Lab Exercise 12 - GENETICS
Student Learning Outcomes
At the completion of this exercise you should:
1. Be able to determine the expected genotype and phenotype frequencies in offspring from a
Mendelian dihybrid cross.
2. Be able to explain why the observed proportions of offspring phenotypes, in a given dihybrid
cross, usually differ from the proportions “expected” (calculated using a Punnett square).
3. Be able to list seven observable Mendelian traits in humans.
4. Be able to describe the genetic basis for the ABO blood trait in terms of codominance and
multiple alleles.
5. Be able to describe sex-linked and traits and how they differ from the inheritance of most
other traits.
,Question 1. Before starting the lab work, define the terms below. If necessary, look these up in
your biology textbook and print the definitions in the spaces below:
This refers to the different forms of genes
A. Allele
B. Trait Refers to a genetically determined characteristic
C. Gene A gene is a segment of DNA that controls the traits of an organism
D. Dominant This is the genetic allele that masks the other if both are present
E. Recessive This is the genetic allele that is masked by the other if both are present
and its traits are observed only if homozygous.
This refers to a condition where an individual is in a position to inherit
F. Homozygous
the same DNA sequence of a particular gene
The state in which organisms inherit different forms of a gene from a
G. Heterozygous
parent gene
Genotype refers to a complete set of inheritable genes that can be passed
H. Genotype
down from one individual to the other. It also refers to an organism's
genetic makeup.
These are the physical properties of an organism that can be seen or
I. Phenotype
observed like their appearance, organism behaviors, and development.
J. Genetics Refers to the study of heredity
I. Genotype and Phenotype Frequencies in a Dihybrid Cross
All characteristics of an organism result from hereditary factors called genes. For each gene,
there typically are more than one variant, which are called alleles. When Gregor Mendel crossed
his true-breeding green pea plants with true-breeding yellow pea plants, the offspring plants,
called the F1 generation, produced only yellow peas. Since these plants must have received an
allele for green peas from one parent (as well as the allele for yellow peas), the yellow allele
apparently was dominant over the green allele, which is therefore said to be the recessive allele.
This was verified by the results of several subsequent experiments, including the reappearance of
,plants which produced green peas in the F2 generation (the offspring from a cross of two F1
plants).
Sometimes we have incomplete dominance, wherein a heterozygous individual (possessing
two different alleles for the same trait) shows an intermediate phenotype. For example, a plant
with an allele for red flowers and an allele for white flowers may produce pink flowers. The
heterozygous individual is also sometimes called a hybrid.
Today, we will explore the inheritance pattern in a dihybrid cross. The name refers to the fact
that we will observe two sets of alleles, independent of each other and each governing a separate
trait. The traits are eye color and hair form, human characteristics which have been somewhat
generalized for the purpose of our demonstration.
Our demonstration of the dihybrid cross will assume that the genes and alleles for eye color and
hair form follow Mendel's Laws, the law of segregation and the law of independent
assortment. Therefore, we assume that the pairs of parental alleles each separate during meiosis
and the individual alleles are randomly distributed to the gametes. Also, the genes for eye color
and hair form are assumed to be independent of one another (in other words, they are not
"linked" on the same chromosome). Their alleles are thus allowed to randomly mix during
meiosis, and all possible combinations should be equally represented in the gametes.
In the following discussions, we will adhere to a certain convention of genetic notation.
Dominant alleles will be represented by italicized capital letters (e.g., B as the allele for brown
eyes), and recessive traits will be represented by italicized lowercase letters (e.g., b as the allele
for blue eyes). Thus, for diploid organisms, genotypes will be shown with two alleles (e.g., bb ,
Bb, or BB ). For the gametes, which are haploid, genotypes will be shown with one allele (e.g.,
B or b ).
A. Eye Color and Hair Form
[Neither eye color nor hair form is a truly monogenic trait. Both are influenced by several genes
and environmental factors. For the purposes of this lab however, we will treat both as monogenic
with no environmental influence.]
1. Eye Color: The inheritance of eye color is actually quite complex and the discussion given
here has been oversimplified. In blue eyes, there is no pigment in the front part of the iris,
and a person with blue eyes is homozygous recessive for non-pigmented irises (bb). The
presence of pigment is controlled by the dominant allele B. For simplicity, we shall
recognize the homozygous dominant BB and the heterozygote Bb as brown eyes; and bb
as blue eyes.
, 2. Hair Form: The form of hair is dependent primarily upon its shape in cross-section. Straight
hair is rounded, while wavy, curly, and extremely curly hair show progressive degrees of
flattening. No doubt a number of genes are involved in hair form, but generally the evidence
indicates that there is one pair of alleles which can produce the difference between curly and
straight hair. The heterozygote shows wavy hair, which is an example of incomplete
dominance. For the sake of communication, let AA be the genotype for curly hair, Aa for
wavy hair, and aa for straight hair.
Student Learning Outcomes
At the completion of this exercise you should:
1. Be able to determine the expected genotype and phenotype frequencies in offspring from a
Mendelian dihybrid cross.
2. Be able to explain why the observed proportions of offspring phenotypes, in a given dihybrid
cross, usually differ from the proportions “expected” (calculated using a Punnett square).
3. Be able to list seven observable Mendelian traits in humans.
4. Be able to describe the genetic basis for the ABO blood trait in terms of codominance and
multiple alleles.
5. Be able to describe sex-linked and traits and how they differ from the inheritance of most
other traits.
,Question 1. Before starting the lab work, define the terms below. If necessary, look these up in
your biology textbook and print the definitions in the spaces below:
This refers to the different forms of genes
A. Allele
B. Trait Refers to a genetically determined characteristic
C. Gene A gene is a segment of DNA that controls the traits of an organism
D. Dominant This is the genetic allele that masks the other if both are present
E. Recessive This is the genetic allele that is masked by the other if both are present
and its traits are observed only if homozygous.
This refers to a condition where an individual is in a position to inherit
F. Homozygous
the same DNA sequence of a particular gene
The state in which organisms inherit different forms of a gene from a
G. Heterozygous
parent gene
Genotype refers to a complete set of inheritable genes that can be passed
H. Genotype
down from one individual to the other. It also refers to an organism's
genetic makeup.
These are the physical properties of an organism that can be seen or
I. Phenotype
observed like their appearance, organism behaviors, and development.
J. Genetics Refers to the study of heredity
I. Genotype and Phenotype Frequencies in a Dihybrid Cross
All characteristics of an organism result from hereditary factors called genes. For each gene,
there typically are more than one variant, which are called alleles. When Gregor Mendel crossed
his true-breeding green pea plants with true-breeding yellow pea plants, the offspring plants,
called the F1 generation, produced only yellow peas. Since these plants must have received an
allele for green peas from one parent (as well as the allele for yellow peas), the yellow allele
apparently was dominant over the green allele, which is therefore said to be the recessive allele.
This was verified by the results of several subsequent experiments, including the reappearance of
,plants which produced green peas in the F2 generation (the offspring from a cross of two F1
plants).
Sometimes we have incomplete dominance, wherein a heterozygous individual (possessing
two different alleles for the same trait) shows an intermediate phenotype. For example, a plant
with an allele for red flowers and an allele for white flowers may produce pink flowers. The
heterozygous individual is also sometimes called a hybrid.
Today, we will explore the inheritance pattern in a dihybrid cross. The name refers to the fact
that we will observe two sets of alleles, independent of each other and each governing a separate
trait. The traits are eye color and hair form, human characteristics which have been somewhat
generalized for the purpose of our demonstration.
Our demonstration of the dihybrid cross will assume that the genes and alleles for eye color and
hair form follow Mendel's Laws, the law of segregation and the law of independent
assortment. Therefore, we assume that the pairs of parental alleles each separate during meiosis
and the individual alleles are randomly distributed to the gametes. Also, the genes for eye color
and hair form are assumed to be independent of one another (in other words, they are not
"linked" on the same chromosome). Their alleles are thus allowed to randomly mix during
meiosis, and all possible combinations should be equally represented in the gametes.
In the following discussions, we will adhere to a certain convention of genetic notation.
Dominant alleles will be represented by italicized capital letters (e.g., B as the allele for brown
eyes), and recessive traits will be represented by italicized lowercase letters (e.g., b as the allele
for blue eyes). Thus, for diploid organisms, genotypes will be shown with two alleles (e.g., bb ,
Bb, or BB ). For the gametes, which are haploid, genotypes will be shown with one allele (e.g.,
B or b ).
A. Eye Color and Hair Form
[Neither eye color nor hair form is a truly monogenic trait. Both are influenced by several genes
and environmental factors. For the purposes of this lab however, we will treat both as monogenic
with no environmental influence.]
1. Eye Color: The inheritance of eye color is actually quite complex and the discussion given
here has been oversimplified. In blue eyes, there is no pigment in the front part of the iris,
and a person with blue eyes is homozygous recessive for non-pigmented irises (bb). The
presence of pigment is controlled by the dominant allele B. For simplicity, we shall
recognize the homozygous dominant BB and the heterozygote Bb as brown eyes; and bb
as blue eyes.
, 2. Hair Form: The form of hair is dependent primarily upon its shape in cross-section. Straight
hair is rounded, while wavy, curly, and extremely curly hair show progressive degrees of
flattening. No doubt a number of genes are involved in hair form, but generally the evidence
indicates that there is one pair of alleles which can produce the difference between curly and
straight hair. The heterozygote shows wavy hair, which is an example of incomplete
dominance. For the sake of communication, let AA be the genotype for curly hair, Aa for
wavy hair, and aa for straight hair.
