Marble Heredity 2009
Purpose/Objectives:
The purpose of this activity is to demonstrate the process of how genes are passed
from parents to offspring, the concept of dominant and recessive traits, and the difference
between genotype and phenotype. Using marbles to represent a genetic trait, you will
illustrate what can happen when the genes from two parents combine to produce a new
combination of genes in their offspring.
By the end of this exercise, you should be able to:



Explain how traits are passed from parents to offspring.
Identify what controls the inheritance of traits in organisms.
Explain what is meant by genotype and phenotype.
Background:
Heredity is the passing of physical characteristics, or traits, from parents to offspring.
Traits, such as stem height or hair color, vary between individuals and are determined by
genetic material inherited from each parent. Scientists use the term "gene" for the unit of
genetic material that controls a specific trait. Alleles are the different forms of a gene. For
example, a short pea plant displays the "short" allele for stem height, while a tall pea plant
displays the "tall" allele for stem height.
An organism inherits one allele from each parent, so every gene is controlled by a
combination of two alleles. A dominant allele is one whose trait always shows up in the
organism when that allele is present. A recessive allele is hidden or masked whenever the
dominant allele is present. The allele combination or genetic makeup of an organism is
called its genotype, while its physical appearance is called its phenotype. Genotypes are
represented by 2 letters, one for each allele (i.e. AA, Aa, or aa). The dominant allele is
usually a capital letter (i.e. A) and the recessive allele is usually a lower-case letter (i.e. a). A
genotype may be homozygous for a trait if an organism has two identical alleles, or
heterozygous for a trait if an organism has two different alleles.
Gregor Mendel is considered the "Father of Genetics" because his studies on the
inheritance of certain traits in pea plants formed the foundation for the scientific study of
heredity. Mendel reached several conclusions about the "laws" of inheritance from his
experiments. These conclusions are that: (1) alleles control the inheritance of traits (2) alleles
occur in pairs (3) each parent contributes one allele (4) one allele can hide or mask another.
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Marble Heredity 2009
Vocabulary:
Heredity
Trait
Genetics
Gene
Alleles
Dominant allele
Recessive allele
Phenotype
Genotype
Homozygous
Heterozygous
Materials:
Per group of 6:



4 solid colored marbles
4 clear marbles
4 paper bags
Methods/Procedure:
You will be working in groups of 6. Designate four group members as "parents" and two
group members as "offspring."
1. Each parent takes two marbles of the same type (i.e. both soild or both clear). These
marbles represent the parents' alleles for a trait. Record the "Marble Type", "Genotype",
and "Phenotype" in Table 1 for all four parents.
*2. Your instructor will tell you which parents should pair together.
3. Each parent puts both their marbles into a paper bag and shakes the bag to mix.
 Make a prediction about what kind of F1 offspring you expect from each parent pair.
4. Each parent draws one marble from their paper bag without looking. Record the result in
Table 1 under "Marble Passed to F1 Offspring."
5. Each F1 offspring inherits the two marbles drawn from one pair of parents. Record the
"Marble Type," "Genotype," and "Phenotype" for both F1 offspring in Table 2.
6. Each F1 offspring puts both their marbles into a paper bag and shakes the bag to mix.
 Make a prediction about what kind of F2 offspring you expect from each F1 pair.
7. Each F1 offspring draws one marble from their paper bag. Record the result in Table 2
under "Marble Passed to F2 Offspring."
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Marble Heredity 2009
8. Complete the information for the F2 offspring in Table 3 based on the marbles drawn by
the F1 offspring.
Name: _____________________ Period: ____
Data/Results:
Solid marbles are dominant to clear marbles. The symbol for a solid marble allele is capital
"S", and the symbol for a clear marble allele is lower-case "s".
***Data in the tables below is for cross between parents homozygous for different alleles!
TABLE 1
Parents
Marble Type (Allele Symbol)
Genotype
Phenotype
(SS, Ss, ss)
(Solid, Clear)
Allele Passed to
F1 Offspring
Solid (S)
Solid (S)
SS
Solid
Solid (S)
Clear (s)
Clear (s)
ss
Clear
Clear (s)
Solid (S)
Solid (S)
SS
Solid
Solid (S)
Clear (s)
Clear (s)
ss
Clear
Clear (s)
Marble Type (Allele Symbol)
Genotype Phenotype
(SS, Ss, ss)
Allele Passed to F2
(Solid, Clear)
Offspring
Solid (S)
Clear (s)
Ss
Solid
Solid OR Clear (S OR s)
Solid (S)
Clear (s)
Ss
Solid
Solid OR Clear (S OR s)
TABLE 3
F2
Offspring
F1 Offspring
TABLE 2
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Marble Type (Allele Symbol)
(
)
(
Genotype
Phenotype
(SS, Ss, ss)
(Solid, Clear)
) SS, Ss, OR ss
Solid OR Clear
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Marble Heredity 2009
Name: _____________________ Period: ____
Questions:
1. Make a prediction about what kind of F1 offspring you expect from each parent pair.
Parent A
Parent B
Expected F1 Offpsring
Solid (S), Solid (S)
Solid (S), Solid (S)
Only solid phenotype, only SS genotype
Clear (s), Clear (s)
Clear (s), Clear (s)
Only clear phenotype, only ss genotype
Solid (S), Solid (S)
Clear (s), Clear (s)
Only solid phenotype, only Ss genotype
2. Make a prediction about what kind of F2 offspring you expect from each F1 pair.
F1 Offspring A
F1 Offspring B
Expected F2 Offpsring
Solid (S), Solid (S)
Clear (s), Clear (s)
Only solid pheotype, only Ss genotype
Solid (S), Clear (s)
Solid (S), Clear (s)
Mostly solid phenotype, some clear
phenotype; SS, Ss, and ss genotypes
3. How many different kinds of offspring are possible when homozygous parents are crossed?
Only one kind of offspring is possible, regardless of whether both parents
are homozygous for the same allele.
4. What different kinds of offspring are possible when two heterozygous parents are crossed?
Both phenotypes and all three different genotypes (SS, Ss, and ss).
5. On Tables 1, 2, and 3 above, circle any genotypes that are heterozygous.
6. In humans, the allele for free earlobes (F) is dominant over the allele for attached ear
lobes (f).
a. What possible genotypes could a person with the free earlobe phenotype have?
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Marble Heredity 2009
FF or Ff
b. What possible genotypes could a person with the attached earlobe phenotype have?
Only ff
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