Mendelian Heredity

Lab 2: Mendelian Heredity

Lab Objectives:

1. Assess the role of Mendelian inheritance in maintaining biological variation

2. Infer potential genotypes when presented with information about a phenotype 3. Contrast genotype with phenotype and understand their roles in evolution

4. Understand and solve Punnett squares and pedigrees.

Lab Activity: Mendelian Baby Game & Personal Genetic Survey (Modified From: Dr. Laura Greer Vick, Peace College)

Purpose: To illustrate how Mendelian inheritance maintains variation in a population.

As Charles Darwin developed his theory of Natural Selection during the 1840s and 1850s, he was troubled by a major inconsistency that he was at a loss to explain. He recognized that the fact of variation – that all individuals in a species are not identical – could play a vital role in determining differential reproductive success among individuals as their environments shifted. But the maintenance of variation, the very stuff on which natural selection is able to act, through multiple generations was not clear to him. In particular, variation seemed doomed to disappear over time either through the effects of blending inheritance or due to natural selection itself.

In the mid-19th century there was no conception of the molecular level of inheritance. The reigning reproductive theory at the time believed that a general ‘formative force’ acted on the vital fluids of organisms to produce more advanced organisms. It was generally believed that offspring were created through a rudimentary blending of these fluids in two mating organisms. Darwin attempted to formulate a theory of reproduction, known as pangenesis, which posited that every cell in the body contributed minute particles he named gemmules to the production of offspring. However, this idea still did not get around blending inheritance. If organisms tended to become more and more blended together over time, how could they maintain variants? Darwin did not have an answer. Furthermore, his theory of natural selection itself deemed that some variants would be more successful than others, leading to the nonrandom elimination of some variation. If natural selection worked in the way Darwin claimed it did, it would be reducing the very raw material it needed in order to cause evolution.

Unbeknownst to Darwin and biologists in general at the time, rules of heredity that did not rely on blending inheritance and that allowed for the maintenance of variants through multiple generations had already been proposed in 1866. Gregor Mendel’s principles of segregation and independent assortment indicated that some traits are discontinuous, or discrete, and cannot blend over the course of several generations. Furthermore, his breeding experiments with multiple generations of pea plants showed that rare variants that seemed to disappear from one generation could reappear in later generations. It was the fusion of Mendel’s principles of inheritance with Darwin’s theory that culminated in “The Modern Synthesis” of the mid-20th century, and laid the groundwork for the modern science of biology. The purpose of this lab is to illustrate how some basic discrete traits are inherited and how sexual reproduction maintains variation within populations during each generation.

Instructions: Congratulations! You have just been hired as the newest genetic engineer at Mendelian Babies! The mission of this company is to aid in the natural conception of beautiful, healthy infants by insuring that only the most compatible genetic materials are combined. It is your job to match mothers-to-be with one of the available sperm donors to give them the best probability of having babies with the traits they desire. You must first complete the job training and then you can be matched with potential mothers-to-be and create the perfect offspring!

Genetic Engineer Training (3 pts):

1. Blood Type: Three alleles, A, B, and O, may contribute to an individual’s blood type. Alleles A and B are dominant over O, but neither is dominant over the other – a case of co-dominance. For example, an individual with blood type O has a genotype of OO, while an individual with blood type A can have a genotype of AA or AO. An individual with blood type AB has inherited both of the dominant alleles and has a genotype of AB.

a) Construct a Punnett Square for a mating between individuals with the following genotypes:





b) Write out each genotype (e.g., AB) that might result from this cross, and its likelihood of occurring (¼, ¾, etc.):

c) Write out each phenotype (e.g., ‘Blood type O’) resulting from this cross, and its likelihood of occurring:

2. What role does the environment play in creating a phenotype from a given genotype? Does natural selection, the process whereby some individuals have greater reproductive success than others, act on the basis of an individual’s genotype or their phenotype? Chapter 4 of your textbook contains additional information on these subjects.

3. Pedigree: In a pedigree, squares represent males and circles represent females. Horizontal lines connecting males and females represent mating, and vertical lines extending downward from a couple indicate their children.

In the pedigree below, shaded-in squares and circles represent individuals who are homozygous recessive for cystic fibrosis (and therefore display the condition). Some of the genotypes of the individuals in the pedigree below are filled in. Fill in the rest of the pedigree

Genetic Engineering: Your First Client (4 pts)

You receive a cheat sheet explaining alleles for the 10 most commonly requested phenotypes:

1 (Blood Groups): A & B are co-dominant. Both A & B are dominant to O.

2 (Blood Groups): Rh+ is dominant to Rh-.

3 (Ability to roll tongue): R (ability to roll tongue) is dominant to r (inability to roll tongue).

4 (Earlobes): EE individuals have free-hanging earlobes, Ee individuals have attached earlobes, while earlobes of ee individuals are soldered (directly affixed).

5 (Ability to taste PTC): P (the allele which allows an individual to taste PTC (phenylthiocarbamide) is dominant over p.

6 (Tay-Sachs): The dominant T allele produces normal individuals, Tt individuals are carriers, and tt individuals have the disease.

7 (Cystic fibrosis): C is dominant. Cystic fibrosis is a Mendelian recessive disorder, so individuals with the cc genotype have cystic fibrosis. Cc individuals do not have any dangerous symptoms of the disease, but they are carriers of the cystic fibrosis allele.

8 (Hypercholesterolemia): H is dominant and produces hypercholesterolemia, while hh individuals are normal.

9 (Sex Chromosomes & Hemophilia): XX = females, XY = males. H is dominant (no hemophilia). XhXh individuals are females with “classic hemophilia”. XhY are males with classic hemophilia.

10 (Brachydactyly): The dominant allele B produces brachydactyly (short fingers). The bb condition produces normal length fingers. (Note: although the B allele is dominant, this condition is relatively uncommon).

Additionally, you receive a brief genetic profile of the mother to be:

After having a lengthy discussion with your first client, Ivy Winters, you have determined that her phenotypic trait wish list comprises the following:

1. A girl

a. She wants to enter her little girl into beauty pageants. She already has the name Pandora chosen for the child so she does not want a boy (even though you pointed out a son could perform in pageants as well).

2. No Tay-Sachs

a. Ivy doesn’t want her child to have a disease that progressively destroys nerve cells in the brain and spinal cord and can cause death. But she knows that she herself is a carrier for the disease.

3. No ability to taste PTC

a. She has heard that being able to taste it ruins some delicious foods. (Although you know that PTC is not found in any of the food that humans eat, you recognize that some studies have shown genetic associated sensitivity towards bitter-tasting small molecules has the potential to influence an individual’s food choice so you accept her statement.)

4. Ability to roll tongue

a. Ivy insists that this will be the best talent for Pandora to use to win all the beauty pageants.

5. Blood type B

a. She has read that people with this blood type can read body language and have other methods of deciphering others. This will give little Pandora the ability to read the judges and adjust her performance to gain more points in her pageants.

You’re not completely on board with all of the reasoning behind the traits this woman wants, but you can’t fail your first client. You pull the three most fitting sperm donors from the database.

Relevant Genetic Material (Alleles) of Each Sperm Donor

Now you must assess which sperm donor is the best match for this client.

For questions 1-5, Explain your reasoning and show your work where applicable (HINT: use what you learned in your job training to find the genotype probabilities for potential offspring).

1. Ivy wants a girl. Which sperm donor will be best for this?

2. Children with severe Tay-Sachs disease usually only live into early childhood. It is therefore undesirable. Which sperm donor will be most likely to keep Pandora from having Tay-Sachs?

3. Which sperm donor will be most likely to create a baby that will not be able to taste PCT?

4. Pandora is expected be able to roll her tongue. Which sperm donor has the best probability for this?

5. Which sperm donor will be most likely to create a baby with blood type B?

6. Overall, which sperm donor should you match with Ivy? Bear in mind both Ivy’s preferences and the overall viability of the child. Explain your reasoning.

After Lab Activity: Personal Genetic Survey

Many observable genetic traits are the result of several or many genes acting together. Stature is a complex, multi-gene trait, and is called continuous because it is expressed as a continuum of phenotypes (tall, short, and all heights in between). However, the phenotypic (observable) expression of some traits is a better reflection of an individual’s genotype because these traits are either present or absent. Such a trait is known as discrete. As genetics have advanced, a greater appreciation for the complexity of gene expression has developed. It’s now understood that most traits relate to more than a single gene and are affected by factors such as sex, age, environment, or life history. However, it is still useful to consider the connections between the genotype and the phenotype, as you will for some of your own traits in this exercise.

Instructions: For each of the traits listed below, identify your phenotype and your possible genotypes on the following table. If you have a dominant phenotype, you may be homozygous or heterozygous for that trait. If you have the recessive phenotype, you must have two recessive alleles (bb) unless the character is a sex-linked trait. Remember that if the trait is sex-linked, a male inherits only a single allele from his mother, rendering him essentially homozygous dominant or recessive based on that single allele (see Trait #4 below).

Trait #1:

Tongue-rolling – The ability to roll the tongue into a U-shape is not characteristic of all people. Individuals who cannot do it are thought to be homozygous recessive for that trait.

Possible phenotypes: roller, non-roller Possible genotypes: RR, Rr, rr

Trait #2:

Mid-phalangeal Hair – Hair on the middle segment of the fingers is present in some individuals but absent in others. Absence of mid-phalangeal hair indicates a homozygous recessive genotype for the trait.

Possible phenotypes: hair, no hair Possible genotypes: MM, Mm, mm

Trait #3: “Hitchhiker’s Thumb” – Hyperextention of the end of the thumb is known as having a “hitchhiker’s thumb”. A homozygous recessive genotype allows for the hyperextension.

Possible phenotypes: non-hitchhiker’s thumb, hitchhiker’s thumb

Possible genotypes: HH, Hh, hh

Trait #4: Short Index Finger – An index finger that is shorter than the ring finger is inherited as a homozygous recessive trait in females; yet if a male receives a single copy of this recessive gene, the trait is expressed. This occurs because it is a sex-linked trait carried on the X chromosome.

Possible phenotypes: long index finger, short index finger

Possible genotypes, female: II, Ii, ii

Possible genotypes, male: IO, iO*

*Where ‘O’ stands for the missing allele not contributed by the Y chromosome

Personal Genetic Survey Results Table (1 pts)