genes
the unit of heredity; a segment of DNA located at a particular place on a chromosome that encodes the information for the amino acid sequence of a protein and, hence, particular traits
hybrids
an organism that is the offspring of parents differing in at least one genetically determined characteristic; also used to refer to the offspring of parents of different species
dominant
an allele that can determine the phenotype of heterozygotes completely, such that they are indistinguishable from individuals homozygous for the allele; in the heterozygotes, the expression of the other (recessive) allele is completely masked
law of segregation
the principle that each gamete receives only one of each parent's pair of alleles of each gene
true-breeding
pertaining to an individual all of whose offspring produced through self-fertilization are identical to the parental type. True-breeding individuals are homozygous for a given trait
genotype
the genetic composition of an organism; the actual alleles of each gene carried by the organism
phenotype
the physical characteristics of an organism; can be defined as outward appearance (such as flower color), as behavior, or in molecular (such as glycoproteins on red blood cells)
punnett square method
a method of predicting the genotypes and phenotypes of offspring in genetic crosses
test cross
a breeding experiment in which an individual showing the dominant phenotype is mated with an individual that is homozygous recessive for the same gene. The ratio of offspring with dominant versus recessive phenotypes can be used to determine the genotype of the phenotypically dominant individual
law of independent assortment
the independent inheritance of two or more traits, assuming that each trait is controlled by a single gene with no influence from gene(s) controlling the other trait; states that the alleles of each gene are distributed to the gametes independently of the alleles for other genes; this law is true only for genes located on different chromosomes or very far apart on a single chromosome
linkage
the inheritance of certain genes as a group because they are parts of the same chromosome. Linked genes do not show independent assortment
genetic recombination
the generation of new combinations of alleles on homologous chromosomes due to the exchange of DNA during crossing over
sex chromosomes
either of the pair of chromosomes that usually determines the sex of an organism; for example, the X and Y chromosomes in mammals
autosomes
a chromosome that occurs in homologous pairs in both males and females and that does not bear the genes determining sex
sex-linked
referring to a pattern of inheritance characteristic of genes located on one type of sex chromosome (for example X) and not found on the other type (for example Y); in mammals, in almost all cases, the gene controlling the trait is on the X chromosome so this pattern is often called X-linked. In X-linked inheritance, females show the dominant trait unless they are homozygous recessive, whereas males express whichever allele, dominant or recessive, that is found on their single X chromosome
incomplete dominance
a pattern of inheritance in which the heterozygous phenotype is intermediate between the two homozygous groups
multiple alleles
many alleles of a single gene, perhaps dozens or hundreds, as a result of mutations
codominance
the relation between two alleles of a gene, such that both the alleles are phenotypically expressed in heterozygous individuals
polygenic inheritance
a pattern of inheritance in which the interactions of two or more functionally similar genes determine phenotype
pedigrees
a diagram showing genetic relationships among a set of individuals, normally with respect to a specific genetic trait
carrier
an individual who is heterozygous for a recessive condition; a carrier displays the dominant phenotype but can pass on the recessive allele to offspring
albinism
a recessive hereditary condition caused by defective alleles of genes that encode the enzymes required for the synthesis of melanin, the principle pigment in mammalian skin and hair; albinism results in white hair and pink skin
sickle-cell anemia
a recessive disease caused by a single amino acid substitution in the hemoglobin molecule. Sickle-cell hemoglobin molecules tend to cluster together, distorting the shape of red blood cells and causing them to break and clog capillaries
Huntington disease
an incurable genetic disorder, caused by a dominant allele, that produces progressive brain deterioration, resulting in the loss of motor coordination, flailing movements, personality disturbances, and eventual death
nondisjunction
an error in meiosis in which chromosomes fail to segregate properly into daughter cells
Turner syndrome
a set of characteristics typical of a women with only one X chromosome; women with Turner syndrome are sterile, with a tendency to be very short and to lack typical female secondary sexual characteristics
trisomy X
a condition of females who have three X chromosomes instead of the normal two; most such women are phenotypically normal and fertile
Klinefelter syndrome
a set of characteristics typically found in individuals who have two X chromosomes and one Y chromosomes; these individuals are phenotypically males but are sterile and have several female-like traits, including broad hips and partial breast development
Jacob syndrome
a set of characteristics typically of human males possessing one X and two Y chromosomes (XYY); most XYY males are phenotypically normal, but XYY males have a higher-than-average incidence of high testosterone levels, severe acne, and above average height