Mendel’s Laws and Genetic Mutations: Inheritance Principles
Mendel’s First Law: Law of Uniformity
Mendel’s first law, the law of uniformity, concerns the first filial generation. The offspring resulting from crossing two pure strains (homozygous) forms a group of hybrids that have uniformity, both in terms of genotype and phenotype. This law is based on the crossing of two varieties homozygous for a character (homozygous dominant and homozygous recessive, AA and aa), which gives rise to a uniform F1 generation (Aa), with the same phenotype as the dominant parent.
Mendel’s Second Law: Law of Segregation
Mendel’s second law, the law of segregation, concerns the second filial generation. Individuals of the F2 generation, resulting from crossing F1 hybrids with each other, are phenotypically different from each other. This is because of the disjunction or segregation of the factors responsible for these characters. These factors are initially found together in the hybrid and then separate and are shared among the various gametes. This is based on selfing or mating between individuals of the F1 hybrid (Aa). After meiosis in male and female individuals of the F1 generation, alleles A and a separate (segregate) from each other to form gametes and then rejoin again at fertilization in accordance with the laws of chance and probability. For this reason, in the F2 generation, dominant and recessive phenotypes reappear in a 3:1 ratio, respectively. The genotypic combinations are homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa), which are in a 1:2:1 proportion.
Mendel’s Third Law: Law of Independent Assortment
Mendel’s third law, the law of independent assortment, states that antagonistic characters are inherited independently of each other. This is because the alleles responsible for these characters (which Mendel called factors of heredity) are transmitted to offspring separately and combined in all possible ways. This law governs the simultaneous transmission of two characters with dominant inheritance, whose genes are located on different pairs of homologous chromosomes. From the parental phenotypes, disjunction or separation between the two characters occurs (as a result of meiosis) so that each is transmitted to offspring separately, independently, and combined in all possible ways (e.g., the yellow phenotype can be associated with smooth or rough texture, and the same is true for green). This results in four phenotypes that appear in a 9:3:3:1 ratio.
Experimental Crosses and Data Analysis
- Hits: Count the number of offspring at each crossing and apply mathematical calculations to analyze the data.
- In experimental crosses, the observed frequencies of each phenotype should be calculated for comparison with the expected frequencies and theoretical probabilities.
- The results derived from the crosses above are true in all cases dealing with the transmission of a character governed by a gene with two allelic forms and dominant inheritance.
Consanguinity
Consanguinity refers to the degree of genetic relationship between two individuals derived from a common ancestor. If this ancestor had a pathological recessive mutation, it is normal that it has been transmitted to the most immediate descendants, turning them into, at least, carriers. The problem with inbreeding, for example, between first cousins (who share a common set of grandparents and therefore share one-eighth of their genes), is that it increases the risk of having children affected by some anomaly. This anomaly manifests when a child inherits the mutated allele in a homozygous recessive state.
Mutations
Mutations are changes or variations in genetic material that occur spontaneously or are induced by mutagens. They are heritable only when they affect germ cells; if they affect somatic cells, they disappear with the individual in which they appear. If a mutation affects a dominant character, it is easily detected. However, if it is recessive, it is more difficult to detect because it may run through heterozygous carriers. These carriers reproduce and transmit the recessive allele, which is only manifested in a homozygous recessive state, particularly in people who have repeatedly engaged in inbreeding.
Specific Gene Mutations
Specific gene mutations are alterations in the DNA sequence of a gene, usually affecting a single base pair. They modify the amino acid sequence of the protein and, therefore, its biological activity.
Chromosomal Mutations
Chromosomal mutations are produced by altering the normal sequence of gene fragments that make up a chromosome. There are several causes of these mutations:
- Inversion: A segment inverts its sequence.
- Duplication: The same sequence is repeated.
- Deletion: A fragment of the chromosome is deleted.
- Translocation: Fragments are exchanged between chromosomes.
Other factors responsible for changes in genetic information are transposons, first described by Barbara McClintock. These are mobile or jumping genes capable of changing their position in the genome and “jumping” from one chromosome to another. The abandonment of their initial position and their reintegration into another chromosome often causes genetic variations.
Genomic Mutations
Genomic mutations affect the genome and give rise to a variation in chromosome number. The most common is aneuploidy, which occurs when an individual accidentally has one chromosome more or less in relation to their diploid condition. Monosomies occur when there is only one homologous chromosome instead of two, and trisomies occur when there are three homologous chromosomes instead of two. In the human species, aneuploidy arises spontaneously, causing changes that may affect autosomes or sex chromosomes. The following table lists the syndromes (sets of symptoms) most characteristic of these mutations.