Chapter 8 Human Karyotypes and Chromosome Behavior Jones

Chapter 8 Human Karyotypes and Chromosome Behavior Jones and Bartlett Publishers © 2005

Chromosome stability • The normal chromosome complement of mitotic cells of an organism can be photographed in metaphase. • Actual cut-outs or digital images can be arranged to pair homologous chromosomes. • In humans, chromosome 1 is the longest.

Human karyotype

Human karyotypes: terms

The banding patterns of human chromosomes The short arm of a chromosome is called “p” (for ‘petite’) and the long arm “q”. A, B, C, etc. refer to grouping of chromosomes by size.

The 3 shapes of chromosomes based on the location of the centromere

• Acentric chromosomes lack a centromere. • Dicentric chromosomes have two centromeres. – These form a bridge during cell division, or the chromosome can break. Chromosome 2 in humans.

The proposed mechanism for the creation of the long human chromosome 2 by fusion of two shorter chromosomes

Similarity between chromosomes of the great apes

In human females, one of the 2 X-chromosomes is inactivated early in development Dosage compensation: In Drosophila males, the X chromosome has its transcription increased. C. elegans females decrease transcription of both X chromosomes. In mammals, one X is inactivated (Barr body). Calico cat example.

X-inactivation Barr bodies

Calico Cat Female cats heterozygous for orange and black hair color develop patches with either X inactivated. (The white area is another gene, S for white spotting and is autosomal. )

Pseudoautosomal inheritance • At the tips of both arms of the X chromosome are several megabases that are not inactivated. • These have homologous regions in the Y chromosome, so crossing over can occur. • These pseudoautosomal regions (PARp and PARq) have a 20 -fold higher recombination rate than in true autosomal chromosomes.

Proposed steps in the creation of the modern Y-chromosome by rearrangement of segments in an ancient Y-chromosome

Distribution of Y-chromosome haplotypes, presumed to have descended from Genghis Khan Haplotype: the allelic form of each of a set of linked genes present in a single chromosome.

Human chromosomal anomalies and spontaneous abortion

The probability of a child with Down syndrome increases with Mother’s age

Pairing and segregation of 3 homologous chromosomes in meiosis I This kind of chromosome pairing occurs in trisomics (2 n+1) or triploids (3 n)

Nondisjunction

Environmental risks for aneuploidy in humans • Suggested risks: radiation, smoking, alcohol consumption, oral contraceptives, fertility drugs, pollutants, pesticides, etc. • Maternal-age effects on meiosis – maybe estrogen has a role. • Bisphenol A is an estrogen mimic found in polycarbonate plastics. • In mice, aneuploid oocytes increased from 1. 4% to 11. 6%