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one of two alternate forms of a gene that can have the same locus on homologous chromosomes and are responsible for alternative traits


threadlike structures made of DNA molecules that contain the genes


situation in which both alleles of a gene contribute to the phenotype of the organism

color blindness

a variety of disorders marked by inability to distinguish some or all colors

cystic fibrosis

the most common congenital disease

dihybrid cross

hybridization using two traits with two alleles each

down syndrome

a condition of retardation and associated physical disorders caused by an extra chromosome in one's genetic makeup

dominant allele

an allele whose trait always shows up in the organism when the allele is present


the biochemical units of heredity that make up the chromosomes; a segment of DNA capable of synthesizing a protein

genetic code

the ordering of nucleotides in DNA molecules that carries the genetic information in living cells

genetic variation

the number and frequency of alleles that are present in a particular population


the particular alleles at specified loci present in an organism


the branch of biology that studies heredity and variation in organisms

genotypic ratio

the ratio of the genotypes that appear in offspring


an organism that has two different alleles for a trait


corresponding or similar in position or structure or function or characteristics


having identical alleles at corresponding chromosomal loci

incomplete dominance

one allele is not completely dominant over the other allele


A picture of all the chromosomes in a cell arranged in pairs

Mendel's laws

laws of dominance,segregation,and independent assortment

monohybrid cross

hybridization using a single trait with two alleles `

multiple alleles

three or more forms of a gene that code for a single trait


something that comes into existence as a result


ancestry of a purebred animal


what an organism looks like as a consequence of its genotype

phenotypic ratio

a ratio that shows the varied outcomes that results from a genetic cross and is based on physical appearance alone


phenylketonuria (disease due to lack of an enzyme in infants)

polygenetic trait

trait controlled by two or more genes

Punnet square

A chart that shows all the possible combinations of alleles that can result from a genetic cross

recessive alleles

An allele that is masked when a dominant allele is present.

recombinant DNA

genetically engineered DNA made by recombining fragments of DNA from different organisms

sex-linked trait

Characteristic such as red-green color blindness, controlled by genes on the X chromosome also called an X linked trait.

sickle cell anemia

a genetic disorder in which erythroctyes take on an abnormal curved or "sickle" shape

Tay-Sachs disease

recessive disorder that is of the nervous system and the child usually dies by the age of 4

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