Basic concepts of genetics
Heredity is the ability of living organisms to transmit traits to their offspring. Variability is the ability to acquire new traits during ontogeny. These two properties form the foundation of the existence and evolution of living systems.
- Gene is a discrete unit of heredity, a segment of a DNA molecule that encodes a specific protein and determines the development of a trait.
- Locus is the physical position of a gene on a chromosome.
- Genotype is the complete set of genes of an organism.
- Phenotype is the set of morphological and physiological traits, the manifestation of the genotype.
- Genome is all the genetic information of a cell contained within the haploid set of chromosomes and organelles.
- Gene pool is the set of all genes in a population.
- Allele is one of the alternative forms of a gene arising from mutations; in a diploid organism, there are two alleles of each gene on homologous chromosomes.
Allelic genes are located at the same loci of homologous chromosomes and control the same trait; non-allelic genes are at different loci and control different traits. A dominant allele (capital letter) masks the expression of a recessive allele (lowercase letter) in a heterozygote. An organism carrying identical alleles is a homozygote; different alleles make it a heterozygote.
Mendel's laws
In 1865, Gregor Mendel published "Experiments on Plant Hybrids", describing inheritance patterns in the garden pea. He developed the hybridological method and introduced genetic symbols.
- Law of uniformity of first-generation hybrids: crossing two homozygotes differing in one pair of alternative traits results in a uniform F1 generation displaying the dominant trait.
- Law of segregation: crossing two F1 heterozygotes results in an F2 phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1. This is explained by the hypothesis of purity of gametes: hereditary factors do not blend and enter gametes individually.
- Law of independent assortment: during a dihybrid cross, traits are inherited independently, yielding a phenotypic ratio of 9:3:3:1.
In 1936, Ronald Fisher showed that Mendel's data were statistically too close to expected values. Mendel's success is attributed to his choice of the pea: self-pollination, clear alternative traits, the monogenic nature of the studied traits (seed color and shape), and large progeny.
Chromosomal theory of heredity
The Mendel paradox stimulated the study of chromosomes. In 1902–1903, W. Sutton and T. Boveri independently demonstrated that chromosomes carry heredity. In the 1910s–1920s, Thomas Morgan proved the linear arrangement of genes on chromosomes.
Morgan's law: genes located on the same chromosome are preferentially inherited together and form a linkage group. Morgan also discovered sex-linked inheritance using white eye color in Drosophila, which appeared predominantly in males.
Were constructed genetic maps — diagrams of the relative arrangement of genes and the distances between them, expressed in centimorgans (morgans). One morgan corresponds to a 1% probability of recombination between alleles in meiosis.
Morgan's model organism was Drosophila melanogaster: four pairs of chromosomes (XY and three autosomes), a short life cycle (~2 weeks to sexual maturity), high fertility, ease of maintenance, and a large number of spontaneous mutations.
Types of crosses
- Monohybrid involves one pair of traits; di- and polyhybrid involve two or more pairs.
- Backcross is the crossing of a hybrid with one of the parental forms; used in breeding to fix traits.
- Test cross is the crossing of an individual with an unknown genotype with a recessive homozygote to determine its genotype.
- Reciprocal cross involves two crosses with the sexes of the carriers of the analyzed trait reversed; used to study sex-linked inheritance.
Pure line is a genetically homogeneous group of individuals that stably transmits traits to offspring without segregation.
Crossing over
Crossing over is the exchange of corresponding segments between homologous chromosomes; the primary source of combinatorial variability. It occurs in prophase I of meiosis.
- Synapsis: homologous chromosomes approach each other and form bivalents; each consists of two sister chromatids.
- Breakage: at crossover points (chiasmata), enzymes cut the DNA of non-sister chromatids.
- Exchange and reunion: fragments are spliced crosswise with the chromatid of the homolog.
As a result, recombinant gametes with new allele combinations and non-recombinant gametes with parental combinations are formed. For example, chromosomes AB and ab yield Ab and aB.
Organization of the eukaryotic nuclear genome
Nuclear genome is the total DNA of the cell nucleus. It includes unique and repetitive sequences.
Unique sequences are present in one or a few copies:
- Coding DNA comprises exons that carry information for amino acid sequences or functional RNAs.
- Non-coding DNA: introns within genes; regulatory sites—promoters, enhancers, silencers; pseudogenes.
- Cis-elements regulate genes on the same chromosome; trans-elements are proteins and transcription factors that bind to cis-elements.
Repetitive sequences can account for more than half of the eukaryotic genome:
- Mobile genetic elements ("jumping genes") are transposons and retrotransposons capable of moving and causing mutations.
- Tandem repeats: satellite DNA (short repeats in centromeres and telomeres), mini- and microsatellites used in DNA fingerprinting.
The C-value paradox is the lack of a direct correlation between genome size and organism complexity: some amphibians and plants have larger genomes than humans due to repetitive sequences. Alternative splicing explains how a single gene encodes multiple proteins: pre-mRNA exons are spliced in different combinations, increasing protein diversity without increasing the number of genes.
Key points
- Heredity and variability are fundamental properties of living systems; gene, locus, genotype, phenotype, and allele are basic concepts of genetics.
- Mendel's three laws describe F1 uniformity, 3:1 segregation in F2, and independent assortment with a 9:3:3:1 ratio.
- Hypothesis of purity of gametes: hereditary factors are transmitted to gametes individually without blending.
- Morgan's chromosome theory: genes are located linearly on chromosomes and form linkage groups; distances are measured in centimorgans.
- Sex-linked inheritance was first described using Drosophila as a model organism.
- Test crosses, backcrosses, and reciprocal crosses are tools for identifying genotypes and modes of inheritance.
- Crossing over in prophase I of meiosis creates new allele combinations and provides combinatorial variability.
- The eukaryotic genome combines unique coding regions, regulatory elements, and numerous repeats; alternative splicing expands the cell's protein repertoire.