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Genetics - Study of Heredity

Science, Grade 6

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Study Guide Genetics - Study of Heredity Science, Grade 6

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GENETICS: THE STUDY OF HEREDITY Mendel’s Work Gregor Mendel was a high school teacher from Austria who developed the foundation of genetics. He worked with pea plants in the monastery where he lived in the 17th century. Without his work the study of heredity would not be where it is today. Gregor wondered how and why pea plants had different physical characteristics, otherwise known as traits. Some of the pea plants grew short while others grew tall, and some had green pods while others had yellow pods. He began to notice that some of the pea plants had similar traits of their parents. He hypothesized that the parents passed down traits to their offspring, which is called heredity. It was this work that laid the foundation of genetics. The P Generation A purebred is an offspring with the same form of a trait as its parent. This is achieved by letting plants with a certain trait reproduce for many generations. Mendel crossed a purebred short stem pea plant with a purebred tall stem plant. He called it the P generation or the parent generation. Mendel called the first generation the F1 generation. The F stands for the filial, or ‘son’ in Latin, generation and the 1 stands for which generation it is from. All of the offspring in the F1 generation were tall. Mendel allowed the offspring (F1 plants) to grow and reproduce. The results of the F2 generation were a mix of tall and short plants. Dominant and Recessive Alleles From Mendel’s work on peas, he was able to hypothesize that the traits existed in pairs and that each parent contributed one trait to the offspring. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
What is an Allele? Mendel reasoned further that one trait can be masked by another trait in the parents. For example, the purebred parents for green and yellow pod colors F1 generation will all have a green pod. So the yellow pod color will not show up at all in the F1 generation. In the F2 generation the yellow pod color comes back at a ratio of about 3:1. An allele is the different forms of a particular trait or gene. The gene that controls the pod color has one allele for green and another allele for yellow. Each generation of pea plants inherits one allele from each parent, a total of 2 alleles, for each physical characteristic. A dominant allele is the allele that will always show up in an organism if the allele is present in the organism. So in the pea pod example, the green allele is the dominant allele. A recessive allele is the allele that will be masked if the dominant allele is present. The only possible way that the recessive allele will show up is if the dominant allele is not present at all. Lesson Checkpoint: What is a dominant allele? Hybrids A hybrid is the F1 generation that has two different forms of an allele. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Probability and Heredity Probability is the chance that a certain event will occur. If you were to flip a coin into the air and let it hit the ground, which side would it land on? There are two different possibilities, heads or tails. It is equally probable that the coin will land on the heads side as it is that it will land on the tails side. This translates to… the probability of a coin landing on heads or tails is 1 in 2. There is a 50% chance that the coin will land on the heads side and there is a 50% chance that it will land on the tails side totaling 100%. Mendel’s probability illustrated that probability can be used to predict an organisms traits. Lesson Checkpoint: What is the probability of a coin landing on tails? Punnett Squares A Punnett Square is a tool that scientists use to predict the probability of an organism’s genetic makeup. They show the different combinations of alleles that are possible with a particular trait and show the ratio’s of those traits. Probability of this Punnett Square: GG = 25%, Gg = 50%, and gg = 25% Phenotypes and Genotypes There are four important terms that geneticists use on regular basis and they are phenotype, genotype, homozygous, and heterozygous. A phenotype is the physical appearance of a particular organism. A pea plant can have one of two different physical appearances for pod color: either green or yellow. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
The genotype is the genetic make-up of a particular organism. An organism has two separate alleles for a trait. The allele combination is the genotype. A homozygous organism is one that has two of the same alleles for a particular trait. A pea plant with the alleles GG has a green pod while a pea plant with the alleles gg has a yellow pod. Both are said to be homozygous (Homozygous = the same). A heterozygous organism is one that has two different alleles for a particular trait. A pea plant with the alleles Gg has a green pod in every circumstance and are considered to be heterozygous (heterozygous = different). Chromosomes and Inheritance A chromosome is a strand of DNA that contains genes which define the traits of the individual. Chromosomes are passed on from parents to their offspring through the sex (eggs or sperm). These sex cells contain ½ of the total number of chromosomes as do the other cells that make up the rest of the body. Two sex cells come together to make one new cell of the offspring. For example, each human body cell has a total of 46 chromosomes. Human sex cells have 23 chromosomes. When the egg (23) from the female, comes together with a sperm (23) from the male, the result is a cell with a total of 46 chromosomes which has a different combination of chromosomes from both of the parents. The Chromosome Theory of Inheritance These concepts led to the chromosome theory of inheritance that states that genes are carried via chromosomes from the parents to their offspring. Each sex cell contains exactly half of each parent’s total number of chromosomes through a process known as meiosis. All organisms have a specific number of chromosomes that are different according to their particular species. Each chromosome is made up of thousands of different genes and on those chromosomes are genes that have two alleles on opposite chromatid that can either be the same or different from one another. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
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