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Agenda Spring 2001

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Monohybrid (One-Gene)

 

Introduction

Ever wonder how your sister came out with blue eyes, when both of your parents have brown eyes? Was it the mailman? No, it was genetics! Today you will learn how to use a Punnett Square to figure out the probability of certain genotypes, and therefore, how likely will someone have a certain trait.

Some Ideas Refreshed:

For each trait you have 2 alleles, one comes from each parent: The dominant allele BLOCKS the recessive allele (for ALL cases, unless labeled "mixed dominance")

Part I: First, lets practice some vocabulary.

Practice with terms. For all questions, use these facts: the trait is fur color (f). Black fur is dominant over gray fur.

  1. Write the letter of the dominant allele. _________
  2. Write the letter of the recessive allele. _________
  3. Write the genotype for gray fur (2 alleles!). _________
  4. Write the genotype for Black fur (2alleles!). _________ or _________
  5. Write out the homozygous dominant genotype. _________
  6. Write out the heterozygous genotype. _________
  7. Write out the homozygous recessive genotype. _________
  8. Write the phenotype for #5. _________
  9. Write the phenotype for #6. _________
  10. Write the phenotype for #7. _________

Part II: How to make a Punnett Square. Here is an example:

  1. Make a tic-tac-toe board.
  2. X out the top left corner.
  3. Write the mom’s alleles across the top.
  4. Write the dad’s alleles down the side.
  5. Match the column & rows for each box.
  6. Write the capital letter first.

Problem: Mix a heterozygous mouse (mom) with a homozygous recessive (dad) mouse. How will the kids come out?

  1. List probabilities of different genotypes

· 2 Ff = 1 Ff 4 total 2 · 2 ff = 1 ff 4 total 2

(Black fur is dominant over gray)

Pinky (Dad of Brain)

Brain (Son of Pinky)

  1. List probabilities of different phenotypes:

 

˝ Ff = ˝ Black Fur ˝ ff = ˝ Gray Fur

***FOR EXTRA CREDIT: After reading #1-4, predict Brain’s mama’s genotype.

Now, practice using fur color in the following section: (still for mouse fur color)

 

Part III: Now its you’re turn to make a Punnett Square. Still using fur color, do the following problems.

  1. If the mother is homozygous recessive and the father is homozygous dominant.

a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. If the mother is heterozygous, and the father is heterozygous.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. If the mother is heterozygous, and the father is homozygous dominant.

    a)Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. If the mother is homozygous recessive, and the father is heterozygous.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

Part IV: Lets do some more Punnett Square practice using flower traits.

If you are experimenting with orchids, where red flowers are dominant over white flowers. (Use the letter "R"). Make the following crosses:

  1. Cross red homozygous orchids with white homozygous orchids.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. Cross Red heterozygous orchids with heterozygous orchids.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. Cross homozygous recessive orchids with red heterozygous orchids.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. Cross Red homozygous orchids with Red heterozygous orchids.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

Part V: Now for more practice using traits from people.

Now look lets look at people, using eye color. Brown eyes are dominant over blue/green. (Use the letter "B"). Make the following crosses:

  1. Mix two homozygous dominant parents together.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

     

 

  1. Mix two heterozygous parents together.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. Mix a homozygous dominant mom with a heterozygous dad together.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. If Kevin has green (blue) eyes, and both his parents have brown eyes, what must their genotypes be? (Look at Part III #1-3 for help)

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

Part VI: Are you now ready for a challenge?

"Sex-linked traits" come from genes that are located on the X chromosome, and not on the y. (Remembers women are XX and men are Xy). For example, baldness is a recessive sex-linked trait. Now you write the allele letter as a part of the X chromosome, like this:

XB or Xb; y stays blank.

The players: B is not bald, b is bald.

Genotype:

XBXB

XBy

XBXb

Xby

XbXb

Phenotype:

Not bald female

Not bald male

Not bald "carrier" female

Bald male

Bald female

Example:

 

˝ XBXb — Not bald carrier female.

˝ XBy — Not bald male.

 

  1. Cross (mate) a bald male to a carrier female.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

  1. Cross a not bald male to a carrier female.

    a) Write the genotype probabilities.

    b) Write the phenotype probabilities.

 

If you’re a guy, which member of your family should you look at to tell if you are going to go bald? Why? (Use a Punnett Square in your answer).