Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance

the theory that the behavior and movement of an organism's chromosomes, during meiosis, accounts for Mendel's two laws (of segregation and independent assortment).

Autosomal Chromosomes (Autosomes)

chromosomes derived from somatic cells; most of an organism’s chromosomes are autosomal; e.g., in humans, 22 of our 23 homologous pairs are autosomal.

Sex Chromosomes

chromosomes derived from germ cells; only a few of an organism's chromosomes are sex chromosomes; e.g., in humans, only 1 homologous pair (the X and Y chromosome) are sex chrom.

Linked Genes

genes that are located on the same chromosome; Because they’re close together, they usually get inherited as a pair instead of separating; crossing over can shuffle them, creating new gene combinations.

Sex-Linked Genes

genes that are located specifically on the sex chromosomes (i.e., the X or Y chromosome).

Recombinants

offspring possessing new combinations of traits that are inherited from both parents; recombinants are phenotypically distinct from their parents because they show new trait combinations made by mixing features from both parents.

Unlinked Genes

genes located on separate chromosomes; indep. assortment accounts for recombination of unlinked genes.

Gene Mapping

a technique that allows geneticists to use recombination and crossing over data in order to determine a gene's exact position on a chromosome; the technique determines the loci of genes.

X-Linked Genes

sex-linked genes that are located exclusively on the X-chromosome (there is no counterpart on the Y).

Barr Bodies

a condensed, inactive X-chromosome in the nucleus of cells of mammalian females.

Nondisjunction

a chromosomal error that can occur during meiosis I or II; the members of a homologous pair of chromosomes or sister chromatids fail to separate; results in abnormal gametes.

Aneuploidy

the condition of an offspring having an abnormal number of chromosomes due to an abnormal gamete (caused by nondisjunction) fertilizing a normal gamete.

Trisomy

type of aneuploidy in which the organism has an extra copy of one particular chromosome; the chromosome is found in triplicate within each cell; this condition is often expressed as 2n + 1.

Monosomy

type of aneuploidy in which the organism has only a single copy of one particular chromosome; the organism is missing a copy of the chromosome in question; this condition is often expressed as 2n - 1.

Polyploidy

general term for the condition in which an organism possesses more than two complete sets of chromosomes; less dangerous and debilitating than aneuploidy; common in plants, rare in animals.

Triploidy

type of polyploidy in which the organism possesses three complete sets of chromosomes (3n).

Tetraploidy

type of polyploidy in which the organism possesses four complete sets of chromosomes (4n).

Mosaicism

condition in which an individual is composed of two populations of genotypically distinct cells, both coming from a single fertilized egg; usually results from errors in embryonic mitosis.

Chimerism

when one person has two sets of genotypically distinct cells from different zygotes. It happens because two embryos merge into a single individual, creating mixed cell populations.

Deletions

a type of structural alteration to chromosomes; a chromosomal segment breaks away, leaving the chromosome without that particular genetic material (the chromosome is “missing a piece”).

Duplications

structural alteration to chromosomes; occurs when a DNA segment breaks off one member of a homologous pair and attaches to its partner. This makes the second chromosome carry an extra copy of that segment, which can sometimes lead to useful mutations.

Inversions

a type of structural alteration to chromosomes; a chromosomal segment breaks away and then reattaches to the original chromosome, but in reverse order (the genetic material is now in the improper position on the chromosome).

Translocations

type of structural alteration to chroms.; a chromosomal segment breaks away from one chrom. and then reattaches to a nonhomologous chrom. (causing the nonhomologue to have extra material that it normally does not contain).