Download January 30th – 31st, 2012

January 30th – 31st, 2012
Bellringer:
Engage
(5 min)
Look at the picture below of a mother, father and their children – quickly identify some
traits of the children that are common to the mother and common to the father.
1.
Extend
(5 min)
2.
We know that our germ cells (sex cells) are produced through meiosis. What
might happen if meiosis goes wrong? What type of effect might this have on a
baby?
What happens when two chromosomes fail to separate during meiosis? (What is
this called?)
How did I get this face (It’s a Toss Up)
OBJECTIVES
Apply the principles of segregation and independent assortment to the inheritance of characteristics
Determine phenotypes and genotypes
Define allele, gene, homozygous, heterozygous
Define and give an example of dominant and recessive alleles, codominance, incomplete dominance, X-linked (sexlinked) traits, and polygenic traits
The purpose of this lab is to illustrate:
1. Mendel's Law of Segregation
2. Mendel's Law of Independent Assortment
3. the relationship between genotype and phenotype
4. a variety of patterns of inheritance
INTRODUCTION
As you have learned from meiosis, you received one half of your genes from your mother and the other half
from your father. The chromosomes went through segregation from one another when they separated and
made sex cells. Each one of the sex cells carried a gene at a locus (a specific point). When fertilization took
place the homologous chromosomes were restored and a new set of genetic material was introduced into the
egg from the sperm. The sperm cells were also segregated during meiosis and depending upon which egg was
fertilized by what sperm actually gives you your individual genetic compliment called the genotype. Human
variation happens not only because of the large number of traits that exist within our species, but also
because of the random mixing of alleles that occurs during sexual reproduction. The expression of the genes
which we see physically is called the phenotype. There are many patterns of inheritance – some are
relatively simple and straightforward, while others are more complex. Many alleles (forms of a gene) are
dominant, which means that they mask or hide the expression of other alleles, called recessive alleles.
Recessive alleles are expressed only when their dominant counterparts are not present – in other words, when
we inherit two copies of the recessive allele, one from each parent. Remember, when we have two copies of
the same allele, our genotype is homozygous for that trait.
For example if you have a round face and inherited a R from mom and a R from dad , you are homozygous
dominant for that trait. Dominant traits can be expressed even if there is only one. The genotype Rr is still a
round face. The large R is dominating the r’s expression. Even though you are heterozygous (Rr) for the trait.
Recessive alleles are expressed only in the homozygous condition (rr).
If our genotype carries one copy of the dominant trait and one copy of the recessive trait then it is called
heterozygous. In individuals with two different alleles for a trait (heterozygous for the trait), neither allele
completely masks the presence of the other, and the result is a combination of both. Other traits show
continuous variation throughout a population and do not result in a distinctly dominant or recessive
phenotype. These traits are thought to be controlled by many genes and are called polygenic traits. Another
kind of interaction between genes is called epistasis. An example of this is the interaction between the gene
that codes for red hair and the gene that codes for dark versus blonde. Thus, a person may be a strawberry
blonde, or have dark auburn hair, depending on how these two genes interact. Some traits are carried on the
X chromosome, and there is no counterpart on the Y chromosome. Because females have two X
chromosomes, a recessive allele might be masked by the presence of a dominant allele. However, because
males have only one X chromosome, recessive traits are often expressed since there is no other allele to mask
them. These traits are called sex-linked, or more accurately, X-linked. Other traits may be found only on the
Y chromosome, so they are found only in males. These traits, such as beard growth, are sex-limited , which
means their expression is confined to one gender or the other (in this example, males). Most of the traits used
in this activity have been simplified and represent a simplified model. In this exercise you will be using coins to
determine how traits are passed from parents to offspring.
Now that you have read the above article lets analyze, evaluate, and create:
(Answer these questions on your own paper)
Analyze:
 Can you distinguish between phenotype, genotype, allele, gene, homozygous, heterozygous, dominant and

recessive alleles, codominance, incomplete dominance, X-linked (sex-linked) traits, and polygenic traits?
Design a questionnaire to gather information from your classmates (save this, we will be using it in class on
Friday)
Evaluate:
 Evaluate someone else’s understanding of the terms mentioned above and of this reading by using your
questionnaire. (We will also be rotating and doing this on Friday.)
Create:
 Create a list of ideas from this reading that are still confusing to you, that you still do not understand. Create a
list of things that could help you to better understand this passage when reading it again.
Now, you are ready to begin the activity for today:
It’s A Toss Up
(Student Handout)
Purpose: To explore how traits are passed from parent to offspring
Background:
Heredity is the passing of traits or characteristics from parent to offspring. The units of heredity are the genes that are
found on chromosomes in the cells. In this activity, you will observe the results of how different gene combinations
produce certain traits. Before starting, discuss these ideas with the group and write the answers in your own words:
1. What do the terms dominant and recessive mean?
2. Explain the difference between the genotype and the phenotype of an individual.
3. How are dominant and recessive genes written or abbreviated in a genotype?
4. How can you tell by looking at the genotype of the individual if he/she is homozygous or heterozygous for that
trait?
Materials/Equipment:
2 coins
Facial Features Chart
Pencil
Safety Considerations: Always follow lab safety procedures.
Procedure:
1. Work in teams of three. Assign one group member to toss for the female parent, one for the male, and one to be
the offspring. The offspring will record the traits that result from the tosses and sketch the facial features that he
or she has inherited from the parents on the observation sheet.
2. Have the team member who is representing the male parent flip a coin to determine the sex of the offspring. If
the coin lands heads, the offspring is female. If it land tails, the offspring is male. Record the sex of offspring 1
in the sketch box provided.
3. From now on, heads will represent a dominant gene, and tails will represent a recessive gene. Both coins should
be flipped at the same time but only once for each trait. Record the genotype and phenotype that result from the
coin toss for the first trait.
4. Continue to flip both coins for each facial trait. Use the completed list of phenotypes to sketch the resulting
offspring.
5. Next, each team member should be assigned a different role and repeat Steps 1-4 so that a different member will
sketch the next offspring. Finally, switch roles a final time using Steps 1-4 to determine traits for the third
offspring.