Biology, Chapter 13: Chromosomes, Mapping and the Meiosis-Inheritance Connection.
Chromosomal Theory of Inheritance: a theory that states hereditary traits are carried on chromosomes.
Thomas Hunt Morgan: an American geneticist who studied macro mutation in the fruit fly Drosophila Melanogaster. Morgan made important contributions to the genetics field.
Testcross: a mating between a phenotypically dominant individual of unknown genotype and a homozygous "tester", done to determine whether the former is homozygous or heterozygous for the relevant gene.
Sex Linkage: a property of some animals referring to sex-linked genes, which are traits determined by genes on the X chromosome that are absent on the Y chromosome.
Sex Chromosome: chromosome implied in the determination of gender. In humans, the sex chromosomes are the X and Y chromosomes.
Autosome: any eukaryotic chromosome that is not a sex chromosome. Autosomes are present in the same kind and number in both male and female members of animal species.
Dosage Compensation: a phenomenon in which expression of genes is kept constant in both male and female individuals, despite the differences in sex chromosome numbers. In mammals, inactivation of one of the X chromosomes in female cells accomplishes dosage compensation.
Barr Body: a condensed and inactivated X chromosome that is seen in the nuclei of cells during interphase. The Barr body is depicted as a deeply staining structure and is present on individuals with more than one X chromosome.
Genetic Mosaic: an organism whose individual cells may express different alleles, depending on which chromosome is inactivated.
Maternal Inheritance: a mode of uniparental inheritance dependent from the female parent. An example in humans is the heritage of mitochondria and organelles from the mother.
Crossover: also known as "crossing over", this process occurs in meiosis. Crossover is the exchange of chromatid segments between homologous chromosomes that are responsible for genetic recombination.
Recombination Frequency: the value obtained by dividing the number of recombinant progeny by the total progeny in a genetic cross.
Linked Genes: a pair of genes that are physically close together on the chromosome and as such tend to segregate together. Their recombination can be used to produce a map of genetic distance for a chromosome.
Map Unit: each percentage of recombination frequency between two genetic loci (the zone where genes are located in the chromosome). This unit is also termed "centimorgan" (cM).
Anonymous Marker: genomic markers placed in a chromosome that do not cause a detectable phenotype but can be visualized using molecular techniques.
Polymorphism: the presence of more than one allele of a gene at a frequency higher than that of newly arising mutiations in a population.
Sickle Cell Anemia: a condition that causes poor blood circulation due to abnormal hemoglobin molecules in red blood cells. This trait is recessive.
Cystic Fibrosis: a disease that produces mucus clogs in the liver, lungs and the pancreas. Cystic fibrosis is caused by a deficiency in chloride ion transport mechanisms.
Tay-Sachs Disease: degeneration of the central nervous system during infancy caused by a defective Hexosaminidase A enzyme.
Phenylketonuria: occurs when the brain fails to complete its development during infancy and is treatable with dietary restriction. The cause is a deficiency in the Phenylalanine Hydroxylase enzyme.
Hemophilia: failure of blood cells to clot properly, leading to elevated risks of heart disease and damage to other organs. The blood-clotting factor VIII appears damaged in these patients.
Huntington Disease: a condition in which brain tissue begins to deteriorate during middle age and is caused by inhibition of brain cell metabolism.
Muscular Dystrophy: causes muscles to waste away due to the degradation of the myelin coating of the nerves that stimulate muscle activity.
Hypercholesterolemia: an excessive level of cholesterol in the bloodstream that can lead to subsequent heart diseases. Hypercholesterolemia's cause is the presence of abnormal receptors on cholesterol cells.
Nondisjunction: the failure of homologues or sister chromatids to separate properly during mitosis or meiosis, resulting in an aneuploid cell or gamete.
Aneuploidy: the condition of an organism whose cells have lost or gained a chromosome. Down syndrome, which is caused by the presence of three 21 chromosomes, is an example of aneuploidy.
Monosomic: describes the condition in which a chromosome has been lost due to nondisjunction during meiosis, producing a diploid embryo with only one of these autosomes.
Trisomic: describes the condition in which an additional chromosome has been gained due to nondisjunction during meiosis, producing a diploid embryo with three of these autosomes. In humans, trisomic individuals survive only if the autosome is small.
Genomic Imprinting: describes an exception to Mendelian genetics in some mammals in which the phenotype caused by an allele is exhibited when the allele comes from one parent, but not from the other.
Genetic Counseling: the process of evaluating the risk of genetic defects occurring in offspring testing for these defects in unborn children and providing the parents with information about these risks and conditions.
Amniocentesis: indirect examination of a fetus by tests on cell cultures grown from fetal cells obtained from a sample of amniotic fluid.
Chorionic Villi Sampling: abbreviated CVS, it is a technique in which fetal cells are sampled from the chorion of the placenta rather than from the amniotic fluid.
