Download bsaa genetic variation in corn worksheet

PRINCIPLES OF HEREDITY:
VARIATION IN CORN
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 1
Student Learning Objectives. Instruction in this lesson should result in
students achieving the following objectives:
1 Explain other types of relationships between alleles and how to determine the
probable outcome of these relationships.
2 Explain how to determine the genotype of an unknown individual.
3 Demonstrate how the probability is determined for dihibrid crosses.
4 List four examples of mutations and explain how mutations can change the
genetic make-up of an organism.
5 Explain how humans have manipulated the genetic make-up of organisms.
Anticipated Problem: What are other types of relationships between alleles and how do
we determine the probable outcome of these relationships?
I. In genetics there are relatively few examples of complete dominance relationships
among alleles. There is a great degree of genetic variation between alleles.
A. Incomplete dominance is a relationship where the heterozygous individual will have a
phenotype in between the parents. Japanese Four O’Clock flowers are an example of
incomplete dominance. In this type of a plant a homozygous dominant (RR) flower is
red, a heterozygous (Rr) flower is pink, and a homozygous recessive (rr) flower is white.
B. Codominance is when a heterozygous offspring will express both alleles for a gene.
For example, roan coat color in shorthorn cattle and in horses is where the animals have
red hairs and white hairs present in their coat. The following letters are used to represent
these alleles: RR = red, RR’ = roan; and R’R’ = white.
C. Of course not all traits are a matter of simple dominance or recessiveness. Many traits
such as the number of fruit, size of the fruit, size of the plant, and overall yield are
affected by many different genes. Polygenic Traits are those that are governed by more
than one gene. The phenotype that is observed is a result of all the genotypes for that
trait that are present.
D. Multiple alleles are genes that have more than two different alleles that trait. For
example, blood types in humans have three different alleles—IA, IB, and i. These three
alleles form six different genotypes. Coat color in rabbits and human eye color are also
examples of multiple allele traits.
Anticipated Problem: How is the genotype of an unknown individual determined?
II. A testcross is a procedure that scientists use to determine the genotype of an unknown
individual. If an organism possesses the dominant phenotype, they do not know if it is
homozygous dominant or heterozygous. In order to determine the unknown genotype,
they cross the unknown with a homozygous recessive.
A. For example, a purple flowered pea plant could be PP (homozygous dominant) or
heterozygous (Pp). The purple flowered plant is crossed with a white flowered plant (pp)
to determine the genotype of the first pea plant.
1. If 100% of the offspring have purple flowers, then the unknown is homozygous
dominant or PP. If half of the offspring have purple flowers and half have white
flowers, then the unknown is heterozygous or Pp.
2. If the unknown is crossed with anything other than a homozygous recessive, the
results could be inconclusive. An unknown purple flowered pea plant crossed with
another purple flowered pea plant could result in offspring that all have purple flowers,
but that does not guarantee that they both are homozygous dominant.
B. Test crosses are performed in parent seed research departments to ensure that plants
are pure for particular traits. Only pure strains can be used to develop hybrid crops.
Anticipated Problem: How is the probability determined when considering two different
sets of traits?
III. A dihybrid cross is one where two different sets of traits are considered. For example,
round versus wrinkled seeds and yellow versus green seeds in peas. In this case a 16
square punnett square is used. Each trait is distributed independently of the other. After
determining the genotype of the parents, then determine all of the possible combinations
of the two traits that are to be combined.
Anticipated Problem: What is a mutation and how does can it change the genetic code
of an organism?
IV. Selection is the process of breeding plants that are selected for a particular
characteristic. This leads to the dominance of certain genetic traits. Producers can select
the traits that they want and may select for traits that are a result of a mutation. A
mutation is when the DNA is changed or varies in an organism. This results in the
development of a new trait that did not exist in the parents. Natural mutations have been
found in the “Gala” apple resulting in new varieties called “Royal Gala” and “Imperial
Gala.” Mutations cannot be predicted and there are several types of mutations.
A. There are a number of different types of mutations.
1. Gene mutations involve changes in the gene and not in the entire chromosome.
a. Point mutations involve the substitution of one nucleotide for another nucleotide
on the DNA molecule. For example, GTATCC becomes GGATCC.
b. Frameshift mutations result from either the insertion or deletion of a nucleotide
in the DNA sequence. Because DNA is read as a series of condons (a sequence
of three nucleotides), this changes the DNA sequence from that point forward.
For example, GTATCC becomes GTTATC or GATCC.
2. Examples of chromosome mutations include deletion, inversion, translocation, and
nondisjunction.
a. Deletion is when a piece of a chromosome breaks off losing part of the genetic
information.
b. Inversion is when a piece of the chromosome breaks off and reattaches itself to
the same chromosome.
c. Translocation is when a piece of a chromosome breaks off and reattaches itself to
a different chromosome.
d. Nondisjunction is when a chromosome does not separate from its homologue
(one of a pair of chromosomes—i.e.—humans have two #1 chromosomes, etc.)
during meiosis. This results in one gamete receiving two copies of the chromosome
and the other receiving none of this particular chromosome. In humans, Down Syndrome
is a result of the offspring receiving three copies of chromosome 21 and Turner’s
Syndrome is a result of the offspring receiving only one sex (number 23) chromosome.
B. When a mutation occurs in a gamete (egg or sperm), it is referred to as a germ-cell
mutation. When this occurs, the mutation can be passed on to the offspring.
C. When non-reproductive cells experience a mutation, the change will only affect that
organism; it cannot be passed on their offspring.
D. Lethal mutations result in death. A plant or part of a plant lacking chlorophyll is
called an albino. Albinism is usually lethal in higher plants.
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 6
E. Some mutations are beneficial and result in the change of a species causing evolution
to occur. One example of a beneficial mutation can be found in Hereford cattle. The
allele for polled is dominant and is a result of a mutation that resulted when horned
Herefords were crossed.
F. Mutations can be caused by radiation (x-ray or nuclear), chemicals, environment, or by
accident.
Anticipated Problem: How have humans changed or manipulated the genetic make-up
of organisms?
V. Because selection requires a great deal of time to produce new offspring and the
environment can affect the outcome, several techniques have been developed to
maximize production. Researchers can use a variety of breeding techniques to manipulate
and select for a variety of beneficial traits.
A. Hybridization is the breeding of two pure lines resulting in offspring that possess the
best characteristics of the two parent strains. Hybridization has been used for the past
century and results in hybrid vigor. Examples of hybrid vigor or heterosis include faster
growth, greater vigor, increased disease resistance, and other beneficial characteristics.
Hybrid seed corn is probably the most visible example of hybridization. If the offspring
of a hybrid cross are allowed to reproduce, the hybrid vigor will probably be lost.
B. Tissue culture is a method used by plant researchers to produce a large number of
offspring by using a few cells from the parent. A small slice of cells (explant) is cut off of
the parent, placed in a growing medium that contains proper nutrients and hormones, and
the cells develop into an entirely new plant. The new plant is a clone of the parent. This
is beneficial for creating a large number of plants in a short amount of time when the
plant is unique in nature. For example, a blue rose was developed through years of
research. Using tissue culture allowed the blue rose to be mass produced rather than
trying to use traditional breeding techniques which would require an enormous amount of
time.
C. A transgenic plant is one that has been produced through the process of genetic
engineering. Genetic engineering takes DNA from one organism and inserts it into the
DNA or another organism. Canola is an example of a transgenic plant. A variety of
canola contains DNA from a flounder which allows the canola to be grown in colder
regions stretching the growing season an additional month. Other plants have been
modified to include genes to resist certain diseases or microorganisms.
D. Plants can frequently have more than two sets of chromosomes in their cells. This
happens in nature and can also be induced by man.
1. Haploids are cells that contain one copy of each chromosome in the nucleus. The
egg and sperm cells are haploid cells. This is referred to as 1n where n represents the
number of different chromosomes.
2. Diploid cells are ones that contain two copies of each chromosome in the nucleus.
In animals all cells except the sex cells are diploids. Animals that possess more or less
than the diploid number of chromosomes are considered mutations. This is
referred to as 2n.
3. However, it is very common in plants for them to have more than two copies of each
chromosome. This is referred to as polyploidy. Over 1/3 of plant species are estimated
to be polyploidy.
a. Corn and cultivated barley are examples of diploid agronomic crops. Apples and
bananas can be either 2n or 3n. Alfalfa, potatoes, and cotton are tetraploid or
have four copies of each chromosome. Wheat is hexaploid; it has six copies of
each chromosome. Strawberries contain 8 copies and boysenberries contain 7
copies of each chromosome.
b. Polyploidy can be caused by nondisjunction during meiosis (the chromosomes
did not separate during cellular division) or by artificial means. Scientists have
learned that the application of colchicine which comes from the root of the
Autumn crocus can be applied to seeds or seedlings to cause the doubling of
chromosomes. Irradiation and chemicals can also cause polyploidy. Polyploidy
has been created in snapdragons, marigolds, and watermelon.
c. Polyploidy can be valuable in plant production. Some plants will experience an
increase in cell size and an increase in the size of the fruit. Some plants will be
sterile or have a difficult time reproducing and an increased rate of death can
result. Seedless grapes, citrus, and watermelon are examples of triploid plants.
d. This characteristic makes it hard to study the genetics of plants.
PRINCIPLES OF HEREDITY:
VARIATION IN CORN
Part One: Matching
Instructions: Match the word with the correct definition.
a. chromosome mutation
b. gene mutation
c. haploid
d. mutation
e. polygenic traits
f. polyploidy
g. testcross
h. transgenic plant
_______1. Results from the change in the DNA of an organism
_______2. Cells that contain more than two copies of each chromosome
_______3. A plant that has been created through man’s manipulation of the genetic information
_______4. A procedure used by scientists to determine the genotype of an unknown
_______5. Deletion, inversion, translocation, and nondisjunction are examples of this type of
mutation
_______6. Cells that contain one copy of each chromosome
_______7. Traits that are governed by more than one gene
_______8. A mutation in the gene and not in the entire chromosome
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. ________________ is a change in the genetic code that results in death.
2. A method used by plant researchers to produce large quantities of cloned plants is called
___________________.
3. The agriculture industry commonly uses _____________________ which is the breeding of
two pure lines resulting in offspring that possess the best characteristics of both parents.
4. Cells that contain two copies of each chromosome are referred to as _________________.
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 12
Part Three: Multiple Choice
Instructions: Circle the letter of the correct answer.
_______1. What type of mutation is the result of a piece of the chromosome breaking off and
reattaching itself to the same chromosome?
a. point
b. inversion
c. nondisjunction
d. translocation
_______2. What type of mutation is the result of a piece of the chromosome breaking off and
reattaching itself to a different chromosome?
a. deletion
b. inversion
c. nondisjunction
d. translocation
_______3. What type of mutation is the result of a piece of the chromosome breaking off causing
part of the genetic information to be lost?
a. deletion
b. inversion
c. nondisjunction
d. translocation
_______4. What type of mutation is the result of the chromosomes not separating during meiosis
causing the cell to not receive enough chromosomes or to receive too many chromosomes?
a. deletion
b. inversion
c. nondisjunction
d. frameshift
_______5. What type of mutation is the result of one nucleotide being substituted for another?
a. point
b. inversion
c. frameshift
d. translocation
_______6. What type of mutation is the result of either the insertion or deletion of a nucleotide in
the DNA sequence?
a. deletion
b. point
c. nondisjunction
d. frameshift
Part Four: Short Answer
Instructions: Answer the following questions.
1. Why is it important to study genetics?
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 13
2. Japanese 4 O’clock Flowers are an example of incomplete dominance. Cross a homozygous
dominant plant with a heterozygous plant. Complete the Punnett Square, list each different
genotype, list each different phenotype, and list the ratios of each genotype and phenotype. Use
a “R” to represent the dominant red color and “r” to represent the recessive white color.
3. Perform a dihybrid cross between two pea plants—ggRR × GgRr. Complete the Punnett
Squares, list each different genotype, list each different phenotype, and list the probability of each
genotype and phenotype. Use the following alleles: G = yellow pea seeds, g = green pea seeds,
R = smooth pea seeds, and r = wrinkled pea seeds.
4. What is the purpose of a testcross?
5. Assume that you have been contracted to grow garden pea seed for a local company. They
are only interested in purchasing yellow colored pea seeds for a niche market. Yellow pea seeds
are completely dominant to green pea seeds. To insure the maximum amount of profit, you want
to know that you are planting seed that is pure for the yellow pea seed color. Show how to
calculate a testcross—complete the Punnett Squares, list each different genotype, list each
different phenotype, and list the probability of each genotype and phenotype. Explain how to
determine the genotype of the unknown based on the phenotypes that could be produced.
6. Compare and contrast haploid, diploid, and polyploid.
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 14
Assessment
TS–A
Technical Supplement
PRINCIPLES OF HEREDITY:
VARIATION IN CORN
1. How are genes distributed into gametes and recombined into zygotes?
Genes are distributed into gametes through a process called meiosis. In this process,
which differs in detail, but not in the end result between male and female parts of the
plant, the genes on each locus in an individual are distributed at random to the pollen
cells or egg cells in a plant. Thus, if a plant has the genotype Bb at a locus, it contains one
B allele and one b allele at that locus. In the pollen from that plant, one half of the pollen
grains will have the B allele and other half the b allele. Likewise, the egg cells will
contain the alleles B and b for that particular trait. When two plants which are each Bb
are crossed, the pollen and egg cells will combine randomly into zygotes. Unless outside
forces change the probability, ¼ of the progeny will be BB, 2/4 Bb, and ¼ bb. Thus, if a
trait is dominant, both BB and Bb individuals will look alike with ¾ of the offspring
showing the dominant characteristic and ¼ showing the recessive characteristic.
2. What are dominant and recessive genes?
A dominant gene, more precisely a dominant allele, is one which is expressed in an
individual regardless of the other allele at the same locus in an individual. In contrast,
a recessive allele is one which is not expressed in the presence of a dominant allele. In the
example of the albino corn seedlings, the dominant allele allows normal development of
green seedlings with normal growth in individuals which carry the recessive allele for
albinism. Only when both recessive alleles for albinism are present at a locus does the
plant lack chlorophyll and die.
3. What is the most probable genotype for the ears thatwere evaluated? What is
the most probable genotype of the parents of the ears that were evaluated?
Illinois Biological Science Applications in Agriculture Lesson B3–2 • Page 45