Trenham Spring 2008 Basic Genetics for BIS 1BIn 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 haploidgamete), 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 geneand locusare inter-changeable for this course. Agene Athe A locusorDepiction of a chromosome with one genemarked on it.-1-
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