Chapter 14 - Mendel and the Gene Idea

Mendel and the Gene Idea

Gregor Mendel

The father of genetics; Austrian monk who discovered basic principles of heredity using pea plants.

Heritable factors

Mendel’s term for what we now call genes—units of inheritance passed from parent to offspring.

Blending hypothesis

The old idea that parental traits mixed together to form an intermediate in offspring; disproved by Mendel.

Particulate hypothesis

The idea that parents pass on discrete, unchanged units (genes) that determine traits.

Trait

A specific variant of a character (e.g., purple or white flower).

Character

A heritable feature that varies among individuals (e.g., flower color).

Model organism

An organism used for experiments to understand biological principles; Mendel used Pisum sativum (pea plant).

True-breeding

Organisms that produce offspring identical to themselves when self-pollinated (homozygous).

Hybridization

The mating or crossing of two true-breeding varieties.

F₁ generation

The first filial generation; hybrid offspring of the parental (P) generation.

F₂ generation

Offspring of the F₁ generation; used to reveal recessive traits and segregation ratios.

Law of Segregation

Two alleles for a gene separate during gamete formation; each gamete carries only one allele.

Law of Independent Assortment

Alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes).

Gene

A heritable unit that controls a specific trait; sequence of DNA coding for a protein or RNA.

Allele

Alternative version of a gene.

Dominant allele

The allele that determines phenotype when present; masks recessive.

Recessive allele

The allele that is masked in heterozygotes; expressed only when homozygous.

Homozygous

Having two identical alleles for a gene (PP or pp).

Heterozygous

Having two different alleles for a gene (Pp).

Phenotype

The physical or physiological expression of a gene (observable trait).

Genotype

The genetic makeup (allele combination) of an organism.

Carrier

An individual heterozygous for a recessive disorder; can pass it to offspring without showing symptoms.

Punnett square

A diagram used to predict the genetic outcomes of a cross.

Test cross

Crossing an individual with an unknown genotype showing the dominant trait to a homozygous recessive individual to determine its genotype.

Genotypic ratio

Ratio of different allele combinations (e.g., 1:2:1 for PP:Pp:pp).

Phenotypic ratio

Ratio of visible traits (e.g., 3:1 purple:white).

Monohybrid cross

Cross involving one character (e.g., flower color).

Dihybrid cross

Cross involving two characters (e.g., seed color and shape).

Independent assortment

Random distribution of allele pairs of different genes into gametes.

Probability

Likelihood that a specific event will occur.

Multiplication rule

The probability of two independent events both occurring = product of their probabilities.

Addition rule

The probability of one or another mutually exclusive event occurring = sum of their probabilities.

Complete dominance

One allele completely masks the effect of the other (classic Mendelian pattern).

Incomplete dominance

Heterozygote shows an intermediate phenotype between the two homozygotes (e.g., red + white → pink).

Codominance

Both alleles are fully expressed in the heterozygote (e.g., MN blood group).

Multiple alleles

More than two allele forms exist in a population (e.g., ABO blood group).

Epistasis

One gene masks or alters the expression of another gene (e.g., Labrador coat color).

Polygenic inheritance

Two or more genes affect one phenotype additively (e.g., height, skin color).

Pleiotropy

One gene affects multiple traits (e.g., sickle-cell disease).

Quantitative characters

Traits that vary continuously across a population due to polygenic inheritance.

Continuous variation

Phenotypes that range smoothly between extremes (height, skin tone).

Discontinuous variation

Distinct phenotypic categories controlled by few genes (e.g., flower color).

Gene interaction

When expression of one gene is influenced by another gene (includes epistasis).

Gene–environment interaction

Phenotype influenced by environmental conditions (e.g., hydrangea color affected by soil pH).

Pedigree

A family tree diagram showing inheritance patterns over generations.

Autosomal dominant trait

Trait appears in every generation; affected individuals usually have one affected parent.

Autosomal recessive trait

Trait can skip generations; affected individuals often have carrier parents.

Carrier detection

Using pedigree or genetic testing to determine if an individual carries a recessive allele.

Chromosome theory of inheritance

Genes are located on chromosomes; their behavior during meiosis explains Mendel’s laws.

Linkage

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

Crossing over

Exchange of genetic material between homologous chromosomes; can unlink linked genes.

Recombination

Production of offspring with new allele combinations different from either parent.

Meiosis I

Stage of cell division when homologous chromosomes separate; explains segregation and assortment.