Info

figure 16-1

A bell curve illustrates that most members of a population have similar values for a given, measurable trait. Only a few individuals display extreme variations of the trait.

Fish length

Fish length figure 16-1

A bell curve illustrates that most members of a population have similar values for a given, measurable trait. Only a few individuals display extreme variations of the trait.

figure 16-2

Many varied but similar phenotypes occur within families because members of a family share some alleles but not others.

figure 16-2

Many varied but similar phenotypes occur within families because members of a family share some alleles but not others.

Causes of Variation

What causes variation in traits? Some variations are influenced by environmental factors, such as the amount or quality of food available to an organism. Variation is also influenced by heredity. Some variations occur as a range of phenotypic possibilities (such as a range of body sizes), whereas others occur as a set of specific phe-notypes (such as two possible flower colors).

To consider variability, think about phenotypes within a single human family. Two parents, each with a distinct genotype, may produce several children. In the picture of the family in Figure 16-2, the two young-adult brothers are not identical to each other, even though their genotypes are combinations of the genotypes of the same two parents. Both young men resemble their father, though in different traits. The baby resembles his young father, his grandfather, and his uncle. Thus, these males representing three generations look similar but not identical.

What causes genes to vary? Variations in genotype arise in three main ways. (1) Mutation is a random change in a gene that is passed on to offspring. (2) Recombination is the reshuffling of genes in a diploid individual. Recall that recombination occurs during meiosis by independent assortment and crossing-over of genes on chromosomes. (3) The random pairing of gametes occurs because each organism produces large numbers of gametes. So, the union of a particular pair of gametes is partly a matter of chance.

Scientists are still exploring other causes of variation in traits. For example, the expression of some genes depends upon the presence or absence of other genes or factors in the environment. The net result of having many alleles of many genes is the variety of unique genotypes and phenotypes that we see in populations.

Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment