Mendelian Genetics

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Part I: Mendelian Genetics

In this assignment, you will use a useful tool, the Punnett square, to predict the probabilities of offspring gender and genotypes and phenotypes of different matings based on parental genetic makeup. Please answer all of the bulleted questions and tasks as you read through this assignment and submit them as a Word document to the assignment drop box titled “Punnett Squares Assignment.”

Each person has two copies of each chromosome, one from each parent. Your genome is exactly one-half your father’s genome and one-half your mother’s genome. After sperm and egg meet, the baby carries both copies of each gene in every cell for the rest of his or her life…except when eggs or sperm are produced. The eggs or sperm receive only one copy of each chromosome and the cycle starts all over again. Exactly which half will the baby get? That is the random part.

Sexual reproduction relies on chance to determine what type of offspring will result. A couple anxiously awaits a boy or girl and a dog breeder anxiously awaits the colors and markings of the puppies to be born. Although there is a random element involved, offspring from a mating will follow mathematical laws of probability based on the genetic makeup of the mother and father.

Watch this video to learn about Punnett squares. Please recall that dominant alleles mask recessive alleles and each baby has two copies of each gene, one from each parent.

 

http://www.clemson.edu/glimpse/?p=1175

  • Complete a Punnett square for the cross Bb x bb, where B is brown eyes and b is blue eyes.
  • What percentage of offpring will be BB? Bb? bb?
  • What percentage will have blue eyes and what percentage will have brown eyes?

Part II: Sex Determination

A male carries an X chromosome and a Y chromosome. A female carries two X chromosomes.

After meiosis, sperm and egg are produced. Sperm have a 50% chance of carrying and X chromosome and a 50% chance of carrying a Y chromosome. Eggs have a 100% chance of carrying an X chromosome, because females ONLY carry X chromosomes.

Sperm:                                             Eggs:

(X)       (X)           (Y)                            (X)                 (X)           (X)

(Y)          (Y)             (X)                                (X)           (X)            (X)

We can represent four potential scenarios if these two parents produce offspring neatly in a Punnett square. The possible sperm are placed above the top of the Punnett Square (circled in blue) and the possible eggs are placed along the left side beside the Punnett Square (circled in red). Each box is then filled in with the letter of each column and row.

punnett square gender

  • From the Punnett Square above, what is the % chance that offspring will be male? What is the percent chance that offspring will be female?

Part III: Codominance, Multiple Alleles and Blood typing

Review the embedded Amoeba Sisters video before completing the following questions.

 

Multiple alleles means that there are more than 2 alleles which can be inherited in a population.  Remember though, each individual only receives 2 alleles, one from each parent.

ABO blood typing uses 3 alleles. From your course notes answer the following two questions:

  • Which two are dominant? _________  Write the two dominant allele genotypes, using the capital letter I, and then a superscript for each of them. ______________________
  • Which allele is recessive? __________  Write the genotype (letters) for the recessive trait, beginning with the lower case i. ____________.

Complete a Punnett square for the following scenario:

Mom is heterozygous for blood type B and Dad is blood type AB. (make sure to put the parent alleles in the correct places outside of the square, and fill in each of the boxes within the square)

  • Place the Punnett square in your word document and answer the following questions:
  • What are all the possible blood types of their offspring? __________________________
  • What is the probability of their child having blood type AB? ________  Blood type O? _____________

Part IV: Deleterious Recessive Traits

A recessive trait is one where the individual must have two recessive alleles for the phenotype of the trait to be visible.  The recessive trait can be carried from generation to generation through heterozygous individuals. A person who is heterozygous for a harmful or deleterious trait is said to be a “carrier” of the trait.

If an individual receives 2 recessive alleles for a harmful or deleterious trait, let us say for our problem below, they will have the “illness”.

Solve the problem of the parents provided below.  Perform the Punnett Square and answer the questions below.  Only the answers are required in the answer sheet that you upload.

Mom and Dad are both heterozygous for the harmful or deleterious recessive trait.

  • What is the probability of their offspring having the illness?  ____________%
  • What is the probability of their offspring being a “carrier”? _____________%
 
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