GENETICS is the study of the patterns of inheritance.

A MONOHYBRID CROSS  is an experimental cross in which only one characteristic is examined.  A DIHYBRID CROSS is an experimental cross in which two characteristics are studied.

GENES are units of information about specific traits, and they are passed from parents to offspring.  Each gene has a specific location (or locus) on a chromosome. A GENOME is a set of all the genes necessary to specify an organism’s characteristics.

PHENOTYPE refers to an organism’s observable traits.  It is determined by both genetic makeup and the environment.  The GENOTYPE refers to the genetic makeup of an individual.

In DIPLOID individuals, each chromosome of an homologous pair contains the same gene for a particular trait  However, the information about this gene may vary.  For example, the gene for flower color on one chromosome may specify the color red, while the same gene on the homologous chromosome may specify the color blue.  We call these alternative forms of a gene ALLELES.

If the two alleles of a pair are identical, we say that the individual is HOMOZYGOUS for the gene in question.  If the two alleles are different, the individual is said to be HETEROZYGOUS for the specific gene.  An individual having a hidden, recessive allele is called a CARRIER.

We say that an allele is DOMINANT when it can mask the effect of the other allele.  In this case, the other allele is said to be RECESSIVE.  By convention, we represent the dominant allele by a capital letter, and its corresponding recessive allele is represented by a lowercase letter.

Mendel’s first law is  also called the PRINCIPLE OF SEGREGATION.  This states that diploid cells have pairs of genes, each member of the pair on homologous chromosomes.  During meiosis, these copies of each gene separate, or segregate, so that each gamete formed has only one copy of the gene.

Mendel’s second law is also known as the PRINCIPLE OF INDEPENDENT ASSORTMENT.  This law simply means that the inheritance of alleles for one trait in no way contributes to, or influences the inheritance of alleles for a different trait.  In other words, each trait is inherited independently of another.

PROBABILITY is the chance that an event will happen.  It is usually expressed as a percentage or fraction.  Probability is NOT the same as possibility.  The PRODUCT RULE states that the probability of two or more independent events occurring simultaneously is the product of their individual probabilities.

INCOMPLETE DOMINANCE is an inheritance pattern in which neither allele of a given gene masks the presence of the other.  In this case, the heterozygote has a phenotype that is intermediate to that of either the homozygous dominant or homozygous recessive.  An example of this is snapdragon color.

CODOMINANCE is an inheritance pattern in which a heterozygote fully expresses both alleles.  An example of this is blood types in humans.

POLYGENIC INHERITANCE is a pattern of inheritance seen when more than one gene controls a particular trait.  Polygenic inheritance examples in humans include height, body built, skin color, etc.  Polygenic inheritance generally results in continuous variation.

PLEIOTROPIC INHERITANCE is a pattern of inheritance seen when a single gene controls more than one phenotypic trait.  This usually is seen in certain disease states called “syndromes”.  A syndrome is a medical term used for a group of related symptoms.  An example of pleiotropic inheritance would be sickle cell anemia.

EPISTASIS is a pattern of inheritance in which the effects of one gene pair mask the effects of other gene pairs.  An example of epistatic inheritance is the shape of squash, or color pattern in Labrador retrievers.

A MUTATION is any heritable change in the nucleotide sequence of DNA.

NONDISJUCTION is the failure of homologous chromosomes (if it happens in meoisis I) or sister chromatids (if it happens in meiosis II) to separate during gamete formation.

A PEDIGREE is a family tree.  Careful examination of a pedigree can lead to an understanding of the mode of inheritance of a particular trait.