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the genetic transmission of characteristics from parent to offspring


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

locus (loci - plural)

the physical location of a gene on a chromosome


carrying two copies of the same allele of a given gene; also called "true-breeding"


carrying two different alleles of a given gene; also called a "hybrid"


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


the union of sperm and egg from the same individual


the union of sperm and egg from two individuals of the same species


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


an allele that is expressed only in homozygotes and is completely masked in heterozygotes

law of segregation

the principle that each gamete receives only one of each parent's pair of alleles of each gene


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


the genetic composition of an organism; the actual alleles of each gene carried by the organism


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


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

X chromosomes

the female sex chromosomes in mammals and some insects

Y chromosomes

the male sex chromosome in mammals and some insects


a chromosome that occurs in homologous pairs in both males and females and that does not bear the genes determining sex


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


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


a situation in which a single gene influences more than one phenotypic characteristic


a diagram showing genetic relationships among a set of individuals, normally with respect to a specific genetic trait


an individual who is heterozygous for a recessive condition; a carrier displays the dominant phenotype but can pass on the recessive allele to offspring


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


a recessive, sex-linked disease in which the blood fails to clot normally


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

trisomy 21/Down syndrome

a genetic disorder caused by the presence of three copies of chromosome 21; common characteristics include mental retardation, distinctively shaped eyelids, a small mouth with protruding tongue, heart defects, and low resistance to infectious disease

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