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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 structure within the nucleus containing the genetic information that is passed from one generation of cells to the next


A condition in which neither of two alleles of a gene is dominant or recessive.

color blindness

genetic inability to distinguish differences in hue

Cystic Fibrosis

A genetic disorder that occurs in people with two copies of a certain recessive allele; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.

dihybrid cross

a cross between individuals that have different alleles for the same gene

dominant allele

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

Down Syndrome

a congenital disorder caused by having an extra 21st chromosome


sequence of DNA that codes for a protein and thus determines a trait

genetic code

the term for the rules that relate how a sequence of nitrogenous bases in nucleotides corresponds to a particular amino acid

genetic variation

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


the science of heredity and of the mechanisms by which traits are passed from parents to offspring


the genetic makeup of an organism

genotype ratio

the ratio of the genotypes that appear in offspring


term used to refer to an organism that has two different alleles for the same trait


corresponding or similar in position or structure or function or characteristics


term used to refer to an organism that has two identical alleles for a particular trait

incomplete dominance

situation in which one allele is not completely dominant over another


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

Mendel's Laws

Unit character-traits controlled by factors (genes) which occur in pairs, Principle of Dominace, Principle of Segregation, and Principle Independent Assortment

monohybrid cross

a cross between individuals that involves one pair of contrasting traits

multiple alleles

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


Product of reproduction, a new organism produced by one or more parents


a diagram that shows the occurrence of a genetic trait in several generations of a family


The physical traits that appear in an individual as a result of its gentic make up.

phenotypic ratio

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


Absence of an enzyme, both recessive alleles must be inherited for the disorder to be displayed

polygenic trait

trait controlled by two or more genes

Punnett Square

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

recessive allele

An allele that will not determine the phenotype unless the genotype is homozygous in that allele

recombinant DNA

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

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

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


a characteristic that an organism can pass on to its offspring through its genes

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