In the example above, the phenotype of the heterozygote is the same as that of one of the homozygotes; in cases where it is different from both, the two alleles are said to be co-dominant.
a chromosome at the beginning of the cell cycle, in G1 phase; the single long bar centromere represents one long double helix of DNA a chromosome near the end of the cell cycle, in metaphase; it is duplicated and condensed, consisting of mother father two identical sister chromatids (each containing one DNA double helix) joined at the centromere.
“p” arm “q” arm A normal diploid chromosome set, as seen in a metaphase spread, prepared pair of autosomes by bursting open a cell at metaphase and staining the scattered maternal 1 chromosomes. In the example shown schematically here, there are three pairs of autosomes (chromosomes both parents, regardless of sex) and two sex chromosomes—an X from the mother and a Y from the father. The maternal 2 numbers and types of sex determination are variable from one class of organisms to another, as is the X number of pairs of autosomes. For simplicity, the cycle is shown for only sex chromosomes one chromosome/chromosome pair.
MEIOSIS AND GENETIC RECOMBINATION The greater the distance chromosome, the greater is the chance that they will be
RECOMBINATION site of crossing-over occurring at a site between them. If two genes are thus reassorted in x% of gametes, they are said to be separated on a chromosome by a genetic map AB distance of x map units (or genotype ab haploid gametes (eggs or sperm) x centimorgans).
POINT MUTATION: maps to a single site in the genome, corresponding to a single nucleotide pair or a very small part of a single gene
INVERSION: inverts a segment of a chromosome lethal mutation: causes the developing organism to die prematurely. conditional mutation: produces its phenotypic effect only under certain conditions, called the restrictive conditions. Under other conditions—the permissive conditions—the effect is not seen. For a temperature-sensitive mutation, the restrictive condition typically is high temperature, while the permissive condition is low temperature.
loss-of-function mutation: either reduces or abolishes the activity of the gene. These are the most common class of mutations. Loss-of-function mutations are usually recessive—the organism can usually function normally as long as it retains at least one normal copy of the affected gene. null mutation: a loss-of-function mutation that completely abolishes the activity of the gene.
TWO GENES OR ONE?
Given two mutations that produce the same phenotype, how can we tell whether they are mutations in the same gene? If the mutations are recessive (as they most often are), the answer can be found by a complementation test.
COMPLEMENTATION: MUTATIONS IN TWO DIFFERENT GENES
DELETION: deletes a segment of a chromosome
TRANSLOCATION:  breaks off a segment from one chromosome and attaches it to another gain-of-function mutation: increases the activity of the gene or makes it active in inappropriate circumstances; these mutations are usually dominant. dominant-negative mutation: dominant-acting mutation that blocks gene activity, causing a loss-of-function phenotype even in the presence of a normal copy of the gene. This phenomenon occurs when the mutant gene product interferes with the function of the normal gene product. suppressor mutation: suppresses the phenotypic effect of another mutation, so that the double mutant seems normal. An intragenic suppressor mutation lies within the gene affected by the frst mutation; an extragenic suppressor mutation lies in a second gene—often one whose product interacts directly with the product of the frst.
In the simplest type of complementation test, an individual who is homozygous for one mutation is mated with an individual who is homozygous for the other. The phenotype of the offspring gives the answer to the question.