BasicGeneticsSp08 - Trenham Basic Genetics for BIS 1B...

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Trenham Spring 2008 Basic Genetics for BIS 1B In BIS 1B we are interested in how the physical characteristics of an organism (the phenotype ) are determined by the genetic characteristics of the organism (the genotype ). We also want to understand how the parental genotype and phenotype are passed to the offspring. The offspring resemble the parents, but they are not identical to them, nor are they a simple average of the mother and father's phenotypes. To understand the resemblance between parents and offspring, we need to understand genetic inheritance. Genetic inheritance depends on the genetic information that parents give their offspring in the form of chromosomes inside an egg or a sperm. If we start with an egg or a sperm (each one is a haploid gamete), we find copies of half of the mother's chromosomes in the egg and copies of half of the father's chromosomes in the sperm. When the egg and sperm fuse to make a zygote , the two haploid (1N) gametes form a diploid (2N) cell. This zygote is the first cell in a new organism and it has all the genetic information needed to create a new individual. This cell will be copied many times to produce a multicellular organism. Chromosome number varies from organism to organism; for example, humans have 23 pairs of chromosomes, so 2N = 46, and a human egg or sperm will be haploid where 1N = 23. Each chromosome is made of DNA and contains genetic instructions for making proteins and other chemical compounds that make up bodies, send nerve impulses, and maintain a living organism. The DNA in a chromosome is divided into particular regions called genes. Each gene is responsible for making a product. The physical location where a gene sits is called a locus (plural = loci), meaning that the terms gene and locus are inter-changeable for this course. A gene A the A locus or Depiction of a chromosome with one gene marked on it. -1-
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So, chromosomes contain genes, and genes are instructions that can be transcribed and translated (read and followed) to produce a particular product. The combined action of all the genes in the genotype produces the physical characteristics (the phenotype) of the organism. These genes are divided among the different chromosomes present in the cell. Half of the chromosomes came from the mother (via the egg) and half came from the father (via the sperm). What instructions came from each parent? Why do biologists talk about pairs of chromosomes? It turns out that each egg contains one copy of every chromosome that will be in the new organism and each sperm also has a copy of every chromosome that will be in the new organism. So the new individual has 2 copies of every chromosome and therefore 2 copies of every gene. These 2 copies together form a pair of chromosomes and we call them a homologous pair of chromosomes, meaning that each chromosome in the pair has the same genes on it.
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This note was uploaded on 04/09/2008 for the course BIS 1b taught by Professor Kimsey during the Spring '08 term at UC Davis.

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BasicGeneticsSp08 - Trenham Basic Genetics for BIS 1B...

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