Heredity and Evolution
Polydactyly- having extra digits (fingers and toes)
Mendel- resolved the question of heredity through his study of pea plants; concept of
segregation, recessiveness and dominance
Selective Breeding- a practice whereby animal or plant breeders choose which individual
animals or plants will be allowed to mate based on the traits (such as coat color or body size)
they hope to produce in offspring. Animals or plants that don’t have the desirable traits aren’t
allowed to breed.
Hybrids- offspring of parents who differ from each other with regard to certain traits or certain
aspects of genetic makeup; heterozygotes
Principle of Segregation- genes (alleles) occur in pairs because chromosomes occur in pairs.
During gamete formation, the members of each pair of alleles separate, so that each gamete
contains one member of each pair.
Recessive- describing a trait that isn’t expressed in heterozygotes; also refers to the allele that
governs the trait. For a recessive allele to be expressed, an individual must have two copies of it
(i.e. the individual must be homozygous).
Dominant- describing a trait governed by an allele; dominant alleles prevent the expression of
recessive alleles in heterozygotes (this is the definition of complete dominance).
Gene- a segment of DNA that directs the production of a specific protein, part of a protein, or
any functional product
Locus- the position on a chromosome where a given gene occurs; the term is frequently used
interchangeably with gene
Alleles- alternate forms of a gene; alleles occur at the same locus on both members of a pair of
chromosomes, and they influence the same trait, but because they’re slightly different from one
another, their action may result in different expressions of that trait. The term allele is sometimes
used synonymously with gene.
Homozygous- having the same allele at the same locus on both members of a pair of
Heterozygous- having different alleles at the same locus on 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. 2
Phenotype- the observable or detectable physical characteristics of an organism; the detectable
expressions of genotypes, frequently influenced by environmental factors
Principle of IndependentAssortment- 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.
Random Assortment- the chance distribution of chromosomes to daughter cells during meiosis;
along with recombination, a source of genetic variation (but not new alleles) from meiosis
Mendelian traits- characteristics that are influenced by alleles at only one genetic locus;
examples include many blood types, such asABO. Many genetic disorders, including sickle-cell
anemia, and Tay-Sachs disease are also Mendelian traits.
Antigens- large molecules found on the surface of cells; several different loci govern various
antigens on red and white blood cells (foreign antigens provoke an immune response).
Codominance- the expression of two alleles in heterozygotes; in this situation, neither allele is
dominant or recessive, so they both influence the phenotype.
Mendelian traits are described as discrete or discontinuous because their phenotypic expressions
don’t overlap; instead, they fall into clearly defined categories. They don’t show continuous
Polygenic- referring to traits that are influenced by genes at two or more loci. Examples include:
stature, skin color, eye color, and hair color. Many (but not all) polygenic traits are influenced by
environmental factors such as nutrition and exposure to sunlight.
Polygenic traits actually account for most of the readily observable phenotypic variation seen in
humans, and they’ve traditionally served as a basis for racial classification.
Pigment- in reference to polygenic inheritance, molecules that influence the color of skin, hair,
Mitochondrial Inheritance- all mitochondrial traits are inherited from the mother (only one
parent); meiosis and recombination don’t occur; therefore, all variation in mtDNAamong
individuals is caused by mutation. Geneticists have used mutation rates to: investigate
evolutionary relationships between species, trace ancestral relationships within the hu