Heredity

Diploid cell

a cell containing two complete sets of chromosomes (2n), one from each parent.

Haploid cell

a cell containing a single set of chromosomes (n), like a gamete.

Homologous chromosomes

matching pairs of chromosomes (one from each parent) that carry the same genes at the same loci.

Sex cells (gametes)

reproductive cells (sperm and egg) that are haploid.

Gregor Mendel

the scientist known as the father of genetics, who established the basic laws of inheritance through pea plant experiments.

Trait

a specific characteristic that can vary among individuals (e.g., eye color).

Genes

segments of DNA that code for a particular trait.

Locus

the specific physical location of a gene on a chromosome.

Alleles

different versions of the same gene.

Homozygous

having two identical alleles for a given gene.

Heterozygous

having two different alleles for a given gene.

Phenotype

the observable physical or biochemical characteristics of an organism.

Genotype

the genetic makeup of an organism (its allele combination).

Dominant

an allele whose trait is expressed even when only one copy is present.

Recessive

an allele whose trait is only expressed when two copies are present (no dominant allele masking it).

Parent, or P generation

the original parents in a genetic cross.

Filial, or F1 generation

the first generation of offspring from a cross.

F2 generation

the second generation of offspring, produced by crossing F1 individuals.

Law of Dominance

Mendel's principle that some alleles are dominant and mask recessive ones in heterozygotes.

Law of Segregation

Mendel's principle that allele pairs separate during gamete formation, so each gamete gets only one allele per gene.

Law of Independent Assortment

Mendel's principle that genes for different traits are inherited independently of each other (for genes on different chromosomes).

Monohybrid cross

a genetic cross tracking the inheritance of a single trait.

Punnett square

a diagram used to predict the possible genotypes/phenotypes of offspring from a cross.

Dihybrid cross

a genetic cross tracking the inheritance of two different traits simultaneously.

Product Rule

the probability of two independent events both occurring equals the product of their individual probabilities.

Sum Rule

the probability of either of two mutually exclusive events occurring equals the sum of their individual probabilities.

Test cross

a cross between an individual of unknown genotype and a homozygous recessive individual, used to determine the unknown genotype.

Linked genes

genes located close together on the same chromosome that tend to be inherited together.

Recombinants

offspring with new combinations of alleles, different from either parent, resulting from crossing-over.

Percentage of recombination (recombination frequency)

the percentage of offspring that are recombinants, used to measure the distance between linked genes.

Map units (centimorgans)

units used to measure genetic distance between genes based on recombination frequency.

Autosomes

chromosomes that are not sex chromosomes (in humans, 22 of the 23 pairs).

Sex chromosomes

the chromosomes that determine an organism's sex (X and Y in humans).

Color blindness

a sex-linked recessive trait affecting the ability to distinguish certain colors.

Hemophilia

a sex-linked recessive disorder affecting blood clotting.

Sex-linked traits

traits controlled by genes located on the sex chromosomes (usually the X chromosome).

Carrier

an individual who has one copy of a recessive allele (often disease-causing) but doesn't show the phenotype.

Barr body

an inactivated, condensed X chromosome found in the cells of female mammals.

Incomplete dominance (blending inheritance)

inheritance pattern where the heterozygote phenotype is an intermediate blend of both alleles.

Codominance

inheritance pattern where both alleles in a heterozygote are fully and separately expressed.

Polygenic inheritance

inheritance where a trait is controlled by multiple genes, producing a range of phenotypes.

Non-nuclear inheritance

inheritance of genetic material located outside the nucleus (e.g., mitochondrial DNA), typically passed from the mother.

Pedigree

a diagram showing the inheritance of a trait across generations of a family.

Phenotypic plasticity

the ability of an organism to change its phenotype in response to environmental conditions, without a change in genotype.

Meiosis

the type of cell division that produces haploid gametes from diploid cells, reducing chromosome number by half.

Gonads

the reproductive organs that produce gametes (testes and ovaries).

Testes

the male gonads that produce sperm.

Ovaries

the female gonads that produce eggs.

Germ cells

cells that give rise to gametes through meiosis.

Meiosis I

the first division of meiosis, separating homologous chromosomes and reducing chromosome number by half.

Meiosis II

the second division of meiosis, separating sister chromatids (similar to mitosis).

Synapsis

the pairing of homologous chromosomes during prophase I of meiosis.

Tetrad (bivalent)

the structure formed by a pair of homologous chromosomes (four chromatids) during synapsis.

Crossing-over (recombination)

the exchange of genetic material between homologous chromosomes during synapsis, creating new allele combinations.

Alleles

(see above) different versions of the same gene.

Gametogenesis

the process of producing gametes through meiosis.

Spermatogenesis

the process of producing sperm cells in the testes.

Oogenesis

the process of producing egg cells in the ovaries.

Polar bodies

small, non-functional cells produced during oogenesis alongside the egg, containing extra genetic material.

Ovum

a mature female gamete (egg cell).

Nondisjunction

the failure of chromosomes to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

Down syndrome

a genetic condition caused by an extra copy of chromosome 21 (trisomy 21), typically resulting from nondisjunction.

Aneuploidy

having an abnormal number of chromosomes (extra or missing) due to nondisjunction.

Translocation

a chromosomal mutation where a segment of one chromosome breaks off and attaches to a different, non-homologous chromosome.