Introduction to Genetics

The work of Gregor Mendel

• Born in 1822 in Czechoslovakia
• Augustinian Monk and high school teacher
• Worked exclusively with pea plants in the monastery garden
• Published his work in 1868

Vocabulary

• Gene - segment of DNA that codes for a particular protein

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Trait – characteristic that a living thing can pass on to its young

Vocabulary (continued)

• Locus: A specific location on a chromosome. Alternatively, the position on a chromosome at which the gene for a particular trait resides.

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• Allele: one of a number of different forms of the same gene for a specific trait.

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Vocabulary (continued)

• Genotype: genetic makeup of an organism (the DNA)

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• Phenotype: physical characteristics of an organism (what you see)

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• Homozygous = Organism that has two identical alleles for a particular trait. ex. YY or yy

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• Heterozygous = Organism that has two different alleles for the same trait and is said to be hybrid for that particular trait
• . ex. Yy
• Hybrids: Organism with two different alleles for a particular trait

Classic Genes & Dominance

• Mendel identified 7 traits that were easy to identify
• Crosses of parents with the same traits were called true breeds.
• Crosses of parents with different traits were called hybrids

Mendel’s First Experiments

• Mendel wanted to find out how traits were passed from parents to offspring

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• Some of the problems he had to overcome where: how to keep the plants from self-fertilizing, how to keep track of the traits, how to keep the experiment controlled

How to prevent self-fertilization!

Mendel’s first cross of true breeds

• The true breeding parents are called P Generation

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• The first generation of hybrids are called F1

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• The second generation of hybrids are called F2

The Law of Segregation

• The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.

What he discovered from that first Experiment

• Dominant: form of a gene that is expressed even if present with a contrasting recessive allele. Will express themselves if only one allele is present

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• Recessive: description of a form of a gene that is only expressed in the homozygous state. Will only express themselves if both alleles are present.

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Classic genetics The Law of Dominance

• The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.

Genotypes versus Phenotype

• T represents the allele for tallness in plants, and t represents the allele for shortness. Tall is dominant (in this plant)
• Homozygous dominant: TT (both alleles are dominant)
• Heterozygous: Tt (One allele is dominant and one is recessive)
• Homozygous recessive: tt (both alleles are recessive)

Challenge

Work with the person sitting next to you and try to figure out the following problem.

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• How could you tell the difference between a plant that was TT versus Tt?

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Punnett square example

Figure 14.5 Genotype versus phenotype

Figure 14.6 A testcross

Mendel’s law of segregation (Layer 1)

Mendel’s law of segregation (Layer 2)

Punnett square review:

Probability and Statistics

• The likelihood of a particular event occurring. Chance
• You cannot predict the precise outcome of an individual event.
• Can be expressed as a fraction or a percent.
• Example: coin flip.
• Probability resets after each flip.