Biology Chapter 9: Patterns of Inheritance

hybrid

An offspring of parents of two different species or of two different varieties of one species; an offspring of two parents that differ in one or more inherited traits; an individual that is heterozygous for one or more pairs of genes.

F1 generation

The offspring of two parental (P generation) individuals; F1 stands for first filial.

carrier

An individual who is heterozygous for a recessively inherited disorder and who therefore does not show symptoms of that disorder but who may pass on the recessive allele to offspring.

pedigree

A family genetic tree representing the occurrence of heritable traits in parents and offspring across a number of generations. A pedigree can be used to determine genotypes of matings that have already occurred.

true-breeding

Referring to organisms for which sexual reproduction produces offspring with inherited traits identical to those of the parents. The organisms are homozygous for the characteristics under consideration.

law of segregation

A general rule in inheritance (originally formulated by Gregor Mendel) that individuals have two alleles for each gene and that when gametes form by meiosis, the two alleles separate, each resulting gamete ending up with only one allele of each gene; also known as Mendel's first law of inheritance.

cystic fibrosis

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

Huntington's disease

A human genetic disease caused by a single dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.

rule of addition

A rule stating that the probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways.

character

A heritable feature that varies among individuals within a population, such as flower color in pea plants or eye color in humans.

homozygous

Having two identical alleles for a given gene.

hemophilia

A human genetic disease caused by a sex-linked recessive allele; characterized by excessive bleeding following injury.

test cross

The mating between an individual of unknown genotype for a particular characteristic and an individual that is homozygous recessive for that same characteristic. The testcross can be used to determine the unknown genotype (homozygous dominant versus heterozygous).

achondroplasia

A form of human dwarfism caused by a single dominant allele; the homozygous condition is lethal.

ABO blood groups

Genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of red blood cells. The ABO blood group phenotypes, also called blood types, are A, B, AB, and O.

codominant

Inheritance pattern in which a heterozygote expresses the distinct trait of both alleles.

P generation

The parent individuals from which offspring are derived in studies of inheritance; P stands for parental.

incomplete dominance

A type of inheritance in which the phenotype of a heterozygote (Aa) is intermediate between the phenotypes of the two types of homozygotes (AA and aa).

complete dominance

A type of inheritance in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

phenotype

The expressed traits of an organism.

polygenic inheritance

The additive effects of two or more gene loci on a single phenotypic characteristic.

recessive allele

An allele that has no noticeable effect on the phenotype of a gene when the individual is heterozygous for that gene.

ultrasound imaging

A technique for examining a fetus in the uterus. High-frequency sound waves echoing off the fetus are used to produce an image of the fetus.

red-green colorblindness

A category of common, sex-linked human disorders involving several genes on the X chromosome; characterized by a malfunction of light-sensitive cells in the eyes; affects mostly males but also homozygous females.

linked genes

Genes located near each other on the same chromosome that tend to be inherited together.

Punnett square

A diagram used in the study of inheritance to show recessive allele the results of random fertilization.

sex chromosome

A chromosome that determines whether an individual is male or female.

genotype

The genetic makeup of an organism.

recombination frequency

With respect to two given genes, the number of recombinant progeny from a mating divided by the total number of progeny. Recombinant progeny carry combinations of alleles different from those in either of the parents as a result of crossing over during meiosis.

trait

A variant of a character found within a population, such as purple or white flowers in pea plants.

dominant allele

The allele that determines the phenotype of a gene when the individual is heterozygous for that gene.

cross-fertilization

The fusion of sperm and egg derived from two different individuals.

Duchenne muscular dystrophy

A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.

heterozygous

Having two different alleles for a given gene.

cross

A mating of two sexually reproducing individuals; often used to describe a genetics experiment involving a controlled mating (a "genetic cross").

dihybrid cross

An experimental mating of individuals differing in two characters.

law of independent assortment

A general rule in inheritance (originally formulated by Gregor Mendel) that when gametes form during meiosis, each pair of alleles for a particular characteristic segregate independently of other pairs; also known as Mendel's second law of inheritance.

rule of multiplication

A rule stating that the probability of a compound event is the product of the separate probabilities of the independent events.

inbreeding

Mating between close blood relatives.

F2 generation

The offspring of the F1 generation; F2 stands for second filial.

monohybrid cross

An experimental mating of individuals differing in a single character.

allele

An alternative version of a gene.

sex-linked gene

A gene located on a sex chromosome. In humans, the vast majority of sex-linked genes are located on the X chromosome.

pleiotropy

The control of more than one phenotypic characteristic by a single gene.

amniocentesis

A technique for diagnosing genetic defects while a fetus is in the uterus. A sample of amniotic fluid, obtained by a needle inserted into the uterus, is analyzed for telltale chemicals and defective fetal cells.

self-fertilize

A form of reproduction that involves fusion of sperm and egg produced by the same individual organism.

chorionic villus sampling (CVS)

A technique for diagnosing genetic defects while the fetus is in an early development stage within the uterus. A small sample of the fetal portion of the placenta is removed and analyzed.

chromosome theory of inheritance

A basic principle in biology stating that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.