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uncorrected page proofs
uncorrected page proofs

... ‘naked lady’ is due to the fact that after the leaves of the plant appear in spring they die off, and the flowers appear in autumn on their own (see figure 14.2). This simple but beautiful plant is poisonous. Deaths have occurred, often after a person has mistaken the plant for wild garlic and eaten ...
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... 3. The gene comes in two forms called alleles. If the gene is phonetically called a “wye” gene, then the two alleles can be represented by Y (standing for the yellow phenotype) and y (standing for the green phenotype). 4. A plant can be either Y/Y, y/y, or Y/y. The slash shows that the alleles are a ...
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... How do you get so many differences in traits? • Well, let’s start at the very beginning…a very good place to start. • If you look at the DNA of any cell, you will find that each strand has a pair or partner. • These are called homologous pairs • One of the chromosomes from a pair comes from mom, ...
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File

... Mendel carried out his experiments in three steps: • 1. Mendel allowed each variety of garden pea to selfpollinate for several generations. This method ensured that each variety was true-breeding (purebred) for a particular trait; that is all the offspring would show only one form of a particular ...
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... Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings ...
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Meiosis



Meiosis /maɪˈoʊsɨs/ is a specialized type of cell division which reduces the chromosome number by half. This process occurs in all sexually reproducing single-celled and multi-celled eukaryotes, including animals, plants, and fungi. Errors in meiosis resulting in aneuploidy are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids. In meiosis I, homologous chromosomes pair with each other and can exchange genetic material in a process called chromosomal crossover. The homologous chromosomes are then segregated into two new daughter cells, each containing half the number of chromosomes as the parent cell. At the end of meiosis I, sister chromatids remain attached and may differ from one another if crossing-over occurred. In meiosis II, the two cells produced during meiosis I divide again. Sister chromatids segregate from one another to produce four total daughter cells. These cells can mature into various types of gametes such as ova, sperm, spores, or pollen.Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a zygote with a complete chromosome count containing a combination of paternal and maternal chromosomes. Thus, meiosis and fertilization facilitate sexual reproduction with successive generations maintaining the same number of chromosomes. For example, a typical diploid human cell contains 23 pairs of chromosomes (46 total, half of maternal origin and half of paternal origin). Meiosis produces haploid gametes with one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis. Thus, if a species has 30 chromosomes in its somatic cells, it will produce gametes with 15 chromosomes.
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