Glossary
Allele
One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Example:
For the gene determining flower color, 'purple' and 'white' are different alleles.
Chloroplast
A plastid in green plant cells and other photosynthetic organisms that contains chlorophyll and in which photosynthesis takes place; it also contains its own DNA.
Example:
In plant cells, the chloroplast captures light energy to convert carbon dioxide and water into sugars.
Chromosome
A thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying 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:
Crossing over during meiosis shuffles alleles between homologous chromosomes, creating new combinations on each chromatid.
Diploid
A cell or organism containing two complete sets of chromosomes, one inherited from each parent.
Example:
Most human body cells are diploid, meaning they have 46 chromosomes (23 pairs).
Gamete
A mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote.
Example:
In humans, the sperm and egg are the gametes that combine during fertilization.
Gene
A unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Example:
The gene for eye color determines whether an individual will have blue, brown, or green eyes.
Genetic Variation
The differences in DNA sequences among individuals within a population, which is the raw material for evolution.
Example:
The wide range of human hair colors, eye colors, and heights is a result of genetic variation within the population.
Genotype
The genetic makeup of an organism, referring to the set of alleles an individual possesses for a particular trait.
Example:
A pea plant with the genotype 'Tt' is heterozygous for height, carrying one tall allele and one short allele.
Haploid
A cell or organism containing a single set of unpaired chromosomes.
Example:
Human gametes, like sperm and egg cells, are haploid, each carrying 23 chromosomes.
Homologous Chromosome
A pair of chromosomes (one from each parent) that are similar in length, gene position, and centromere location, and that pair up during meiosis.
Example:
During meiosis, homologous chromosomes pair up and can exchange genetic material through crossing over.
Independent Assortment
Mendel's principle stating that alleles of different genes assort independently of one another during gamete formation.
Example:
Due to independent assortment, a gamete could receive a maternal chromosome 1 and a paternal chromosome 2, or vice versa, leading to diverse combinations.
Law of Segregation
Mendel's principle stating that during gamete formation, the two alleles for a heritable character separate (segregate) from each other so that each gamete receives only one allele.
Example:
According to the Law of Segregation, an individual heterozygous for a trait (e.g., Tt) will produce gametes where half carry the 'T' allele and half carry the 't' allele.
Laws of Probability
Mathematical rules used to predict the likelihood of genetic outcomes in offspring, often applied in Punnett squares.
Example:
Using the laws of probability, one can calculate the chance of an offspring inheriting a specific combination of alleles from a dihybrid cross.
Meiosis
A specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell.
Example:
During human meiosis, a diploid cell in the testes or ovaries divides twice to produce four haploid sperm or one haploid egg cell.
Mitochondria
An organelle found in most eukaryotic cells, responsible for cellular respiration and having its own circular DNA, inherited maternally.
Example:
The mitochondria are often called the 'powerhouses' of the cell because they generate most of the cell's supply of ATP.
Mutation
A permanent alteration in the DNA sequence that makes up a gene or chromosome, serving as the ultimate source of new alleles.
Example:
A point mutation in the hemoglobin gene causes sickle cell anemia, altering the protein's structure.
Nondisjunction
The failure of homologous chromosomes or sister chromatids to separate properly during cell division (meiosis or mitosis).
Example:
Down syndrome (Trisomy 21) is often caused by nondisjunction of chromosome 21 during meiosis, resulting in an extra copy.
Pedigree
A diagram showing the lineage or ancestry of an individual and all the known ancestors, used to trace the inheritance of a specific trait through generations.
Example:
Genetic counselors often use a pedigree to determine the probability of a genetic disorder appearing in future offspring.
Phenotype
The observable physical or biochemical characteristics of an organism, resulting from the expression of its genotype and environmental influences.
Example:
A pea plant with the phenotype of 'tall' could have either a 'TT' or 'Tt' genotype.
Random Fertilization
The unpredictable union of any single sperm with any single egg, contributing significantly to genetic diversity in offspring.
Example:
Even if two parents produce millions of unique gametes, the specific combination that forms a zygote is determined by random fertilization.
Sex-linked
A trait or gene located on one of the sex chromosomes, typically the X chromosome in humans.
Example:
Color blindness is a common sex-linked recessive trait, which is why it affects males more frequently than females.
Sister Chromatid
Two identical copies of a chromosome joined together by a centromere, formed after DNA replication.
Example:
Before a cell divides, each chromosome duplicates, forming two identical sister chromatids that remain attached until anaphase.
Zygote
A diploid cell resulting from the fusion of two haploid gametes; a fertilized egg.
Example:
After a sperm fertilizes an egg, the resulting single cell is called a zygote, which will then undergo mitosis to develop into an embryo.