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Evolution Lecture 7

Evolution Lecture 7 - E volution Lecture 7 Recombination in...

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Evolution Lecture 7 Recombination: in diploid organisms like humans, if you look into the nucleus you will see chromosomes in pairs. We have 23 pairs; other species have 200 etc.. If alleles are carried on these chromosomes, they are transmitted to the gametes. You produce gametes that carry half the number of chromosomes. The chromosomes that each gamete is carried is chosen at random. (see illustration in notes). This is Mendel’s second law: the independent assortment of chromosomes into gametes. Whenever there is sexual reproduction, this is going on. As a result, the gametes combine or the individual could self fertilize. Either way it can produce offspring. Suppose one of the gametes (pollen grain) produces an egg to have a heterozygote individual. By self fertilization, this individual has produced an offspring that is genetically different from itself. different gametes will produce to form different siblings. The independent assortment of chromosomes into gametes, and then the joining of the chromosomes produces genetically variable offspring. They are different from the parents and from one another. The possibilities for producing new genotypes are enormous. But there are further ways to combine genes. There is the process of crossing over: Crossing over occurs when there is a physical exchange of material between the chromosomes.. even within chromosomes, there are ways to produce combinations of genes in offspring that are different from the parents. And every chromosome pair in diploids always goes through one crossing over. It seems necessary to go through meiosis. It doesn’t matter if genes are on the same chromosome, or on different ones as long as they have crossed over. Meiosis + Fertilization = New Genotypes. It creates offspring that are genetically different from parents and from one another even if they are full siblings. Recombination produces new genotypes. Sexual reproduction has evolved a lot. Natural selection is affecting the way sexual reproduction takes place. In humans, the gametes that are formed are very different in size. Sperm are tiny, and eggs are millions of times bigger. This is true with all
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sexually reproducing species. That situation is called anisogamy. The alternative is isogamy. A species that is isogomous produces gametes that are the same size. Anisogamy means that they are not the same size; they are different in size. Some algae and fungi are isogomous. Anisogamy is the rule though. How do you we know if we are looking at the male or female? The female produces the larger gamete. Isogamy is the exception. Bacteria don’t engage in sexual reproduction of this kind. They have mechanisms where they can exchange genetic material but they don’t have sexual reproduction.
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