Conditions and mechanism of the law
For Mendel's first law to work, a strict condition must be met: two homozygous individuals (the parental generation is designated by the Latin letter P) must participate in the crossing. These organisms must differ from each other by only one pair of alternative, i.e., mutually exclusive, traits.
One parent has a dominant homozygous genotype ($AA$), and the second has a recessive homozygous genotype ($aa$). When crossed, they produce hybrids of the first generation (designated as $F_1$).
Each offspring receives one gene from each parent. As a result, a mixed genotype is formed — $Aa$. Such organisms are called heterozygous. Since all offspring receive the same set of alleles ($Aa$), complete genetic and phenotypic uniformity is observed in the first generation.
Complete dominance (using peas as an example)
The classic manifestation of Mendel's first law is associated with the phenomenon of complete dominance. In this situation, the strong (dominant) gene completely suppresses the action of the weak (recessive) gene, leaving it no chance to manifest externally.
Let us consider this process using the crossing of pea plants as an example:
- Parental forms (P): A plant with yellow seeds (genotype $AA$) is crossed with a plant with green seeds (genotype $aa$).
- Offspring ($F_1$): All resulting hybrids inherit the $Aa$ genotype (they are heterozygotes).
- Phenotype of $F_1$: Since the yellow color gene completely dominates over the green color gene, absolutely all seeds in the first generation grow yellow.
Thus, the recessive trait (green color) is preserved hidden in the genotype, but externally all hybrids are uniform and copy the dominant parent.
Incomplete dominance (intermediate inheritance)
In nature, situations are not uncommon where the dominant gene cannot completely suppress the effect of the recessive gene. This phenomenon is called incomplete dominance or intermediate inheritance. It is extremely important to emphasize that the rule of uniformity of first-generation hybrids ($F_1$) is strictly preserved even with incomplete dominance.
Let's analyze this mechanism using the flower color of the four o'clock plant as an example:
- Parental individuals (P): Plants with red flowers (dominant homozygote, $AA$) and white flowers (recessive homozygote, $aa$) are crossed.
- Offspring ($F_1$): Just like with complete dominance, all offspring receive a heterozygous genotype ($Aa$).
- Phenotype of $F_1$: All resulting hybrids bloom with pink flowers. An intermediate trait is observed.
What is the mechanism? Heterozygous organisms ($Aa$) produce significantly less floral pigment than dominant homozygotes ($AA$). The lack of pigment leads to "dilution" of the color, as a result of which the red and white colors produce an intermediate pink result. Nevertheless, the entire array of offspring remains phenotypically and genotypically uniform.
Briefly: answers to topical questions
What are alternative traits in genetics?
Alternative traits represent different varieties of a single trait, the development of which is controlled by alleles — alternative forms of a gene.
Features of genes determining such traits:
- Allelic genes — are located in identical regions (loci) of homologous chromosomes.
- A diploid (somatic) cell contains two alleles, and a gamete contains only one.
Mendel's first law describes the crossing of homozygous individuals that differ precisely in one pair of alternative traits (for example, yellow and green pea color).
Remembering the essence of the law is simple: "Pure opposites produce identical children." If you cross pure lines (AA and aa), all their direct descendants (F1) will be clones of each other in genotype (Aa) and appearance.
Class and exam questions
Does Mendel's First Law hold true in incomplete dominance?
Yes, the law is fully preserved. Despite the appearance of a new intermediate trait, all offspring of the first generation ($F_1$) remain completely uniform both in genotype (all $Aa$) and in phenotype.
What organisms must be crossed to fulfill the law?
Strictly homozygous individuals differing by a single pair of traits must participate in the crossing. One parent must be a dominant homozygote ($AA$), and the other a recessive ($aa$).
Why does crossing red and white four-o'clocks produce pink flowers?
The reason lies in incomplete dominance. Heterozygous hybrids ($Aa$) produce less coloring pigment than homozygous parent plants with red flowers ($AA$), which produces the intermediate pink tint.
What else to study on this topic
- Mendel's second law (Law of segregation)
- Test cross and its application
- Mendel's third law (Law of independent assortment)
- Cytological basis of monohybrid cross
- Multiple allelism
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