Question

How do dominant and recessive alleles affect phenotype?

Answer 1

An allele is a potential sequence of gene coding. A common misconception is that genes are there for unique characteristics. Genes regulate various characteristics of an organism, such as hair colour or eye colour, but the true expression of a characteristic depends on which allele is dominant.

Complete answer:
Humans will have two copies of every gene to store alleles, as well as have two copies of each chromosome.
The two copies of a gene contain an allele, one on each of the chromosomes. Homozygosity happens when, for a specific phenotype, or expressed trait, the two gene copies contain the same allele. When the two alleles are different, heterozygosity exists. Phenotypes may be dominant or recessive either. If a phenotype is dominant, there needs to be only one of the alleles present for it. If it is recessive, both alleles need to be present.
By taking the alleles of the two parents and combining them, Punnett square can be used to predict the probability that a certain genotype will occur in children. Depending on whether the alleles are dominant or recessive, the genotype may affect the phenotype.
The basic square of Punnett is split into four smaller squares. Above the top two squares, the probable genes of one parent are listed, the other next to it going vertically down the left side. Capital letters represent dominant alleles, and recessive alleles are denoted in lower-case letters.
A homozygous dominant gene for brown eyes, for example, would be written as $BB$ and a heterozygous one would be written as $Bb$. A recessive homozygous gene will be written as $bb$. The likelihood of a $BB$ genotype will be 25 percent, $Bb$ 50 percent and $bb$ 25 percent by taking the letters adjacent to each square, for a $Bb$x $Bb$ cross.
In phenotype terms, since brown eyes are a dominant phenotype, the offspring has a 75 percent chance of getting brown eyes.

Note:
Dominance influences the phenotype produced from the genes of an organism, but it does not influence the inheritance of these genes. When the heterozygous phenotype is indistinguishable from that of the homozygous parent, full dominance exists. The heterozygote, however, often shows a phenotype that is an intermediary between both homozygous parents' phenotypes. This intermediate phenotype is called partial or incomplete dominance.