR and Rr  Find the percent chance of each genotype and each phenotype—RR: 50%, Rr: 50%, Red: 100%  Write the ratio of dominant:recessive genotypes—4:0 Punnett Squares: Practice

 Brown is dominant to blue eyes. Using B to represent brown, make a cross between:  Heterozygous x heterozygous  Find the percent chance of each genotype and each phenotype  Write the ratio of dominant:recessive genotypes Punnett Squares: Practice

 Brown is dominant to blue eyes. Using B to represent brown, make a cross between:  Heterozygous x heterozygous—Bb and Bb  Find the percent chance of each genotype and each phenotype—BB: 25%, Bb: 50%, bb: 25%, brown: 75%, blue: 25%  Write the ratio of dominant:recessive genotypes—3:1 Punnett Squares: Practice

 m Additional Resources