We know that alleles are genes that occupy the same place (locus) in chromosomes counterparts. They can also be defined as the alternative form of a gene, since they determine the same trait, however, not in the same way. These alleles can be either dominant or recessive and assume relationships of dominance or recessivity in somatic cells, in which the alleles are paired.
→ dominant allele
The dominant alleles are those that can determine a phenotype even when they appear in a single dose. This means that when we analyze a pair of alleles for a trait, if only one of them is a dominant allele, that trait is expressed.
Imagine, for example, an allele B that determines the black color of the coat of rats, and b that determines the white coat. B is a dominant allele and therefore individuals BB and Bb will have black coat.
→ recessive allele
Recessive alleles, unlike dominant ones, they need to be in pairs for the characteristic to express itself. This means that in heterozygous individuals the recessive allele will not determine the phenotype.
Imagine the same example above where an allele B determines the black color of the coat of rats, and b determines the white coat. White individuals will only be those with the bb genotype.
→ An example of a relationship of dominance and recessivity
In your studies, Mendel worked with pea traits, like the color of the seeds. He noted that by crossing pure plants for green seeds with pure plants for yellow seed, you get a generation with 100% yellow plants. By crossing individuals of this generation, 75% of the yellow plants and 25% of the plants with green seeds were obtained. He then considered that yellow was dominant and green was recessive.
In this situation, we have a cross between VV (Yellow) x vv (Green). In this case, all descendants will be Vv (Yellow). Crossing individuals Vv x Vv, the result of the crossing will be: VV, Vv, Vv, vv, that is, 75% of the seeds will be yellow and 25% will be green. Note that in this example we find that the V allele for yellow seeds is dominant, as individuals VV and Vv are yellow. The v allele is recessive, as only vv plants are green.
ATTENTION:Not all alleles show dominance and recessivity relationships.. An example is the ABO system, in which I alleles are observedTHE, IB and i, of which the I allelesTHE HeyB present a codominance relationship. In this case, individuals ITHE IB have AB blood, as both alleles are expressed. There is also incomplete dominance in which the heterozygous individual has an intermediate phenotype different from homozygous individuals.
By Ma. Vanessa Sardinha dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/biologia/o-que-e-dominancia-recessividade.htm
