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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.

Locus
The 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 assortment
The 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.