HEREDITY Heredity - Continuity of biological traits from one generation to the next. Genetics: is the branch of science that deals with the inheritance of biological characteristics. The study of gene structure and action and the patterns of inheritance of traits from parent to offspring. Genes: Units of inheritance usually occurring at specific locations, or loci, on a chromosome. A gene may be made up of hundreds of thousands of DNA bases.  Genes are responsible for the hereditary traits in plants and animals.

CHROMOSOMES Thread-like, gene-carrying bodies in the nucleus of a cell.  Chromosomes are composed primarily of DNA and protein.  They are visible only under magnification during certain stages of cell division.  Humans have 46 chromosomes in each somatic cell and 23 in each sex cell.

DNA (deoxyribonucleic acid) A large organic molecule that stores the genetic code for the synthesis of proteins.  DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure.  Segments of DNA in chromosomes correspond to specific genes.

Genetic Terminology Allele: An alternative form of a gene that occurs at the same locus on homologous chromosomes, e.g., A, B, and O genes are alleles. Anticodon: A sequence of three bases in tRNA that is complementary to a codon in mRNA. Enables tRNA to sequence amino acids in the order specified by mRNA. Autosome: A non-sex chromosome. Synonymous with somatic chromosomes (chromosome pairs 1-22).

Chromosome: Rod-shaped structures within the cell nucleus that carry genes encoded by DNA. Co-dominant: Genes are co-dominant if both alleles are expressed in the heterozygous state. Codon: A sequence of three bases in DNA or RNA that codes for a single amino acid. Enables specific proteins to be made by specific genes. Crossing over: The exchange of genetic material between members of a pair of homologous chromosomes.

Diploid number of chromosomes: The number of chromosomes found in somatic cells, which in humans is 46. DNA: Deoxyribonucleic acid. Composed of nucleic acids, these molecules encode the genes that allow genetic information to be passed to offspring. DNA polymerases: Enzymes that can synthesize new DNA strands using previously synthesized DNA (or RNA) as a template. Dominant gene: A gene is dominant if it is expressed when heterozygous but its allele is not, e.g. in the tall pea plants the T gene is dominant (expressed in both TT and Tt genotypes) and the t gene is recessive.

Gamete: A reproductive sex cell (ovum or sperm) with the haploid number (23) of chromosomes that results from meiosis. Gene: A segment of a DNA molecule that codes for the synthesis of a single polypeptide. Genome: Term used to denote the entire DNA sequence (gene content) of a gamete, person, population, or species. Genotype: All of the alleles present at the locus (or closely linked loci) of a blood group system, indicating chromosomal alignment if appropriate. Genotypes are indicated by superscripts, underlining, or italics.

Haploid number of chromosomes: The number of chromosomes found in sex cells, which in humans is 23. Heterozygous: The situation in which allelic genes are different, e.g. the Kk genotype in the Kell BGS or the Fya Fyb genotype in the Duffy BGS. Homozygous: The situation in which allelic genes are identical, e.g., the KK genotype or the Fya Fya genotype. Human Genome Project: A worldwide project to map and sequence the human genome. The ultimate goal is to produce the complete nucleotide sequence of every human chromosome.

Karyotype: A photomicrograph (photograph taken through a microscope) of all the chromosomes in a person, arranged in standard classification (from #1 chromosomes through to the sex chromosomes). Linkage: Genes are linked if they are on the same chromosome within a measurable distance of each other and are normally inherited together, e.g., Lutheran and Secretor genes are linked as are the Dd, Cc, Ee sub loci in the Rh BGS. Locus: The location of allelic genes on the chromosome, e.g., A, B, and O genes occur at the ABO locus. (Plural = loci)

Mapping of genes: A variety of processes that include discovering that a gene is linked to another gene (which can serve as a marker for it), assigning genes to particular chromosomes, assigning genes to specific regions on chromosomes, and determining nucleotide sequences on chromosomes. Meiosis: The type of cell division that occurs in sex cells by which gametes having the haploid number of chromosomes are produced from diploid cells. Messenger RNA (mRNA): Type of RNA polymerase using DNA as a template. Contains the codons that encompass the genetic codes to be translated into protein. Mitosis: Cell division that results in the formation of two cells, each with the same number of chromosomes as the parent cells.

Mutation: A permanent inheritable change in a single gene (point mutation) that results in the existence of two or more alleles occurring at the same locus. Nucleic acids: Polymers of phosphorylated nucleosides, the building blocks of DNA and RNA. Nucleoside: The building blocks of RNA and DNA. Compounds consisting of a purine (adenine or guanine) or pyrimidine (thymine or cytosine) attached to ribose (in RNA) or deoxyribose (in DNA) at the 11 carbon.

Pedigree: A diagram representing a family tree. Phenotype: The antigens (traits) that result from those genes that are directly expressed (can be directly antigen typed), e.g., group A in the ABO. Plasmid: Extrachromosomal circular DNA in bacteria. Plasmids can independently replicate and encode a product for drug resistance or some other advantage. Used in molecular genetics as vectors for cloned segments of DNA. Polymerase chain reaction: An in vitro method of amplifying DNA sequences hundreds of millions to billions of times in a few hours.

Polymorphism: The existence of two or more different phenotypes resulting from two or more alleles, each with an appreciable frequency. Most blood group systems are polymorphic. Polypeptides: Polymers of amino acids that form the building blocks of proteins. Population genetics: The branch of genetics that deals with how genes are distributed in populations and how gene and genotype frequencies stay constant or change. Calculations are based on the Hardy-Weinberg law.

Recessive: Genes are recessive if the phenotype that they code for is only expressed when the genes are homozygous, e.g., le le genes, in the Lewis system or h h genes in the ABO BGS. Recombinant: A person who has a new combination of genes not found together on the chromosome in either parent, e.g., an MS/Ns offspring whose parents are Ms/NS and MS/MS. A recombinant results from crossing over in one parent. Recombinant DNA: In molecular genetics, artificially made DNA composed of fragments of DNA from different chromosomes (often from different species) that have been joined together (spliced) by genetic engineering.

Restriction endonucleases: DNA enzymes of bacterial origin that can cleave DNA at internal positions on a strand because they recognize specific sequences (usually 4-6 base pairs). Restriction fragment length polymorphisms (RFLP): Regions of DNA of varying lengths that can be cut out of DNA by restriction endonucleases. Because the fragment lengths vary among individuals, they are polymorphic and can be used as genetic markers. Ribosomal RNA (rRNA): Type of RNA found in ribosomes, the site of protein synthesis in the cytoplasm.

RNA Ribonucleic acid: Nucleic acids that are formed using DNA as a template. Similar to DNA except has ribose in place of deoxyribose and uracil in place of thymine. Sex chromosomes: The chromosomes that determine sex. XX in females and XY in males. Sex-linked: An outdated term for genes on the X chromosome. Historically synonymous for X- linked since, apart from genes essential for male sex determination, the Y chromosome appears to have few recognized gene loci. Somatic chromosome: A non-sex chromosome (soma=body). Synonym is autosome.

Syntenic: Genes are on the same chromosome but are not close enough for linkage to be demonstrated. Transcription: Synthesis of single-stranded RNA by RNA polymerase using DNA as a template. The process in the nucleus whereby DNA is transcribed into mRNA. Transfer RNA (t RNA):Type of RNA that facilitates translation of mRNA into protein. Contains anti codons that provide the molecular link between the codons of mRNA and the amino acid sequences of proteins.

Translation: The process of translating the codon sequence in mRNA into polypeptides with the help of tRNA and ribosomes. Trans position: Genes in the trans position are on opposite chromosomes of a pair of homologous chromosomes. In the genotype CDe/cde, for example, D and c genes are in the trans position.

X-chromosome: The sex chromosome present in double dose in females (XX) and in single dose in males (XY). X-linked: Genes on the X chromosome, e.g., genes for hemophilia A, hemophilia B, and Xga blood group genes. Y-chromosome: The sex chromosome present only in males (XY).