Biology 9: Unit 5 Genetics & Hereditary

Preformation

An early, incorrect idea that organisms develop from tiny, fully formed versions of themselves.

Pangenesis

Darwin’s hypothesis that all parts of the body send information to reproductive cells (incorrect).

Blending hypothesis

The incorrect idea that parental traits mix together like paint in offspring.

Traits

Observable characteristics of an organism.

True-breeding

An organism that produces offspring with the same traits when self-fertilized.

Self-fertilization

Fertilization in which one organism produces both gametes.

Cross-fertilization

Fertilization between gametes from two different individuals.

Gene

A segment of DNA that codes for a specific trait.

Allele

Different versions of the same gene.

Homozygous

Having two identical alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Genotype

The genetic makeup of an organism.

Phenotype

The physical expression of a genotype.

Dominant

An allele that is expressed when only one copy is present.

Recessive

An allele that is masked by a dominant allele and expressed only when homozygous.

Incomplete dominance

A form of inheritance where the heterozygous phenotype is a blend.

Codominance

A form of inheritance where both alleles are fully expressed.

Polygenic trait

A trait controlled by two or more genes.

Punnett square

A diagram used to predict the outcome of genetic crosses.

Probability

The likelihood that a specific event will occur.

Monohybrid cross

A genetic cross involving one trait.

Dihybrid cross

A genetic cross involving two traits.

Testcross

A cross with a homozygous recessive used to determine an unknown genotype.

Sex-linked genes

Genes located on sex chromosomes, usually the X chromosome.

Hemizygous

Having only one copy of a gene instead of two.

Pedigree

A diagram that tracks inheritance of traits through generations.

Chromosomal mutations

Changes in chromosome structure that affect many genes.

Gene duplication

A mutation in which a gene is copied.

Genetic variation

Differences in DNA sequences among individuals in a population.

Gene translocation

A mutation where part of a chromosome attaches to another chromosome.

Nondisjunction

The failure of chromosomes to separate properly during meiosis.