Glossary
Chromosomes
Structures made of DNA tightly coiled around proteins, found in the nucleus of eukaryotic cells, that carry genetic information in the form of genes.
Example:
Humans typically have 46 chromosomes in each somatic cell, organized into 23 pairs.
Crossing Over
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
Example:
During crossing over, a chromosome might swap a segment containing the allele for blue eyes with its homologous partner, creating a new combination of alleles on that chromosome.
Genetic Variation
The differences in DNA sequences among individuals within a population, providing the raw material for evolution.
Example:
The wide range of fur colors in a population of rabbits is an example of genetic variation, allowing some to blend better into different environments.
Genotypes
The genetic makeup of an organism, referring to the specific set of alleles an individual possesses for a particular trait.
Example:
If a pea plant has two identical alleles for tallness (TT), that represents its genotype for height.
Independent Assortment
The random orientation and separation of homologous chromosomes during meiosis I, leading to different combinations of alleles in gametes.
Example:
If a pea plant has genes for both seed color and seed shape, independent assortment means the inheritance of yellow vs. green seeds doesn't influence the inheritance of round vs. wrinkled seeds.
Meiosis
A specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, and is essential for sexual reproduction and genetic variation.
Example:
The formation of sperm and egg cells in humans occurs through meiosis, ensuring that offspring receive half their chromosomes from each parent.
Mitosis
A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typically for growth and repair.
Example:
When you cut your finger, mitosis is the process that replaces the damaged skin cells with new, identical ones.
Mutated Alleles
Variants of a gene that arise from a change in the DNA sequence, potentially leading to altered protein function or new traits.
Example:
Cystic fibrosis is caused by specific mutated alleles that lead to a defective protein involved in chloride transport.
Natural Selection
The process by which individuals with advantageous heritable traits tend to survive and reproduce more successfully than others, leading to evolutionary change.
Example:
In a population of giraffes, those with longer necks might reach more leaves and survive better, leading to natural selection favoring longer necks over generations.
Nondisjunction
The failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis.
Example:
Down syndrome often results from nondisjunction of chromosome 21 during meiosis, leading to an extra copy of that chromosome.
Phenotypes
The observable physical or biochemical characteristics of an organism, resulting from the expression of its genotype and environmental influences.
Example:
A person's blue eyes, curly hair, or blood type are all examples of their phenotypes.
Punnett Squares
A diagram used to predict the probability of offspring genotypes and phenotypes from a genetic cross.
Example:
Using a Punnett square, you can predict that if two parents heterozygous for a dominant trait (like widow's peak) cross, there's a 25% chance their child will not have a widow's peak.
Random Fertilization
The unpredictable fusion of any one sperm with any one egg during sexual reproduction.
Example:
Even if two parents produce millions of genetically unique gametes, random fertilization ensures that each offspring is a unique combination of those possibilities, like winning a specific lottery ticket.