BIOL207 B01 Lec03
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
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 DNA count
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. DNA content 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. DNA content 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
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. 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
a. many chromatids per chromosome b. occurs in only