Chapter 12 The Chromosomal Basis of Inheritance

Wild type phenotype

The phenotype for a character most commonly observed in natural populations, such as red eyes in Drosophila

Mutant phenotype

Traits that are alternatives to the wild type, such as white eyes in Drosophila. These occur due to alleles assumed to have originated as changes, or mutations, in the wild-type allele

Drosophila melanogaster

The fruit fly, a model organism for genetic analysis.

Chromosome theory of inheritance

A basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.

sex chromosomes

chromosomes responsible for determining the sex of an individual

X chromosome

The human sex chromosome that is present in both sexes: one in males and two in females

Y chromosome

The sex chromosome found only in males. When paired with an X chromosome from the mother, it produces a male child.

SRY gene

The sex determining region of the Y chromosome in males. Encodes the testis-determining factor, which turns the primordial gonads into the testes.

sex-linked gene

A gene located on either sex chromosome

X-linked gene

A gene located on the X chromosome; such genes show a distinctive pattern of inheritance

Y-linked gene

A gene located on the Y chromosome

Duchenne muscular dystrophy

A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.

Hemophilia

An X-linked recessive disorder in which blood fails to clot properly, leading to excessive bleeding if injured.

X inactivation

One of two X chromosomes is randomly inactivated and remains coiled as a Barr body; this prevents double expression of genes found on X chromosomes in females.

Barr body

A dense object lying along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome.

Mosaic expression

Some cells have an inactive maternal X chromosome, while others have an inactive paternal chromosome

Linked genes

Genes located on the same chromosome that tend to be inherited together in genetic crosses.

Genetic recombination

The production of offspring with combinations of genes that differ from those found in either parent due to crossing over and independent assortment.

Parental type

An offspring with a phenotype that matches one of the parental phenotypes; also refers to the phenotype itself.

Recombinant type

An offspring with a different combination of alleles from that of either parent, resulting from one or more crossovers in prophase I of meiosis and independent assortment.

Crossing over

Process in which homologous chromosomes exchange portions of their chromatids during meiosis.

Genetic map

An ordered list of the gene loci along a particular chromosome.

Linkage map

A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.

Map units

A measurement of the distance between genes; one map unit is equivalent to a 1 percent recombination frequency.

Nondisjunction

An error in meiosis or mitosis in which members of a pair of homologous chromosomes or sister chromatids fail to separate properly from each other

Aneuploidy

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number

Trisomy

A diploid cell that has three copies of a particular chromosome instead of the normal two

Monosomy

A diploid cell that has only one copy of a particular chromosome instead of the normal two

Polyploidy

A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division

Deletion

A mutational loss of one or more nucleotide pairs from a gene

Duplication

An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated

Inversion

An aberration in chromosome structure resulting from reattachment of a chromosomal fragment in a reverse orientation to the chromosome from which it originated

Translocation

An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome

Trisomy 21: Down syndrome

A human genetic disease usually caused by the presence of an extra chromosome 21; characterized by developmental delays and heart and other defects that are generally treatable or non-life-threatening

Cri du chat

A chromosomal deletion in chromosome 5 that results in children who are mentally retarded, have small heads, and have a cry that sounds like a loud cat; they die at a young age

Chronic myelogenous leukemia

A form of leukemia characterized by the accumulation of abnormal, mature granulocytes (granulocytes are a type of white blood cell)