Genes can have variations. Alternative sequences {allele}| can be at genetic loci. Bateson and Saunders invented name [1902]. Different alleles make different polypeptides and produce different phenotypes, such as blood types O, A, B, and AB. Haploid organisms have one allele at each locus. For diploid organisms, one allele is from father, and one is from mother.
For one organism, all cells have the same gene alleles {genotype}|. Johannsen invented the word [1909].
For one organism, all genes have the same gene alleles from one parent {haplotype}|. Algorithms {haplotyping} can identify such alleles.
Gene alleles can be normal most-common alleles {wildtype}|.
DNA-sequence genetic loci {polymorphic locus} can have sequence variations {polymorphism, DNA}| {polymorphic variation}. For example, genes can have different alleles. Repeated sequences can have different numbers of repeats.
Sequence positions can have different nucleotides {single nucleotide polymorphism} {single-nucleotide polymorphism} (SNP). At genetic loci, alleles can differ by one nucleotide.
Restriction endonucleases cut at different positions for different alleles at polymorphic loci, causing variations in DNA-fragment lengths {Restriction Fragment Length Polymorphism} (RFLP) (FLP). Cutting or non-cutting at restriction-endonuclease sites makes two short DNA fragments or one long DNA fragment.
Small populations, non-random breeding, and mutations can change allele frequencies in populations {population genetics}|.
In populations, heterozygous and homozygous proportion tends to stay constant {balanced polymorphism}. Balanced polymorphism uses habitat efficiently and preserves variation. In isolated groups, gene ratios stay constant even if environment favors one allele.
For genes, populations have number of one allele divided by number of all alleles {allele frequency} {gene frequency}.
Small populations can have chance allele-frequency changes {genetic drift}|.
If individuals have extreme genetic traits, general trait is in species {principle of genetic gradients} {genetic gradient principle}.
In geographic areas, allele frequencies are constant for species with stable populations {Hardy-Weinberg law}.
Allele-proportion change rate depends on allele reproductive advantage {selection pressure}| and on whether allele is recessive or dominant.
variation
Population allele ratio changes by heterozygote superiority, environmental heterogeneity, cycles, agonistic or antagonistic gene linkages, and homozygote selection.
balancing
Natural selection can maintain allele ratios {stabilizing selection} {balancing selection} to maintain variation. Balancing selection happens in large, non-isolated populations with alleles that are neither dominant nor recessive.
directional
Natural selection can change allele ratios {directional selection} {purifying selection} to reduce variation. Directional selection can happen by genetic drift or inbreeding in small isolated populations.
Homologous-chromosome genetic loci can have different alleles {heterozygous alleles}|. Population allele frequencies determine probability that individuals are heterozygous {heterozygosity}.
Homologous-chromosome genetic loci can have same allele {homozygous alleles}|.
Inheritance laws can depend on cell factors {pangenesis, cell}. Darwin invented the word [1868]. De Vries invented the word pangen.
Trait inheritance uses regular processes {Mendel's laws} {Mendel laws}: law of segregation and law of independent segregation.
For all genes, sperm and egg cells have one gene allele {segregation law} {law of segregation, Mendel}, from either father or mother.
Most gene segregations are independent {independent segregation law} {law of independent segregation}, because genes typically are not on same chromosome or are far apart on same chromosome.
For heterozygosity, phenotypes can mix two allele traits {incomplete dominance}, one allele {dominant allele}| can determine phenotype, or both alleles can cause recessive trait.
For heterozygosity, one allele can be dominant and one allele {recessive allele}| can have no affect on phenotype, or both alleles can cause recessive trait.
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Date Modified: 2022.0225