Allele: A particular version of a gene at a location in a chromosome. Also defined as: Alternative forms of a gene. Mutations can create new alleles. Autosome: A chromosome other than a sex chromosome i.e.chromosomes that are not directly involved in determining sex. Humans have 22 pairs of autosomal chromosomes.Carrier: Heterozygous individual who has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder. Characteristic: A feature that can be inherited. Characteristic vs. trait: Characteristic = hair color trait = blond Chromosome: A coiled rod shaped structure of DNA and protein found in the nuclei of all eukaryotic cells and visable during cell division. Humans in their diploid cells, have 2set of chromosomes in 22 autosomal pair and 1 sex pair of chromosomes. In human sperm or egg, you have one set of chromosomes in 22 autosomal chromosomes and 1 sexchromosome.Complete dominance: When different alleles of the same trait are present, one allele expresses itself and masks the expression of all other alleles of the same trait. Dihybrid Cross: A cross involving two pairs of traits. DNA: Deoxyribonucleic acid which is the genetic material of most organisms. Made of 2 nucleotide polymeres (sugar-phosphate-base) bound at their bases in a helix configuration. DNA contains gene and non-gene segments of nucleotides. Diploid: Having 2 set of genes and chromosomes (2n), one set from each parent. Dominance: In a heterozygote, the allele phenotypically expressed. The correct term is complete dominance. The allele that is expressed is called the dominant allele. The allele that is not expressed is called the recessive allele. Dominant alleles are shown with a capital letter; recessive with a small letter. Traditionally, the dominant allele is written first i.e. a big D then little d in a heterozygote, Dd. Fertilization: When a sperm (or pollen in plants) unites with an egg. Since sperm and egg each contain one set of chromosomes, fertilization produces a diploid (2n) cell containing two sets of chromosomes. In humans and plants, this “first” cell is called a zygote. Gametes: Male (sperm or pollen) and female (eggs) sex cells that are usually the result of meiosis. The cell that produces gametes is termed a germ cell. Gene: A unit of heredity located in a particular segment of DNA making up a chromosome.Genotype: The genetic makeup of an organism expressed as the letters of the trait(s) i.e. the collection of an organism’s alleles located in the nucleated cells of that organism. For example: the genotype of a single trait might be Tt, but the genotype of an organism with 3 traits would be the sum of all its traits SsuuMM Germ Cell: A cell that undergoes meiosis cell division and produces gametes. Haploid: A cell containing only one set of chromosomes (N) Heredity: The transmission of genetic characters from parents to offspring, and the effects of this transmission. Homologous chromosomes: Matching chromosomes i.e. chromosomes having the same shape and the same traits at the same loci on the chromosomes. . Homozygous: Having two identical alleles for a given trait. For example, WW or ww. Incomplete Dominance: A type of inheritance in which offspring have an appearance that is intermediate between the phenotypes of the parents. For example: Crossing a whiterat with a black rat produces a grey rat. Meiosis: A two-stage cell division found only in sexually-reproducing organisms that results in gametes with half the chromosome number of the germ cell and therefore, half the genetic material as well. One nucleus in a germ cell starts with two sets of chromosomes (2n). It produces 4 nuclei in 4 gametes, each with one set of chromosomes (n). Monohybrid Cross: A cross involving single pairs traits e.g. TT x tt Mutation: A change in the base pairs of nucleotides of DNA that can be inherited. It is considered a “mistake” often resulting in genetic diversity. If the mutation is made in a non-gene portion of DNA, there is usually no observable effect i.e. no phenotypic expression. A point mutation affects one nucleotide. A substitution mutation is the changing of one base pair of a nucleotide for another. Insertion or deletion mutations involve the insertion or deletion of nucleotides into DNA.Nucleotide: In DNA and RNA, this molecule performs the function of a monomer; chains of this monomer form polymers of nucleic acids, either DNA or RNA. The nucleotide molecule is a complex of a sugar, a phosphate group, and a base. There are 4 possible bases: adenine (A), thymine (T), Cytosine (C), and guanine (G) found in DNA molecules. Pedigree: A family tree showing how traits are passed from generation to generation.Phenotype: The detectable traits, both physical and chemical, of an organism i.e. the expression in words of an observable genotype. Polygenic Inheritance: When a single characteristic is affected by two or more independent genes having similar or additive effects on the characteristic. In humans, genes (alleles) located in as many as 60 different locations on chromosomes affect skin color (infinite color variation). Human height and body form are other characteristics which involve multiple alleles. Probability: The relative likelihood of an event occurring. The probability of 2 events occurring together is the product of their individual probabilities. In genetics, these eventsmust occur on different chromosomes. Punnett Square: A model for establishing the genotypes and probabilities of a genetic cross. Randomness: The occurrence of events with no apparent pattern. Recessive allele: In a heterozygote, the allele that is not expressed. Sex Chromosomes: The chromosomes that are involved in the determination of sex. In humans, these are the X and Y chromosomes. Sex-linked Traits: Traits that result from genes in sex chromosomes. In humans, the majority of sex linked genes (400) are found on the x chromosome. Some 75 are found on the Y chromosome only deal with ‘male’ traits. Single-gene Cross: A genetic cross that involves a single gene e.g. Tt x tt.Somatic cell: Body cell; excludes cells that undergo meiosis (see germ cell) and becomesperm or egg. Trait: Specific version of a characteristic e.g. characteristic = hair shape trait = curlyor trait = wavy or trait = straight Two-Gene Cross: A genetic