Meiosis - Biology 111 Principles of Biology Lecture...

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Biology 111, Principles of Biology Lecture Outlines Virtually ALL multicellular eukaryotes are DIPLOID They have two copies of every chromosome PLOIDY refers to the number of chromosome SETS an organism carries Diploid organisms get one complete set of chromosomes (and the genetic information contained therein) from the mother and one complete set from the father The two copies of any particular chromosome are known as HOMOLOGS, or HOMOLOGOUS CHROMOSOMES Humans have 23 pairs of chromosomes (23 pairs of homologs) Each chromosome carries a different set of genes Two homologs carry the same genes and look the same, but they generally do not carry exactly the same information The eye color gene on one homolog may be blue The same gene on the other homolog may be brown Different versions of a gene are ALLELES Brown and blue are two different alleles of the gene for eye color. Each gene on a particular chromosome is localized at a particular position (LOCUS) Chromosomes may be visualized by isolating condensed chromosomes at metaphase of mitosis. Such a collection of chromosomes is known as a KARYOTYPE (see Figure 13.3): Two Classes of Chromosomes: AUTOSOMES: Major class of chromosomes Chromosome pairs are the same in both males and females (2 homologs of every autosome) Humans have 22 pairs of autosomes SEX CHROMOSOMES: This pair of homologs is different in males and females The sex chromosomes in humans are "X" and "Y"
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Females have two X-chromosomes (XX) Males have one X- and one Y-chromosome (XY) Unlike the autosomes, the X- and Y-chromosome look different from one another The Y-chromosome is much smaller than the X-chromosome and it does not carry as many genes In general, the genes located on the X-chromosome are NOT also located on the Y-chromosome In mammals, a single gene on the Y-chromosome determines maleness -- SRY (Sex-Reversal on the Y chromosome) -- In the absence of a functional SRY gene, an embryo will develop as a female
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Meiosis - Biology 111 Principles of Biology Lecture...

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