Hereditary development

Gene

A specific sequence of nucleotides that serves as a recipe for making proteins, which influence the structure and function of cells.

Mitosis

The process by which a cell's nucleus makes an exact copy of all the chromosomes and splits into two new cells; creates most body cells.

Meiosis

The process by which a gamete's chromosomes duplicate and then divide twice, resulting in four cells with only half the genetic material of the original gamete.

Human chromosome number

46 chromosomes (23 pairs) in the nucleus of normal human cells; each sperm/egg has 23 chromosomes, combining to form 46.

Autosomes

22 of the 23 pairs of chromosomes, which are similar in length between the pair.

Sex chromosomes

The 23rd pair of chromosomes, which may differ in length; XY produces a genetic male, XX produces a genetic female.

Genotype

The sum total of all the genes a person inherits.

Phenotype

The features that are actually expressed (what you see when you look in the mirror).

Homozygous

Receiving the same version of a gene from both mother and father for a given characteristic.

Heterozygous

Receiving a different version of a gene from each parent for a given characteristic.

Dominant gene

A gene that expresses itself in the phenotype even when paired with a different version of the gene.

Recessive gene

A gene that expresses itself only when paired with a similar version of the gene (i.e., in the homozygous condition).

Alleles

Different versions of a gene.

Polygenic

A characteristic that is the result of the combined effect of several genes, rather than just one.

Incomplete Dominance

When a dominant gene does not completely suppress a recessive gene (example: sickle cell disease).

Carrier

A person who has inherited only one copy of a recessive gene disorder; typically unaffected but can pass the gene to offspring.

Monozygotic Twins

Identical twins that occur when a fertilized egg splits apart in the first two weeks of development, resulting in two genetically identical offspring (same genotype and often phenotype).

Dizygotic Twins

Fraternal twins that occur when two eggs are released and fertilized by two separate sperm; share genetic material like any two siblings (different genotype/phenotype).

Sex-Linked Disorder

A genetic disorder where the defective gene is found on the X chromosome; males are at greater risk since they only have one X chromosome.

Sickle Cell Disease

A recessive gene disorder in which red blood cells take on a sickle (C) shape, impairing oxygen transport; requires the recessive gene from both parents to fully inherit.

Cystic Fibrosis (CF)

A recessive disorder affecting breathing and digestion due to thick, sticky mucus buildup, especially in the lungs and digestive system.

Phenylketonuria (PKU)

A recessive metabolic disorder where the individual cannot metabolize phenylalanine; causes intellectual deficits if untreated, but is easily managed with a special diet.

Tay-Sachs Disease

A recessive disorder caused by enzyme deficiency, leading to lipid accumulation in brain nerve cells; typically fatal by age five.

Huntington's Disease

An autosomal DOMINANT disorder affecting the nervous system; fatal, but does not appear until midlife.

Tourette Syndrome

An autosomal DOMINANT tic disorder causing uncontrollable motor and vocal tics and body jerking; usually minor and controllable.

Achondroplasia

An autosomal DOMINANT disorder causing disproportionate short stature, short limbs/fingers, a large head, and specific facial features.

Fragile X Syndrome

A sex-linked disorder from an X-chromosome abnormality; causes learning/behavior problems, and is more severe in males (who lack a healthy second X).

Hemophilia

A sex-linked disorder causing problems with blood clotting, resulting in internal/external bleeding.

Duchenne Muscular Dystrophy

A sex-linked disorder causing progressive muscle weakening, leading to inability to move, wasting, and possible death.

Chromosomal Abnormality

When a child inherits too many or too few chromosomes; the most common cause is maternal age.

Trisomy 21 / Down Syndrome

A chromosomal abnormality with three (rather than two) 21st chromosomes; causes intellectual disability and characteristic physical features.

Trisomy 13 and Trisomy 18

Chromosomal abnormalities involving an extra chromosome 13 or 18; cause multiple birth defects, and affected individuals generally die within the first weeks/months of life.

Turner Syndrome

A sex-linked chromosomal disorder (XO) where part/all of an X chromosome is lost; affects cognitive functioning and sexual maturation in females, with short stature and infertility.

Klinefelter Syndrome

A sex-linked chromosomal disorder (XXY) where an extra X chromosome is present in a male; results in small testes, some breast development, infertility, and low testosterone.

Genetic Counseling

A service that helps individuals identify, test for, and understand potential genetic conditions that could affect them or their offspring.

Behavioral Genetics

The scientific study of the interplay between genetic and environmental contributions to behavior.

Genotype-Environment Correlations

The processes by which genetic factors contribute to variations in the environment (Plomin et al.); includes passive, evocative, and active types.

Passive Genotype-Environment Correlation

When children passively inherit both the genes AND the environment their family provides that supports those genes (e.g., inheriting athletic genes plus an athletic home environment).

Evocative Genotype-Environment Correlation

How the social environment reacts to individuals based on their inherited characteristics (e.g., a shy vs. outgoing temperament shapes how others treat the person).

Active Genotype-Environment Correlation (Niche Picking)

When individuals actively seek out environments that support their genetic tendencies (e.g., a musically inclined child seeking out music lessons).

Genotype-Environment Interactions

Genetic susceptibility to the environment; shown through adoption studies where environmental effects on behavior differ based on genetic risk.

Epigenetics

The study of modifications in DNA that affect gene expression (without changing the DNA sequence itself) and are passed on when cells divide; environmental factors can switch genes on/off.