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Biology (Biological Sciences)
Darren De Lorey

BIOL207 B01 Lec03 2014-01-10 CHAPTER 2 cont’d A. Chromosome abnormalities a. Karyotypes = photograph of a metaphase chromosome; b. Deletions, inversions, insertions, translocations, duplications of large parts of chromosomes are all possible c. Aneuploidy = addition or subtraction of a chromosome from a homologous pair i. E.g. trisomy 21 = Down’s syndrome ii. Gene balance = biochemistry of the cell is thrown off by abnormal ratios of some gene products B. Chromosome number, ploidy, and DNAcount a. “n” = the number of chromosomes in a gamete b. “2n” = the number of chromosomes in a zygote c. number of chromosomes means the total number of chromosomes you count on under the microscope or on a karyotype d. There is no such thing as 4n, 6n, 8n… e. Humans are 2n=46, because you can count 46 chromosomes in a human cell. Wheat is 2n=42, because you can count 42 chromosomes in a wheat cell. f. DNAcontent in a cell can be measured in base pairs, or in picograms (pg), or it can be represented as multiples of the variable “c”. g. DNAcontent changes throughout cell cycle; if gamete is 1c, fertilization doubles this to 2c, then replication doubles this to 4c. h. Note that replication and mitosis changes c-value, but not chromosome number. Fertilization and meiosis change both c-value and chromosome number. i. Many plants and some animals are polyploid j. “x” can be used to define ploidy; e.g. tetraploids are 2n=4x so there would be 4 copies of each chromosome in a karyotype, rather than 2 as in a diploid. k. polyploids may form bivalents or polyvalents, depending how closely related the chromosomes are (autopolyploids vs. allopolyploids) l. 2n=6x=42 bread wheat ; six complete sets of chromosomes m. Stable; chromosomes segregate equally into daughter cells e.g. wheat n=3x=21 n. Polyploids need and even number of chromosome sets to be stable, e.g. 2n=4x, 6x, 8x, are all stable, but 2n=3x is not able to produce fertile offspring. C. Endoreduplication a. many chromatids per chromosome b. occurs in only some c
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