Plant Science

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Unit

• Plant Science

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Problem Area

• Managing Inputs for Plant Growth

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Lesson

• Principles of Heredity Variation in Corn

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Student Learning 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.

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Terms

• albinism
• chromosome mutation
• codominance
• deletion
• dihybrid cross
• diploids
• frameshift mutation
• gene mutation
• haploids

• heterosis
• hybridization
• hybrid vigor
• incomplete dominance
• inversion
• lethal mutation
• multiple alleles
• mutation
• nondisjunction

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Terms cont.

• point mutation
• polygenic traits
• polyploidy
• selection
• testcross
• tissue culture
• transgenic plant
• translocation

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What are other types of relationships between
alleles and how do we determine the probable
outcome of these relationships?

• 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 OClock
  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.

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• 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 RR 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
  allelesIA, 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.

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How is the genotype of an unknown individual
determined?

• 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.

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• 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.

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How is the probability determined when
considering two different sets of traits?

• 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.

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What is a mutation and how does can it change the
genetic code of an organism?

• 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.

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• 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.

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• 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
  chromosomesi.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 Turners Syndrome is a result
  of the offspring receiving only one sex (number
  23) chromosome.

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• 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.
• 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.

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How have humans changed or manipulated the
genetic make-up of organisms?

• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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Review/Summary

• What are other types of relationships between
  alleles and how do we determine the probable
  outcome of these relationships?
• How is the genotype of an unknown individual
  determined?
• How is the probability determined when
  considering two different sets of traits?
• What is a mutation and how does can it change the
  genetic code of an organism?
• How have humans changed or manipulated the
  genetic make-up of organisms?