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sex chromosome one of the pair of chromosomes that determine the sex of an individual autosome any chromosome that is not a sex chromosome sex-linked trait a trait that is determined by a gene found on one of the sex chromosomes, such as the X chromosome or the Y chromosome in humans linked gene pairs of genes that tend to be inherited together chromosome map a diagram of gene positions on a chromosome map unit a frequency of crossing over of 1 percent germ-cell mutation mutations that occur in the gamete cells somatic-cell mutation a mutation that occurs in a body cell lethal mutation a gene or chromosomal mutation that influences the development of an organism in such a way that the organism cannot survive deletion the loss of a part of DNA from a chromosome inversion a reversal in the order of the genes, or of a chromosome segment, within a chromosome translocation the movement of a segment of DNA from one chromosome to another, which results in a change in the position of the segment; also the movement of soluble nutrients from one part of a plant to another nondisjunction the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during mitosis or meiosis II point mutation a mutation in which only one nucleotide or nitrogenous base in a gene is changed substitution a mutation in which a nucleotide or a codon in DNA is replaced with a different nucleotide frameshift mutation a mutation, such as the insertion or deletion of a nucleotide in a coding sequence, that results in the misreading of the code during translation because of a change in the reading frame insertion mutation one or more nucleotides are added to a gene pedigree a diagram that shows the occurrence of a genetic trait in several generations of a family carrier in biology, an individual who has one copy of a recessive autosomal allele that causes disease in the homozygous condition genetic disorder an inherited disease or disorder that is caused by a mutation in a gene or by a chromosomal defect polygenic a characteristic of an organism that is determined by many genes complex character characters that are influenced strongly both by the environment and by genes multiple allele more than two alleles (versions of the gene) for a genetic trait codominance a condition in which both alleles for a gene are fully expressed incomplete dominance a condition in which a trait in an individual is intermediate between the phenotype of the individual's two parents because the dominant allele is unable to express itself fully sex-influenced trait an autosomal trait that is influenced by the presence of male or female sex hormones Huntington's disease a rare hereditary disease of the brain characterized by involuntary movements of the limbs or face, decreasing mental abilities, and eventual death amniosentesis a procedure used in fetal diagnosis in which amniotic fluid is removed from the uterus of the pregnant woman chorionic villi sampling a procedure in which the chorionic villi to are analyzed to diagnose fetal genotypes genetic counseling the process of testing and informing potential parents about their genetic makeup and the likelihood that they will have offspring with genetic defects or hereditary diseases gene therapy a technique that places a gene into a cell to correct a hereditary disease or to improve the genome