# 3: Alleles and Probabilities

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• 3.1: Genes are the Basic Units of Inheritance
• 3.2: Model Organisms Facilitate Genetic Advances
• 3.3: Mendel’s First Law
Mendel’s First Law, also called The Law of Equal Segregation, states that during gamete formation, the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either allele. More than one allele of a gene can be present in an individual since  most eukaryotic organisms have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, chromosomes exist as pairs, with one homolog inherited from each parent.
• 3.4: Biochemical Basis of Dominance
For the majority of genes studied, the normal (i.e. wild-type) alleles are haplosufficient. So in diploids, even with a mutation that causes a complete loss of function in one allele, the other allele, a wild-type allele, will provide sufficient normal biochemical activity to yield a wild type phenotype and thus be dominant and dictate the heterozygote phenotype.
• 3.5: Crossing Techniques Used in Classical Genetics
Mendel also invented several testing and analysis techniques still used today. Classical genetics is the science of solving biological questions using controlled matings of model organisms. It began with Mendel in 1865 but did not take off until Thomas Morgan began working with fruit flies in 1908. Later, starting with Watson and Crick’s structure of DNA in 1953, classical genetics was joined by molecular genetics, the science of solving biological questions using DNA, RNA, and proteins isolated
• 3.6: Dihybrid Crosses
Mendel's Second Law, also called the Law of Independent Assortment, argues that two loci assort independently of each other during gamete formation. The commonly observed 9:3:3:1 phenotypic ratio that is predicted from this law  can also be obtained using Punnett Square
• 3.7: Why Use Nomenclature?
• 3.8: Basic Nomenclature
• 3.9: Linked Genes

Thumbnail:  Example of a Punnett square. In this example in peas, the color yellow is determined by the dominant allele Y and the color green is determined by a recessive allele y. (CC BY-SA 3.0 Unported; Pbroks13 via Wikipedia)

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