Introduction to Punnett Squares I Vocabulary l Dominant
Introduction to Punnett Squares
I. Vocabulary l Dominant- an allele for a gene that masks other alleles; Symbolized by a capital letter l Recessive- an allele that is masked by a dominant allele; Symbolized by a lower case letter.
Vocab Cont. l Homozygous/Purebred- An organism with two of the same alleles for a gene l Heterozygous/Hybrid- An organism with two different alleles for a gene
Vocab Cont. l Phenotype- What a trait appears like for an organism (Picture) l Genotype- the alleles an organism has for a gene written out in symbols
II. Examples of Phenotype & Genotype Phenotype Genotype Homozygous Tall TT Heterozygous Tall Tt Homozygous Short tt
l How can we predict the result of a cross without actually performing the cross? l Scientists use punnett squares
III. Drawing a Punnett Square l For this example, we will use Mendel’s second experiment: F 1 offspring X 1. F 1 offspring = ? Determine what symbols to use: Dominant = Tall =T Recessive = Short =t
2. Write out the genetic cross using symbols F 1 offspring x F 1 offspring = ? Tt x Tt =?
Tt x Tt = ? 3. Draw the Punnett Square
Tt x Tt = ? 4. Add the genotypes from the genetic cross above. Use 1 letter per square T t One goes across the top T t One goes down the side
Tt x Tt = ? 5. Put the symbol on top into the squares below it T t T t
Tt x Tt = ? 6. Put the symbol on the left into the squares next to it T TT t t. T Tt tt
Tt x Tt = ? 7. List the possible genotypes and phenotypes of the offspring T t Genotype Phenotype TT Tall T TT Tt Tt Tall tt Short t t. T tt
Tt x Tt = ? 8. Calculate the probability of phenotypes and genotypes T t Each square represents a T TT Tt ¼ or 25% probability t t. T tt
Tt x Tt = ? T T TT t t. T t Tt tt Genotype TT Tt (t. T) tt Probability ¼ 2/4 = ½ ¼
Tt x Tt = ? T T TT t t. T t Tt tt Phenotype Probability Tall (TT, Tt) ¾ Short (tt) ¼
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