Summarizing Chromosomal Basis of Inherited Disorders

Summary

The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram and allows for the assessment of many chromosomal abnormalities. Disorders in chromosome number, or aneuploidies, are typically lethal to the embryo, although a few trisomic genotypes are viable. Because of X inactivation, aberrations in sex chromosomes typically have milder phenotypic effects. Aneuploidies also include instances in which segments of a chromosome are duplicated or deleted. Chromosome structures may also be rearranged, for example by inversion or translocation. Both of these aberrations can result in problematic phenotypic effects. Because they force chromosomes to assume unnatural topologies during meiosis, inversions and translocations are often associated with reduced fertility because of the likelihood of nondisjunction.

Glossary

aneuploid

individual with an error in chromosome number; includes deletions and duplications of chromosome segments

autosome

any of the non-sex chromosomes

chromosome inversion

detachment, 180° rotation, and reinsertion of a chromosome arm

euploid

individual with the appropriate number of chromosomes for their species

karyogram

photographic image of a karyotype

karyotype

number and appearance of an individuals chromosomes; includes the size, banding patterns, and centromere position

monosomy

otherwise diploid genotype in which one chromosome is missing

nondisjunction

failure of synapsed homologs to completely separate and migrate to separate poles during the first cell division of meiosis

paracentric

inversion that occurs outside of the centromere

pericentric

inversion that involves the centromere

polyploid

individual with an incorrect number of chromosome sets

translocation

process by which one segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome

trisomy

otherwise diploid genotype in which one entire chromosome is duplicated

X inactivation

condensation of X chromosomes into Barr bodies during embryonic development in females to compensate for the double genetic dose