ASB 194 Lecture 3 new.doc

ASB 194 Lecture 3 new.doc - Lecture 3. SOME THINGS DARWIN...

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Lecture 3. SOME THINGS DARWIN DIDNT KNOW ABOUT: I. Heredity and genetics: Darwin did not know of the existence of genes , although he knew that some kind of material, termed germ plasm, was passed from parent to offspring. It was thought that the process of inheritance involved some kind of blending of traits from each parent. Blending was a problem for the theory of natural selection, because if blending occurred, variation (a key component of the theory) would continuously be diluted through mating---eventually every organism would be the same. How could red birds ever evolve from green ones? The first bird with a little red on it would immediately have the trait swamped by the population of birds carrying the green trait. Gregor Mendel’s main contribution was that he discovered and demonstrated experimentally that traits were inherited as distinct, non- blending units: this is called the principle of particulate inheritance. He demonstrated some of these principles with his experiments with pea plants. What Mendel and successive molecular biologists showed is that: 1) Biological traits are coded for by sequences of DNA called genes . 2) Genes for each trait come in pairs. In sexually reproducing organisms, one member of the pair comes from the mother’s ova, the other member from the father’s sperm. The character received from the mother and father can be the same or different. 3) The gametes produced by any adult have only one copy of each gene, and the probability of the gamete carrying either of the two parental copies is 50%. 4) Often one of the two genes for each trait is dominant to the other and determines the final observed trait. The unobserved trait (and the gene that codes for it) is called recessive . Only when two forms of the recessive gene are present in an individual do we observe the recessive form of the trait. 5) Strings of genes are stuck together on structures called chromosomes .
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Humans have 23 pairs of chromosomes each with about 60,000 genes that code for protein products. 6) Different genes for different traits generally assort independently of each other. Thus knowing that the mother has passed on one form of a gene does not tell us which other gene forms she has contributed. (eg. The genes for hair color are passed on independently of the genes for blood type). 7) Genes are DNA and generally code for single protein chains. A single protein rarely results in any observable trait (just as a single ingredient cannot make a cake). But changes in a single gene can change some aspect of the observed trait (just like substituting salt for sugar drastically changes the flavor of a chocolate cake even though it is still a cake). Thus the observed trait, phenotype , is partially determined by the genotype. Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions
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ASB 194 Lecture 3 new.doc - Lecture 3. SOME THINGS DARWIN...

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