Glossary
Allele
A specific variant of a gene, typically found at a particular locus on a chromosome.
Example:
For the gene that determines flower color, 'R' (red) and 'r' (white) are two different alleles.
Carrier
An individual who carries one copy of a recessive allele for a genetic trait or disease but does not express the trait themselves because they also have a dominant allele.
Example:
A woman who has one allele for hemophilia but does not have the disease herself is considered a carrier.
DNA
Deoxyribonucleic acid, a double-stranded nucleic acid that carries the genetic instructions used in the growth, development, functioning, and reproduction of all known organisms and many viruses.
Example:
The unique sequence of nucleotides in your DNA determines everything from your eye color to your susceptibility to certain diseases.
Dihybrid Crosses
A genetic cross between two individuals that are heterozygous for two different traits, used to study the inheritance of two genes simultaneously.
Example:
Predicting the inheritance of both seed color and seed shape in pea plants at the same time requires a dihybrid cross.
Dominant
An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; it masks the expression of the recessive allele.
Example:
In humans, the allele for brown eyes is dominant over the allele for blue eyes, so a person with one brown and one blue allele will have brown eyes.
Genotype
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 the alleles 'Tt' for height, then 'Tt' is its genotype.
Gregor Mendel
An Austrian monk and botanist often referred to as the 'father of modern genetics' for his foundational work on inheritance patterns in pea plants.
Example:
Thanks to Gregor Mendel's meticulous experiments, we understand how traits like flower color are passed from parent to offspring.
Heterozygous
A genotype where an individual has two different alleles for a specific trait, typically one dominant and one recessive.
Example:
If a person has one allele for curly hair and one for straight hair, they are heterozygous for that trait.
Homozygous Dominant
A genotype where an individual has two identical dominant alleles for a specific trait.
Example:
A pea plant with two 'TT' alleles for height is homozygous dominant and will be tall.
Homozygous Recessive
A genotype where an individual has two identical recessive alleles for a specific trait.
Example:
A pea plant with two 'tt' alleles for height is homozygous recessive and will be short.
Law of Independent Assortment
Mendel's second law stating that alleles for different genes assort independently of one another during gamete formation, meaning the inheritance of one trait does not influence the inheritance of another.
Example:
The inheritance of pea plant color (yellow or green) doesn't affect the inheritance of pea plant shape (round or wrinkled), demonstrating the Law of Independent Assortment.
Law of Segregation
Mendel's first law 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:
When a parent produces gametes, the allele for tallness and the allele for shortness separate, ensuring each sperm or egg gets only one, illustrating the Law of Segregation.
Monohybrid Cross
A genetic cross between two individuals that are heterozygous for one particular trait, used to study the inheritance of a single gene.
Example:
Crossing two pea plants that are both heterozygous for flower color (e.g., Rr x Rr) is a classic monohybrid cross.
Non-Mendelian Inheritance
Patterns of inheritance that do not follow the simple dominant/recessive rules established by Mendel, including concepts like incomplete dominance, codominance, and polygenic inheritance.
Example:
Blood type inheritance, involving multiple alleles and codominance, is an example of Non-Mendelian Inheritance.
Phenotype
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 are their phenotype for eye color, a visible trait.
Phenotypic Ratio
The ratio of observable traits (phenotypes) among the offspring of a genetic cross.
Example:
A classic phenotypic ratio of 9:3:3:1 is expected from a dihybrid cross between two heterozygotes.
Punnett Squares
A diagram used to predict the probability of offspring genotypes and phenotypes from a genetic cross by combining the possible gametes from each parent.
Example:
Using a Punnett Square helps visualize the 25% chance of having a child with a specific recessive trait if both parents are carriers.
RNA
Ribonucleic acid, a single-stranded nucleic acid that plays crucial roles in gene expression, acting as a messenger, a ribosomal component, and a transfer molecule in protein synthesis.
Example:
Messenger RNA (mRNA) carries the genetic code from the nucleus to the cytoplasm, where proteins are made.
Recessive
An allele that only expresses its phenotypic effect when two copies are present (i.e., in a homozygous recessive genotype), being masked by a dominant allele in a heterozygous state.
Example:
The allele for cystic fibrosis is recessive, meaning an individual must inherit two copies to develop the disease.
Ribosomes
Cellular organelles responsible for protein synthesis, translating messenger RNA (mRNA) into polypeptide chains.
Example:
Without ribosomes, the cell wouldn't be able to produce the enzymes and structural proteins essential for life.
Sex-Linked Traits
Traits determined by genes located on the sex chromosomes (X or Y), most commonly on the X chromosome.
Example:
Color blindness is a common sex-linked trait, which is why it affects males more frequently than females.