Key Terms
Linked genes
Genes located on the same chromosome. They tend to be inherited together.
Process
During meiosis, homologous chromosomes pair (synapsis) and physically exchange segments at points called chiasmata.
Result
New allele combinations that were not present in either parent.
EXAMPLE
Parent has alleles AB on one chromosome, ab on the other.
Centimorgan (cM)
Unit of genetic distance; 1 cM = 1% recombination frequency.
FORMULA
Recombination frequency = (number of nonparental offspring) / (total offspring)
Karyotype
Number and appearance of an individual's chromosomes.
Karyogram
Photographic image of a karyotype arranged by size and centromere position.
Euploid
Correct chromosome number.
Aneuploid
Incorrect chromosome number (includes monosomy and trisomy).
CAUSE OF ANEUPLOIDY
Nondisjunction — homologous chromosomes or sister chromatids fail to separate during meiosis.
Nondisjunction during meiosis I
Homologs fail to separate.
Nondisjunction during meiosis II
Sister chromatids fail to separate.
WHY SEX CHROMOSOME DISORDERS ARE MILDER
X inactivation. In female mammals, one X chromosome in each cell condenses into a Barr body early in development, becomi
Tortoiseshell cats
A visible consequence of X inactivation. Coat color in cats is X-linked.