Final answer:
In a dominant-recessive system, alleles determine the expression of traits. Homozygous individuals express the trait represented by the alleles they have, while heterozygous individuals only express the dominant allele. This concept applies to both plants, like peas, and humans.
Step-by-step explanation:
In a simple dominant-recessive system, alleles determine the expression of traits. When an individual has two alleles that are the same (homozygous), the trait they represent will be expressed. However, if the individual carries one of each allele (heterozygous), only the dominant allele will be expressed, while the recessive allele will be suppressed.
For example, in flower color in peas, the dominant allele is purple (P) and the recessive allele is white (p). If an individual has the genotype PP, both alleles are purple, and the individual will have purple flowers. If an individual has the genotype Pp, one allele is purple and one allele is white, but only the purple allele will be expressed, resulting in purple flowers. And if an individual has the genotype pp, both alleles are white, and the individual will have white flowers.
This concept applies to humans as well. If an individual receives two dominant alleles, their phenotype will express the dominant trait. If an individual receives two recessive alleles, the recessive trait will be expressed. Individuals with two identical alleles, whether dominant or recessive, are said to be homozygous. Individuals with one dominant allele and one recessive allele are said to be heterozygous, and they will express the dominant trait.