Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 14

Each species has a characteristic number of chromosomes. Cytogenetics links abnormal chromosomes to phenotypes. Chromosome charts, or karyotypes, provide information on genetic disease, exposure to environmental toxins, and evolutionary relationships between species.
Genes on the same chromosome are linked; rather than demonstrating independent assortment, they produce a large number of parental genotypes and a small number of recombinant genotypes.
Genotype predictions and linkage maps are derived from knowing allele configurations (coupling or repulsion) and crossover frequencies, which are directly proportional to distances between genes. The first linkage maps depicted genes on the X chromosome, which are easier to follow because they are present in only one copy in the heterogametic sex.
In humans, the male is hemizygous for genes on the Y chromosome. The female, with two X chromosomes, is homogametic. The SRY gene on the Y chromosome controls other genes that stimulate development of male structures and suppress development of female structures.
A sex-linked trait passes from mother to son because the male inherits his X chromosome from his mother and his Y chromosome from his father. A sex-linked allele may be dominant or recessive. Y-linked traits are very rare.
X inactivation shuts off one X chromosome in the cells of female mammals, equalizing the number of active sex-linked genes in each sex. Early in development, each female cell inactivates one X chromosome. A female is mosaic for sex-linked heterozygous genes on the X chromosome because the X chromosomes are inactivated at random with respect to their origin.
A chromosome consists of DNA, RNA, and proteins. Chromosomes are distinguishable by size, centromere position, satellites, staining patterns, and DNA probes. Dark-staining DNA, called heterochromatin, maintains the chromosome's structure. It surrounds the centromere, comprises the telomeres, and is interspersed with light-staining, protein-encoding euchromatin.
Chromosomal abnormalities may affect number of chromosomes, missing (deletion) or extra (duplication) genetic material, and rearrangements. Polyploid cells have extra chromosome sets, and aneuploids have extra or missing individual chromosomes. A trisomy (one extra chromosome) is less harmful than a monosomy (one absent chromosome). Sex chromosome aneuploidy is less severe than autosomal aneuploidy. Nondisjunction, an uneven division of chromosomes in meiosis, causes aneuploidy.
Chromosomal rearrangements disrupt meiotic pairing, which can delete or duplicate genes. An inversion flips gene order, affecting the phenotype if it disrupts a vital gene. A Robertsonian translocation fuses the long arms of two nonhomologs. In a reciprocal translocation two nonhomologs exchange parts. Translocation carriers have reproductive problems because they produce some unbalanced gametes as a result of the unequal division of chromosome regions during meiosis. Fragile X syndrome causes mental retardation and is associated with a fragile site on the X chromosome.