Genetics Terms Starting With F
Genetics Glossary: F
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F1 Generation
/ ef-WUN jen-er-AY-shun / · Latin: filialis (of a son or daughter) + generatio
F1 Generation is the first generation of offspring produced by a cross between two genetically distinct parental individuals or strains.
In Gregor Mendel’s experiments in the 1860s, crossing two true-breeding parents with contrasting traits produced F1 offspring that all displayed the dominant phenotype. The F1 generation is uniformly heterozygous when the parental lines are homozygous for different alleles at the locus of interest. Crossing F1 individuals with each other produces the F2 generation, in which recessive phenotypes reappear in predictable ratios.
Modern agriculture exploits this predictability by producing F1 hybrid seed that combines desirable traits from two inbred lines, often yielding 15 to 20 percent more than either parent line.
Hybrid seed sold to farmers is often F1 hybrid seed, bred to combine desirable traits from two inbred lines and display hybrid vigor, but the F2 offspring of these plants do not reliably reproduce the same high-performing traits, which is why farmers typically purchase new seed each season.
F1 hybrids are not genetically superior in all respects. Hybrid vigor applies to yield and uniformity, but F1 plants cannot be used to produce seeds that reliably reproduce the same traits in the next generation.
When Mendel crossed true-breeding tall pea plants (Pisum sativum) with true-breeding dwarf pea plants, all F1 offspring were tall, demonstrating that the tall allele is dominant over the dwarf allele. Every F1 plant carried one copy of each allele, yet the dwarf phenotype was completely absent across hundreds of plants examined.
F2 Generation
/ ef-TOO jen-er-AY-shun / · Latin: filialis (of a son or daughter) + generatio
F2 Generation is the second generation of offspring produced by interbreeding individuals of the F1 generation, in which recessive phenotypes reappear in predictable ratios.
Mendel observed that crossing F1 heterozygotes produced F2 offspring in a 3:1 dominant-to-recessive phenotypic ratio for single-gene traits. This generation was critical evidence for Mendel’s laws because it demonstrated that recessive traits are not destroyed by crossing but merely masked in the F1. Analysis of F2 ratios remains a foundational method for determining whether traits follow simple Mendelian inheritance.
For two independently assorting genes, the F2 phenotypic ratio expands to 9:3:3:1, a pattern Mendel confirmed by scoring thousands of plants across multiple trait combinations.
When Mendel counted over 7,000 pea plants in F2 generations across seven traits, every trait showed a ratio close to 3:1, providing strong statistical support for his laws of segregation and independent assortment.
The 3:1 ratio in the F2 generation is a phenotypic ratio, not a genotypic ratio. The underlying genotypic ratio is 1:2:1 for homozygous dominant, heterozygous, and homozygous recessive individuals.
In Mendel's F2 cross for seed color, 6,022 yellow and 2,001 green seeds appeared, a ratio of 3.01:1, matching the predicted 3:1 ratio with remarkable precision. Scoring that many individual seeds allowed Mendel to distinguish a true biological ratio from random chance, giving his model of allele segregation its statistical foundation.
Fitness
/ FIT-nes / · Old English: fitt (suitable)
Fitness is a measure of an organism's ability to survive and reproduce in its environment, quantified as the relative contribution of a genotype to the next generation.
In population genetics, fitness is expressed as a value between zero and one relative to the most fit genotype in the population. Natural selection acts on fitness differences between genotypes, causing alleles associated with higher fitness to increase in frequency over generations. Fitness is not a fixed property of a genotype but depends on the environment, genetic background, and population context.
A genotype with the highest fitness in one environment can have the lowest fitness in another, demonstrating that fitness is always relative to context and never an absolute property.
Fitness in genetics does not mean physical strength or athletic ability. It refers specifically to reproductive success and the ability to pass alleles to the next generation.
The sickle cell heterozygote has higher fitness in malaria-endemic regions of sub-Saharan Africa than either homozygote, illustrating how environmental context determines which genotype is favored by selection. Heterozygote frequency in some populations exceeds 20 percent precisely because the protection against malaria outweighs the cost of carrying one sickle allele.
Founder Effect
/ FOWN-der ih-FEKT / · English: founder + effect
Founder Effect is the loss of genetic diversity that occurs when a new population is established by a small number of individuals from a larger population.
Because founders represent only a subset of the original population’s alleles, the new population begins with reduced genetic diversity and allele frequencies that may differ substantially from the source population. Rare alleles present in the founders can become common in the new population, while common alleles absent in the founders are lost entirely. The founder effect is a form of genetic drift and can lead to elevated frequencies of certain genetic diseases in isolated populations.
The Amish population of Pennsylvania descends from a small group of 18th-century founders, resulting in an exceptionally high frequency of Ellis-van Creveld syndrome, a rare skeletal disorder.
The founder effect is not the same as the bottleneck effect. The founder effect involves establishing a new population, while the bottleneck effect involves a severe reduction in an existing population.
Learn More About Founder Effect →Ashkenazi Jewish populations show elevated frequencies of several genetic diseases including Tay-Sachs and Gaucher disease, attributed to founder effects during population expansion from a small ancestral group. Carrier frequencies for Tay-Sachs in this population reach roughly 1 in 30, compared to about 1 in 300 in the general population.
Frameshift Mutation
/ FRAYM-shift myoo-TAY-shun / · English: frame + shift + Latin: mutatio (change)
Frameshift Mutation is a genetic error caused by inserting or deleting nucleotides in a number that is not a multiple of three, which shifts the reading frame of all downstream codons and typically produces a nonfunctional protein.
Because the genetic code is read in triplet codons, any insertion or deletion that is not a multiple of three shifts every following codon, producing a completely altered amino acid sequence downstream of the mutation site. Frameshift mutations almost always result in a nonfunctional protein and frequently introduce a premature stop codon that truncates the polypeptide. Among point mutations, frameshifts tend to cause the most severe functional disruption because they corrupt the entire downstream coding sequence rather than changing a single amino acid.
The most common mutation causing Tay-Sachs disease is a four-base insertion in the HEXA gene that creates a frameshift, completely abolishing hexosaminidase A enzyme activity.
Are Enzymes Proteins? →A frameshift mutation is not the same as a missense mutation. A missense mutation changes only one amino acid, while a frameshift alters the entire downstream protein sequence.
Building Blocks of Proteins →In Duchenne muscular dystrophy, frameshift deletions in the dystrophin gene prevent production of any functional dystrophin protein, causing severe progressive muscle degeneration. Boys with Duchenne typically lose the ability to walk by their early teens, a consequence of reading-frame disruption across a gene spanning more than 2.4 million base pairs.