BSC 370 Chapter 4 book

gene interaction

The production of novel phenotypes by the interaction of alleles of different genes.

X-linkage

The pattern of inheritance resulting from genes located on the X chromosome.

loss-of-function mutation

A mutation that produces alleles encoding proteins with reduced or no function.

null allele

A mutant allele that produces no functional gene product. Usually inherited as a recessive trait.

gain-of-function mutations

A type of mutation in which the gene product takes on a new function and produces a phenotype different from that of the normal allele and from any loss-of-function alleles.

neutral mutations

A mutation with no perceived immediate adaptive significance or phenotypic effect.

incomplete, or partial dominance

Expressing a heterozygous phenotype that is distinct from the phenotype of either homozygous parent. Also called partial dominance.

threshold effect

Condition where normal phenotypic expression occurs any time a minimal level of wild type gene product is attained.

codominance

A condition in which the phenotypic effects of a gene’s alleles are fully and simultaneously expressed in the heterozygote.

multiple alleles

In a population of organisms, the presence of three or more alleles of the same gene.

H substance

The carbohydrate group present on the surface of red blood cells to which the A and/or B antigen may be added. When unmodified, it results in blood type O.

Bombay phenotype

A rare variant of the ABO antigen system in which affected individuals do not have A or B antigens and thus appear to have blood type O, even though their genotype may carry unexpressed alleles for the A and/or B antigens.

epigenesis

The idea that an organism or organ arises through the sequential appearance and development of new structures, in contrast to preformationism, which holds that development is the result of the assembly of structures already present in the egg.

heterogeneous trait

A mutant phenotype that may occur as the result of a mutation in any one of many genes required for normal expression of the trait during development, e.g., hereditary deafness.

epistasis

The nonreciprocal interaction between nonallelic genes such that one gene influences or interferes with the expression of another gene, leading to a specific phenotype.

Recessive epistasis

a gene interaction in which the homozygous recessive genotype at one locus (aa) masks the expression of alleles at a second locus, producing a characteristic 9:3:4 phenotypic ratio in a dihybrid F₂.

Dominant epistasis

a gene interaction where a single dominant allele at one locus (A_) masks the expression of alleles at a second locus, often yielding a characteristic 12:3:1 F₂ phenotypic ratio in a dihybrid cross.

Complementary gene interaction

a two-gene interaction where both loci must carry at least one dominant allele to produce a particular phenotype. Any genotype missing dominance at either locus (aaB_, A_bb, or aabb) shows the alternative phenotype. The classic F₂ phenotypic ratio is 9:7.

Complementation analysis (complementation test)

genetic test used to determine whether two recessive mutations that produce the same phenotype are in the same gene (allelic) or in different genes (non-allelic).

complementation group

An array of mutations that all test negatively when assayed in a complementation test. Thus, they are alleles in the same gene.

pleiotropy

A condition in which a single mutation causes multiple phenotypic effects.

hemizygous

Having a gene present in a single dose in an otherwise diploid cell. Usually applied to genes on the X chromosome in heterogametic males.

sex-limited inheritance

A trait that is expressed in only one sex even though the trait may not be X-linked or Y-linked. (Milk production)

sex-influenced inheritance

A phenotypic expression conditioned by the sex of the individual. A heterozygote may express one phenotype in one sex and an alternate phenotype in the other sex (e.g., pattern baldness in humans).

Rule of x linked dominant

Affected fathers will always have affected daughters

Rule of x linked recessive

Affected mothers will always have affected sons

penetrance

The frequency, expressed as a percentage, with which individuals of a given genotype manifest at least some degree of a specific mutant phenotype associated with a trait.

expressivity

The degree to which a phenotype for a given trait is expressed.

genetic background

The impact of the collective genome of an organism on the expression of a gene under investigation.

position effect

A change in expression of a gene associated with a change in the gene’s location within the genome.

temperature-sensitive mutations

A conditional mutation that produces a mutant phenotype at one temperature and a wild-type phenotype at another.

conditional mutations

A mutation expressed only under a certain condition; that is, a wild-type phenotype is expressed under certain (permissive) conditions and a mutant phenotype under other (restrictive) conditions.

auxotroph

A mutant microorganism or cell line that through mutation has lost the ability to synthesize one or more substances required for growth.

genetic anticipation

The phenomenon in which the severity of symptoms in genetic disorders increases from generation to generation and the age of onset decreases from generation to generation. It is caused by the expansion of trinucleotide repeats within or near a gene and was first observed in myotonic dystrophy.