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1. The unequal segregation of alleles to gametes during meiosis, resulting in gametes containing one allele being more numerous than gametes containing the other allele for certain genes.
2. The replacement of one part of a chromosome with a copy of the homologous part on the sister chromosome, often as a consequence of DNA repair.
gene conversionDNA RECOMBINATION process that results in one ALLELE in a HETEROZYGOTE being converted into the corresponding allele. Conversion can be a consequence of the mode of repair (see DNA REPAIR) of the base-pair mismatch (HETERODUPLEX) that accompanies recombination in MEIOSIS. The process can result in the four haploid products of meiosis exhibiting an unusual (aberrant) segregation pattern.
the unit of heredity most simply defined as a specific segment of DNA, usually in the order of 1000 nucleotides, that specifies a single polypeptide. Many phenotypic characteristics are determined by a single gene, while others are multigenic. Genes are specifically located in linear order along the single DNA molecule that makes up each chromosome. All eukaryotic cells contain a diploid (2n) set of chromosomes so that two copies of each gene, one derived from each parent, are present in each cell; the two copies often specify a different phenotype, i.e. the polypeptide will have a somewhat different amino acid composition. These alternative forms of gene, both within and between individuals, are called alleles. Genes determine the physical (structural genes), the biochemical (enzymes), physiological and behavioral characteristics of an animal.
The formation of gametes (sperm, ova) involves a process of meiosis, which allows crossing over between four pairs of chromosomes, two derived from each parent, which means that new forms of a particular chromosome are created. Gamete formation also results in cells (gametes) with a haploid (n) set of chromosomes that in fertilization creates a new individual, which is a recombinant of 2n chromosomes, half derived by way of the ovum from the mother and half via the spermatozoa from the father.
Changes in the nucleotide sequence of a gene, either by substitution of a different nucleotide or by deletion or insertion of other nucleotides, constitute mutations which add to the diversity of animal species by creating different alleles and can be used as a basis for genetic selection of different phenotypes. Some mutations, be they a single base change in a single gene or a major deletion, are lethal.
the way in which genes exert their effects on tissues or processes, e.g. by being dominant or recessive, or partially so, being absent, being sex-linked, being involved in chromosomal aberrations.
different forms of a particular gene usually situated at the same position (locus) in a pair of chromosomes.
see gene duplication (below).
the collection of DNA sequences in a given genome. Called also gene library.
responsible for the barred pattern on the feathers of Barred Plymouth Rock birds.
see box (4).
a group of related genes derived from a common ancestral gene, located closely together on the same chromosome. Called also multigene family.
two independent pairs of nonallelic genes, neither of which is functional without the other.
a non-reciprocal exchange of DNA elements during meiosis which results in a functional rearrangement of chromosomal DNA.
dihydrofolate reductase gene; an enzyme required to maintain cellular concentrations of H2 folate for nucleotide biosynthesis, and which has been used as a 'selective marker'; cells lacking the enzyme only survive in media containing thymidine, glycine and purines; mutant cells (dhfr) transfected with DNA that is dhfr′ can be selectively grown in medium lacking these elements.
diversity (D) gene
genes located in diversity (D) segment; contribute to the hypervariable region of immunoglobulins.
one that produces an effect (the phenotype) in the organism regardless of the state of the corresponding allele. Examples of traits determined by dominant genes are short hair in cats and black coat color in dogs.
as a result of non-homologous recombination, a chromosome carries two or more copies of a gene.
see expression (3).
the proportion of the substances or animals in the group which carry a particular gene.
genes located on the Y chromosome and appearing only in male offspring.
immune response (Ir) g's
genes of the major histocompatibility complex (MHC) that govern the immune response to individual immunogens.
see mobile dna.
replacement of a normal gene with a mutant allele, as in gene knockout mice.
one whose presence brings about the death of the organism or permits survival only under certain conditions.
see gene bank (above).
one that has undergone a detectable mutation.
non-protein encoding gene
the final products of some genes are RNA molecules rather than proteins.
when more than one mRNA is transcribed from the same DNA sequence; the mRNAs may be in the same reading frame but of different size or they may be in different reading frames.
total of all genes possessed by all members of the population which are capable of reproducing during their lifetime.
see probe (2).
one that produces an effect in the organism only when it is transmitted by both parents, i.e. only when the individual is homozygous.
regulator gene, repressor gene
one that synthesizes repressor, a substance which, through interaction with the operator gene, switches off the activity of the structural genes associated with it in the operon.
one that produces products which can be measured and therefore used as an indicator of whether a DNA construct has successfully been transferred.
one that is carried on a sex chromosome, especially an X chromosome.
nucleotide sequences coding for proteins.
the insertion of functional genes into cells of the host in order to alter its phenotype, usually used to treat an inherited defect.
tumor suppressor g's
a class of genes that encode proteins that normally suppress cell division that when mutated allow cells to continue unrestricted cell division and may result in a tumor.