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Allele

one of several alternative forms of a particular gene

Autosome

a chromosome that occurs in homologous pairs in both males and females and that does not bear the genes determining sex

Carrier

an individual who is heterozygous for a recessive condition; a carrier displays the dominant phenotype but can pass on the recessive allele to offspring

Cross-fertilization

the union of sperm and egg from two individuals of the same species

Dominant

an allele that can determine the phenotype of heterozygotes completely, such that they are indistinguishable from individual s homozygous for the allele; in the heterozygotes, the expression of the other (recessive) allele is completely masked

Down syndrome

a genetic disorder caused by the presence of three copies of the chromosomes 21; common characteristics include mental retardation, distinctly shaped eyelids, a small mouth with protruding tongue, heart defeats, and low resistance to infectious diseases
-frequency of nondisjunction increases with the age of the parents

gene

unit of heredity; a segment of DNA located at a particular place on a chromosomes that encodes the information for the amino acid sequence of a protein and, hence, particular traits

genotype

the genetic composition of an organism; the actual alleles of each gene carried by the organism

heterozygous

"different pair" carrying two different alleles of a given gene (hybrid)

homozygous

"same pair" carrying two copies of the same allele of a given gene (true breeding)

inheritance

the process by which the characteristics of individuals are passed to their offspring

jacob syndrome

a set of characteristics typical of human males possessing one X and two Y chromosomes (XYY) ; mostly XYY males are phenotypically normal, but XYY males have a higher than average incidence of high testosterone levels, severe acne, and above average height

law of independent assortment

the independent inheritance of two or more traits, assuming that each trait is controlled by a single gene with no influence from gene(s) controlling the other trait; states that the alleles of each gene are distributed to the gametes independently of the alleles for other genes; this law is true only for genes located on different chromosomes or very far part on a single chromosomes

linkage

the inheritance of certain genes as a group because they are parts of the same chromosome. Linked genes do not show independent assortment

multiple alleles

many alleles of a single gene, perhaps dozens or hundreds, as a result of mutations

nondisjunction

an error in meiosis in which chromosomes fail to segregate properly into the daughter cells

pedigree

a diagram showing genetic relationships among a set of individuals, normally with respect to a specific genetic trait

phenotype

the physical characteristics of an organism; can be defined as outward appearance (such as flower color), as behavior, or in molecular terms (such as glycoproteins on red blood cells)

Punnett square method

a method of predicting the genotypes and phenotypes of offspring in genetic crosses

recessive

an allele that is expressed on in homozygotes and is completely masked in heterozygotes

self-fertilization

the union of sperm and eggs from the same individual

sex chromosome

either of the pair of chromosomes that usually determines the sex of an organism; for example, the X and Y chromosomes in mammals

sex linked

referring to a pattern of inheritance characteristic of genes located on one type of sex chromosomes (for example X), and not found on the other type (for example Y), in mammals the gene controlling the traits on the X chromosomes, so this pattern is often called X linked. In X linked inheritance, females show the dominant trait unless they are homozygous recessive, whereas males express whichever allele, dominant or recessive, that is focused on their single X chromosome

trisomy 21

a genetic disorder caused by the presence of three copies of the chromosomes 21; common characteristics include mental retardation, distinctly shaped eyelids, a small mouth with protruding tongue, heart defeats, and low resistance to infectious diseases
-frequency of nondisjunction increases with the age of the parents

trisomy x

a condition of females who have three x chromosomes instead of the normal two; most such women are phenotypically normal and fertile

x chromosome

the female sex chromosome in females and some insects

y chromosome

the male sex chromosomes in mammals and some insects

Klinefelter syndrome

a set of characteristics typically found in individuals who have two X chromosomes and one Y chromosomes; these individuals are phenotypically males but are sterile and have several female- like traits, including board hips and partial breast development

turner syndrome

a set of characteristics typical of a woman with only one I chromosome; women with Turner syndrome are sterile, with a tendency to be very short and to lack typical female secondary sexual characteristics

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