Chapter 4

Heredity and Evolution

Hybrids Offspring of mixed ancestry; heterozygotes.

Principle of segregation Genes (alleles) occur in pairs (because chromosomes occur in pairs). During gamete production, the members of each gene pair separate, so each gamete contains one member of each pair. During fertilization, the full number of chromosomes is restored, and members of gene or allele pairs are reunited.

Recessive A trait that is not expressed in heterozygotes; also refers to the allele that governs the trait. For a recessive allele to be expressed, there must be two copies of the allele.

Dominant A trait governed by an allele that can be expressed in the presence of another, different allele. Dominant alleles prevent the expression of recessive alleles in heterozygotes.

LocusThe position on a chromosome where a given gene occurs. The term is sometimes used interchangeably with gene, but this usage is technically incorrect.

Alleles Alternate forms of a gene. Alleles occur at the same locus on homologous chromosomes and govern the same trait. Because they are different, their action may result in different expressions of that trait. The term is often used synonymously with genes.

Homozygous Having the same allele at the same locus on both members of a pair of chromosomes.

Genotype The genetic makeup of an individual. Genotype can refer to an organism’s entire genetic makeup or to the alleles at a particular locus.

Heterozygous Having different alleles at the same locus on members of a pair of homologous chromosomes.

Phenotypes The observable or detectable physical characteristics of an organism; the detectable expressions of genotypes.

Phenotypic ratio The proportion of one phenotype to other phenotypes in a group of organisms. For example, Mendel observed that there were approximately three tall plants for every short plant in the F2 generation. This is expressed as a phenotypic ratio of 3:1.

Mendelian traits Characteristics that are influenced by alleles at only one genetic locus. Examples include many blood types, such as ABO.

Principle of independent assortmentThe distribution of one pair of alleles into gametes does not influence the distribution of another pair. The genes controlling different traits are inherited independently of one another.

Antigens Large molecules found on the surface of cells. Several loci governing antigens on red and white blood cells are known. (Foreign antigens provoke an immune response in individuals.)

Codominance The expression of two alleles in heterozygotes. Because neither allele is dominant or recessive, both influence the phenotype.

Polygenic Referring to traits that are influenced by genes at two or more loci. Examples: stature, skin color, and eye color. Many polygenic traits are also influenced by environmental factors.

Mitochondrial DNA (mtDNA)DNA found in the mitochondria that is inherited only from the mother.

Microevolution Small changes occurring within species, such as a change in allele frequencies.

Macroevolution Changes produced only after many generations, such as the appearance of a new species.

Gene Flow Exchange of genes between populations.

Genetic drift Evolutionary changes—that is, changes in allele frequencies—produced by random factors. Genetic drift is a result of small population size.

Founder effect A type of genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations.

Gene pool The total complement of genes shared by the reproductive members of a population.