Chapter 23 - Principals of biology

Allele

Alternative form of a gene; alleles occur at the same locus on homologous chromosomes.

locus

Physical location of a trait (or gene) on a chromosome.

law of segregation

Mendelian principle that explains how, in a diploid organism, alleles separate during the formation of the gametes.

phenotype

Visible expression of a genotype (e.g., brown eyes or attached earlobes).

genotype

Genes of an organism for a particular trait or traits; often designated by letters—for example, BB or Aa.

dominant allele

Allele that exerts its phenotypic effect in the heterozygote; it masks the expression of the recessive allele.

recessive allele

Allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele.

homozygous

Possessing two identical alleles for a particular trait.

heterozygous

Possessing unlike alleles for a particular trait.

monohybrid cross

Cross between parents that differ in only one trait.

punnett square

Visual representation developed by Reginald Punnett that is used to calculate the expected results of simple genetic crosses.

test cross

Cross, between an individual with a dominant phenotype and an individual with a recessive phenotype, to determine whether the dominant individual is homozygous or heterozygous.

law of independant assortment

Mendelian principle that explains how combinations of traits appear in gametes; see also independent assortment.

pedigree

Chart of genetic relationship of family individuals across generations.

Tay-Sachs disease

Autosomal recessive genetic disorder that results in a deficiency in the enzyme hexosaminidase A; causes an accumulation of glycolipids in the lysosomes, resulting in a progressive loss of psychomotor functions.

cystic fibrosis (CF)

Genetic disease caused by a defect in the CFTR gene, which is responsible for the formation of a transmembrane chloride ion transporter; causes the mucus of the body to be viscous.

phenylketonuria (PKU)

Autosomal recessive genetic disorder that causes a lack of the enzyme that metabolizes phenylalanine; the accumulation of phenylalanine causes problems with nervous system development and function.

sickle-cell disease

Autosomal recessive genetic disorder that causes a malformation of hemoglobin molecules, causing red blood cells to form a sickle shape; also sometimes called sickle-cell anemia due to the symptoms of the disease.

marfan syndrome

Autosomal dominant genetic disorder of the connective tissue, specifically the fibrillin protein.

huntington's disease

Autosomal dominant genetic disorder that affects the nervous system; results in a progressive loss of neurons in the brain.

osteogenesis imperfecta

A dominant genetic disorder of humans that is characterized by a weakened bone structure.

incomplete dominance

Inheritance pattern in which an offspring has an intermediate phenotype, as when a red-flowered plant and a white-flowered plant produce pink-flowered offspring.

familial hypercholesterolemia

Genetic disorder that causes an accumulation of cholesterol in the blood due to defects in the LDL receptors on the cell surface.

multiple alleles

Inheritance pattern in which there are more than two alleles for a particular trait; each individual has only two of all possible alleles.

codominance

Inheritance pattern in which both alleles of a gene are equally expressed in a heterozygote.

polygenic inheritance

Pattern of inheritance in which a trait is controlled by several allelic pairs.

What is an example of how environmental factors can affect phenotype?

A bunny's fur can change color (phenotype) depending on the temperature outside (environmental factor)