​Diversity of Life

Organisms from the same species tend to have many similar-
ities. Some differences are visible traits, such as wrinkled skin or brown
hair. Others are invisible, such as type I diabetes or sickle-cell
anemia. Differences have the potential to be passed on from
one generation to the next, and change the population.

​Diversity of Life

The diversity of life on Earth relies in part on the variety of
traits within a species. Any difference between individuals
of the same species is a variation. Two friends with different
eye color have a variation (green, brown) of the same trait
(eye color). Variations may be due to DNA inherited from
the parents, exposure to certain environmental factors, or a
combination of both inheritance and environmental factors.

Variations can be helpful, harmful, or neutral. Consider
a population of butterflies avoiding predators. Some have
the same wing color pattern as a poisonous species. When
this variation is passed from one generation to the next,
the offspring are more likely to survive and reproduce. A
harmful variation, on the other hand, threatens a population’s
survival. For example, low blood oxygen levels can be found in
individuals with sickle-cell anemia. Neutral variations, such as
different eye color, do not benefit or harm the population.

The combination of sex chromosomes determines the sex
of the offspring. A human female inherits one X chromosome
from her mother and one X chromosome from her father. A
male receives one X chromosome from his mother and one
Y chromosome from his father. Figure 2 compares the X and
Y chromosomes.

You received 23 chromosomes from your mother and 23
chromosomes from your father. The combination of genes
found on these chromosomes codes for the proteins that
determine your traits.

Chromosomes and Variation

Types of Chromosomes

There are two types of
chromosomes found in every one of your cells. Of the 23 pairs
of chromosomes, one pair is sex chromosomes, while the other
22 pairs are autosomal chromosomes. Sex chromosomes
are the pair of chromosomes carrying genes that determine
whether a person is biologically male or female.

The 22 pairs of chromosomes that are not sex chromosomes
are autosomal chromosomes. You inherit half of your
autosomal chromosomes from your mother and half from
your father. All the pairs of autosomal chromosomes are
homologous chromosomes. This means that the genes for a
trait are located at the same place on each chromosome in the
pair, even though the alleles may be different. Females also
have homologous sex chromosomes, while males do not.

​Chromosomes Size

Chromosomes contain DNA, and each section of DNA that codes for a protein is a gene. For every trait, there is a gene or group of genes that controls the trait by producing proteins through the process of protein synthesis. Because the number of genes found on each chromosome and the length of each gene varies, chromosomes come in different sizes. For example, the X chromosome is almost three times the size of the Y chromosome and contains close to 16 times as many genes. Thus, it codes for more proteins, and determines more traits