Cytogenetics: the subdiscipline within genetics that focuses on
II. Portrait of a Chromosome
Karyotype--- A chart arranging chromosome pictures according to
their size, grouped by pairs of homologs.
Anatomy of a chromosome
Centromeres are the largest constriction of the chromosome
The chromosome region next to the telomere consists of 8,000 - 300,000
Repeats similar to the telomere sequence
Multigene families of genes
(e.g. olfactory receptor genes)
Chromosomes differ in size
Chromosomes carry different genes
III. Visualizing chromosomes
Obtain tissue from person
Cells which contain a normal chromosome constitution are called euploid.
IV. Chromosome Abnormalities
Extra chromosome set
Extra or missing chromosome
one chromosome absent
one chromosome extra
Part of a chromosome missing
Part of a chromosome present twice
Segment of chromosome reversed
Individuals with three copies of each chromosome are triploid.
Polyploidy accounts for 17% of all spontaneous abortions and 3% of
Two sperm fertilize one egg.
Haploid sperm fertilizes diploid egg.
Trisomies and Monosomies
One extra or one missing chromosome results in extra or missing copies
of all of the genes on that chromosome.
Most trisomies and monosomies produce inviable embryos.
Some fetuses with trisomy of smaller autosomes survive to birth with
The frequency of Down syndrome correlates with the age of the mother
Mother’s age Down syndrome frequency
30 ~ 34 1/600
35 ~ 39 1/280
40 ~ 44 1/70
45 ~ 49 1/40
Sex Chromosome Aneuploidy
Inheritance of two chromosomes from one parent is called uniparental
May occur when nondisjunction occurs in both parents (one disomic
gamete and one without homolog)
Loss or nondisjunction of one homolog in early embryo followed by
reduplication of remaining homolog.