At the Completion of This Exercise, the Student Will Be Able To

CHAPTER 15. It’s all in the Genes: Understanding Basic Mendelian Genetics

Student Learning Outcomes

At the completion of this exercise, the student will be able to:

Discuss the role of genetics in biology and in society:
Define the following terms: chromosome, gene, allele, homozygous, heterozygous, genotype, phenotype, dominant trait, and recessive trait.
Discuss expected results and observed results based on the inheritance of one trait.
Explain the inheritance of some common genetic traits in humans.
Discuss basic principles of population genetics.
Construct a simple pedigree.

OVERVIEW

“She has her father’s eyes.” “Do you think I’ll be bald like dad?” ”Can two blue-eyed parents have a brown-eyed child?” “If my black-and-white cat mates wit my neighbor’s yellow cat, will we have any calico kittens?” “How can I increase the chances of my next litter of puppies being champions?” “What are the odds of two carrier parents having a child with cystic fibrosis?” The questions are endless when people begin discussing inheritance. People are naturally curios about how traits are inherited from one generation to the next. That’s genetics!

As we approach the dawn of the age of genetics with its vast potential of understanding the human genome and engineering the gene themselves, it is hard to believe that the science of genetics had its humble origins in an obscure monastery garden in Austria in the mid-1800’s. Our fundamental knowledge of genetics is a result primarily of the experiments conducted by an Austrian monk, Gregor Mendel (Fig. 15.1). His investigation into the inheritance patterns of certain characteristics of pea plants is the classical origin of modern genetics. Ironically, the significance of Mendel’s work was not appreciated until the early 1900s.

Each chromosome consists of thousands of structural and functional units called genes (units of heredity), which are segments of DNA. Today, we are beginning to understand how genes work and where they are located on the chromosomes. As examples, the gene for Alzheimer’s disease was found on chromosome 21 and the gene for cystic fibrosis has been identified on chromosome 7. In a typical human (that’s you, hopefully!) nearly 20,000 genes are responsible for producing the traits that make you human, as well as the characteristics that make you a unique member of the human race. A trait may be controlled by one pair of genes, or it may be controlled by more than one pair of genes.

Figure 15.1 Gregor Mendel (1822 – 1884) has been called “The Father of Genetics.”

Because genes are contained on homologous chromosomes, two members of a gene pair can be alike or different. The possible form that a gene may take is called an allele. If an individual possesses two identical alleles, they are said to be homozygous. If an individual possesses two different alleles, they are said to be heterozygous. An individual‘s genetic make-up, or genotype, in turn influences one’s physical characteristics, the phenotype. In many cases, one allele may take over or prevent or mask the expression of another allele. This allele is called the dominant allele, and the allele that is not expressed is called the recessive allele. Other types of inheritance, such as incomplete dominance and codominance, as well as other variables that influence an

individual’s phenotype will be discussed in lecture.

For simplicity, the following lab has been designed to consider traits that are controlled by only one pair of genes. Keep in mind that the majority of human traits are controlled by more than one pair of genes. In this laboratory experience, students’ knowledge of the fundamental mechanisms of genetics will be reinforced by developing a simple model of inheritance and conducting a human genetic traits survey.

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Last Name, First Name [lab partner N0. 1] Last Name, First Name [lab partner N0. 2]
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Last Name, First Name [lab partner N0. 3] Last Name, First Name [lab partner N0. 4]
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Section group #Date
STUDENT ACTIVITY –UNDERSTANDING HEREDITY

Warming Up: Understanding Heredity

If the genotypes of both parents are known, the expected genotypes and phenotypes of their offspring can be calculated, either by mathematical methods or by using a Punnett square. This handy genetic device is named in honor of the British geneticist Reginald C. Punnett (1875-1967).

Punnett squares make predictions about expected results based upon laws of probability. In nature, however, the expected results may not agree with the observed results. Observed results are those appear in the offspring as a result of random combinations of the genes. This exercise has been developed to help students understand the concepts of expected results and observed results.

Students will work in teams of two. Assume that I pennies, heads are dominant to tails, assigning heads as (H) and tails as (h).

Materials

Paper
Pennies

Procedure 15.1Understanding Heredity

Complete the following Punnett square based upon two heterozygous parents. Record the genotype s and phenotypes of the results below.

1/2H / 1/2h
1/2H
1/2h

Determine the expected genotypes and phenotypes for the Punnett square.

Genotypes:__

Phenotypes:

Using the expected genotypes and phenotypes, predict the genotype and phenotype combinations for 100 tosses.

Genotypes: ______

Phenotypes: ______

Place two pennies in your hand, and then toss them onto the tabletop. Tally the letter combinations below, and record your group’s result, as well as class totals, on the charts (tables) provided.

Group Data:Expected and Observed Genotypes

Expected Genotypes
100 tosses / Observed Genotypes
100 tosses
HH
Hh, hH
hh

Expected and observed Phenotypes

Expected phenotypes
100 tosses / Observed phenotypes
100 tosses
Heads
Tails

Class Data:

Expected and Observed Genotypes

Expected Genotypes
100 tosses / Observed Genotypes
100 tosses
HH
Hh, hH
hh

Expected and observed Phenotypes

Expected phenotypes
100 tosses / Observed phenotypes
100 tosses
Heads
Tails

1.What did the pennies represent in the exercise? Were they an accurate representation?

Why or why not?

2. Why were two coins used?

3.What is the difference between expected and observed results?

4.Compare the observed genotypes and phenotypes, from your group and the class, to th

the expected genotypes and phenotypes.

5. Did you notice any relationship between the number of tosses and the expected and

observed results?

6.In nature, what variables may affect the expected and observed results in a population?

7.What is the advantage of using larger populations in studies?

My scientific studies have afforded me great gratification; and I am convinced that it will not be long before the whole world acknowledges the results of my work.
- Gregor Mendel (1822-1884)
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Last Name, First Name [lab partner N0. 1] Last Name, First Name [lab partner N0. 2]
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Last Name, First Name [lab partner N0. 3] Last Name, First Name [lab partner N0. 4]
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Section group #Date
It’s all in the Genes: Understanding Basic Mendelian Genetics

In order to understand how alleles are passed on from parent to offspring, a Punnet square is often used. A Punnett square – shows the possible combination of alleles (genotypes and phenotypes) of offspring that can result. The following Punnet square is a monohybrid cross since it is examining only one characteristic.

In order to complete a Punnett square, the alleles from one parent are listed on the top of the Punnett square and the alleles from the other parent are listed along the side. It does not matter which parent is along the top and which is listed along the side.

The above Punnett square is a monohybrid cross since it is examining only one characteristic.

Exercise 1: Let’s begin by randomly assign the letter “D” height. Being tall is dominant (D) and being short is recessive (d). Complete the following Punnett square based upon

two heterozygous parents; record the genotypes, phenotypes, percent, and ratio (genotypic, phenotypic)

What is the genotype of each parent? ______

What is the phenotype of each parent? ______

Genotype / Phenotype / % / Genotypic
Ratio / Phenotypic
Ratio

What are the possible genotypes? ______

What are the possible phenotypes? ______

What is the phenotypic ratio? ______

Now let’s try a dihybrid cross which looks at two characteristics insteadof one.

There are several ways to determine these combinations. One way is the (foil method), give information a try.

Exercise 2: Let’s assign the letter “A” for eye color. Let’s assume that in eye color brown eyes are dominant and blue eyes re recessive. Let’s assign the letter “B” for hair color. Let’s assume that in hair color brown hair is dominant and blonde hair is recessive.

* Let’s say that the father is heterozygous for eye color (brown eye) and heterozygous for hair color (brown hair) his genotype would be:“AaBb”.

To figure out the 4 possible combination of alleles the dad could donate used the “foil method”

List four possible combinations of alleles the father can give:

______

* If the mother is heterozygous dominant for eye color (brown eyes) and homozygous recessive for hair color (blonde hair),

What is the mother genotype? ______

Figure out the four possible combinations of alleles the mother could give

______

Using the possible combination of alleles from each parent for eye color and hair to fill in the Punnett square below.

Genotype / Phenotype / % / Genotypic
Ratio / Phenotypic
Ratio

Human Genetics

In this exercise you and your partner will find out the genotype and phenotype of an individual. The alleles necessary to make this individual will be determined by a random toss of dice so, individuals from each group will probably look very different.

For the purpose of this experiment, the heritance of many characteristics has been simplified. For example, eye color, hair color, and texture involve much more complex process than portrayed by this exercise. See your textbook for a more detailed explanation of how these traits are inherited. Since most of the characteristics that will make up your individual occur with the domination of one allele from one parent and the other allele from the other parent, you will only have one chance to determine the genotype for each characteristic.

1)Sex: First look at expected outcome for determining se. Complete the Punnett square below using “XX” (female) as one parent and “XY” (male) as the other.

Male
Female / ½ X / ½ Y
½ X
½ X
Genotype / Phenotype / % / Genotypic
Ratio / Phentypic
Ratio

What percentage of the offspring will be female? ______

What percentage of offspring will be male? ______

At the end of today’s lab, you will be able to compare the expected outcome with the random outcome generated by the class.

Continue working in pairs; each partner will toss a dice in the air to determine the X and Y chromosome. If the sum of your two dice is an even number, then the sex of your individual is female (XX); if the sum of your two dice is an odd number, then the sex of your individual is male (XY), write the genotype in the space below.

Genotype:______Phenotype:______

For the remaining characteristics, you will be looking at homozygous and heterozygous traits. Each trait will be determined to be homozygous recessive, homozygous dominant, or homozygous. Letters representing the alleles will be given to you: the uppercase letter is always dominant and the lowercase letter is always recessive. If both dice rolled are even numbers, it indicates Homozygous Dominantalleles. If both dice are odd, it indicates homozygous recessive alleles; if one is odd and the other even, it indicates heterozygous alleles.

2)Eyebrows: Use the alleles “A” and “a” for this trait. Separate brows are dominant, whereas unibrow (o one continuous brow) is recessive.