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TITLE:  Introduction to Genetics
Presented to grade level 7

The purpose of this lesson is to give the students an introduction to the field of genetics, focusing on the history of the science of genetics with an emphasis on Mendelian genetics. The lesson also includes an overview of the terms used in the field of genetics.
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1. Mendel's Pea experiment.
Why did Mendel choose pea plants?  Pea plants grow and reproduce quickly.  They have a wide variety of different traits, and were easy to breed and cross-pollinate.

What did Mendel do?  First, he tranferred pollen from flowers on short-stemmed plants to the stigmas of flowers on other short-stemmed plants and found out that all the offspring were short-stemmed.  Mendel called these plants true-breeding plants, plants that always produce offspring with the same traits as their parents.  When he crossed tall-stemmed plants with other tall-temmed plants, he found that both tall and short plants resulted.

What were Mendel's conclusions?  Mendel realized that each plant must contain two factors for a particular trait.  He called these factors genes.  Alternative forms of a gene (tall and short) are now called alleles. When he crossed two true-breeding plants with opposite traits (for example tall with short), the offspring showed only one of the traits, which Mendel called the dominant trait.   He called the weaker trait which did not appear in those offspring recessive.

How are the genes inherited?  Mendel realized that each parent could only contribute one of the factors (genes) to the plant in the offspring generation, so the offspring in the first filial generation would get one factor from the female parent and one from the male parent.  The two gene types contributed by a single parent separate into the sex cells according to Mendel's Law of Segregation.  Each gene pair for a trait is inherited independently of the gene pairs for other traits. For example, when a tall plant with yellow seeds forms its sex cells, the genes for stem length separate independently from the genes for seed color. This is referred to as Independent Assortment.

2. Important Terms.
Trait The characteristics of an organism.
Genetics The study of heredity, or the passing on of traits from an organism to its offspring.
True-breeding Organisms that always produce offspring with the same traits as their parents.
P generation The parental generation
F1 generation The first filial generation; the result of crossing two members of the P generation
F2 generation The second filial generation; the result of crossing two members of the F1 generation
Genes The units of heredity
Phenotype The physical appearance of a trait
Dominant allele An alternative gene form that is "stronger", and controls the phenotype despite the presence of a recessive allele.  Represented with a capital letter, for instance T for tall.
Recessive allele An alternative gene form that is "weaker" and that usually does not influence the phenotype in the presence of a dominant allele.  Represented by a lower case letter, for instance t for short.
Pure-bred Organisms that have two of the same allele (TT or tt).
Hybrid Organism with a mix of alleles from different pure-bred types (Tt).
Law of segregation When the parent plant forms sex cells (sperm or eggs), the parent's gene pairs separate, so a parent's sex cell will carry only one allele for a particular trait.
Law of independent
Each gene pair for a trait is inherited independently of the gene pairs for all other traits.
Incomplete dominance Each allele modifies the phenotype to some degree, producing a blended trait.

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LS 13


The student will investigate and understand that organisms reproduce and transmit genetic information to new generations. The key concepts included with this lesson are: the characteristics that can and cannot be inherited, and the historical contributions and significance of discoveries related to genetics.
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Materials needed:  Red and blue paper and a blue or red filter (or cellophane)

1. Complete and Incomplete Dominance.

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Materials needed: Dominant and Recessive Traits Worksheet

1. Dominant and Recessive Traits.

2.  Review sheet.
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Literature .
Local Places to Visit .
Related Web Sites Mendel's Paper on plant hybridization (English)
Mendel and Mendelian Genetics

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Site author:  Dave Wenk
Site maintained by lmj8a@virginia.edu
last updated: March 10, 2000