Transcript Slide 1
Unit Plant Science Problem Area Managing Inputs for Plant Growth Lesson Principles of Heredity: Variation in Corn 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. 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 Terms cont. point mutation polygenic traits polyploidy selection testcross tissue culture transgenic plant translocation 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 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. 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. 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. 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. 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. 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. 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. 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. 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. 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?