GED Prep: Heredity
Molecular Basis of Heredity
What an organism looks like and how it functions is largely determined by its genetic material.
The basic principles of heredity were developed by Gregor Mendel,who experimented with pea
plants in the nineteenth century. He mathematically analyzed the inherited traits (such as color
and size) of a large number of plants over many generations. The units of heredity are genes
carried on chromosomes. Genetics can explain why children look like their parents, and why
they are, at the same time, not identical to the parents.
Phenotype and Genotype
The collection of physical and behavioral characteristics of an organism is called a phenotype.
For example, your eye color, foot size, and ear shape are components of your phenotype. The
genetic makeup of a cell or organism is called the genotype. The genotype is like a cookbook for
protein synthesis and use. Phenotype (what an organism looks like or how it acts) is determined
by the genotype (its genes) and its environment. By environment we don't mean the Earth, but
the environment surrounding the cell. For example, hormones in the mother's body can influence
the gene expression.
Reproduction
Asexual reproduction on the cellular level is called mitosis. It requires only one parent cell,
which, after exactly multiplying its genetic material, splits in two. The resulting cells are
genetically identical to each other and are clones of the original cell before it split.
Sexual reproduction requires two parents. Most cells in an organism that reproduces sexually
have two copies of each chromosome, called homologous pairs—one from each parent. These
cells reproduce through mitosis. Gamete cells (sperm and egg cells) are exceptions. They carry
only one copy of each chromosome, so that there are only half as many chromosomes as in the
other cells. For example, human cells normally contain 46 chromosomes, but human sperm and
egg cells have 23 chromosomes.At fertilization, male and female gametes (sperm and egg) come
together to form a zygote, and the number of chromosomes is restored by this union. The genetic
information of a zygote is a mixture of genetic information from both parents. Gamete cells are
manufactured through a process called meiosis whereby a cell multiplies its genetic material
once, but divides twice, producing four new cells, each of which contains half the number of
chromosomes that were present in the original cell before division. In humans, gametes are
produced in testes and ovaries. Meiosis causes genetic diversity within a species by generating
combinations of genes that are different from those present in the parents.
Alleles
Alleles are alternative versions of the same gene. An organism with two copies of the same allele
is homozygous, and one with two different alleles is heterozygous. For example, a human with
one gene for blue eyes and one gene for brown eyes is heterozygous, while a human with two
genes for blue eyes or two genes for brown eyes is homozygous. Which of the two genes is
expressed is determined by the dominance of the gene.
An allele is dominant if it alone determines the phenotype of a heterozygote. In other words, if a
plant has a gene for making yellow flowers and a gene for making red flowers, the color of the
flower will be determined by the dominant gene. So if the gene for red flowers is dominant, a
plant that has both the gene for red and the gene for yellow will look red. The gene for yellow
flowers in this case is called recessive, as it doesn't contribute to the phenotype (appearance) of a
heterozygote (a plant containing two different alleles). The only way this plant would make
yellow flowers is if it had two recessive genes—two genes both coding for yellow flowers.
For some genes, dominance is only partial and two different alleles can be expressed. In the case
of partial dominance, a plant that has a gene that codes for red flowers and a gene that codes for
white flowers would produce pink flowers.
A Punnett square can be used to represent the possible phenotypes that offspring of parents with
known genotypes could have. Take the example with the yellow and red flower. Let's label the
gene for the dominant red gene as R and the gene for yellow flowers as r. Cross a plant with
yellow flowers (genotype must be rr) with a plant with red flowers and genotype Rr. What
possible genotypes and phenotypes can the offspring have? In a Punnett square, the genes of one
parent are listed on one side of the square and the genes of the other parent on the other side of
the square. They are then combined in the offspring as illustrated here:
The possible genotypes of the offspring are listed inside the square. Their genotype will be either
Rr or rr, causing them to be either red or yellow, respectively.
Sex Determination
In many organisms, one of the sexes can have a pair of unmatched chromosomes. In humans, the
male has an X chromosome and a much smaller Y chromosome, while the female has two X
chromosomes. The combination XX (female) or XY (male) determines the sex of humans. In
birds, the males have a matched pair of sex chromosomes (WW), while females have an
unmatched pair (WZ). In humans, the sex chromosome supplied by the male determines the sex
of the offspring. In birds, the sex chromosome supplied by the female determines the sex.
Plants, as well as many animals, lack sex chromosomes. The sex in these organisms is
determined by other factors, such as plant hormones or temperature.
Identical twins result when a fertilized egg splits in two. Identical twins have identical
chromosomes and can be either two girls or two boys. Two children of different sex born at the
same time can't possibly be identical twins. Such twins are fraternal. Fraternal twins can also be
of the same sex. They are genetically not any more alike than siblings born at different times.
Fraternal twins result when two different eggs are fertilized by two different sperm cells.
When meiosis goes wrong, the usual number of chromosomes can be altered. An example of this
is Down syndrome, a genetic disease caused by an extra chromosome.
Changes in DNA (mutations) occur randomly and spontaneously at low rates. Mutations occur
more frequently when DNA is exposed to mutagens, including ultraviolet light, X-rays, and
certain chemicals. Most mutations are either harmful to or don't affect the organism. In rare
cases, however, a mutation can be beneficial to an organism and can help it survive or reproduce.
Ultimately, genetic diversity depends on mutations, as mutations are the only source of
completely new genetic material. Only mutations in germ cells can create the variation that
changes an organism's offspring.
Biological Evolution
Mutations cause change over time. The result of a series of such changes is evolution, or as
Darwin put it, "descent with modification." The great diversity on our planet is the result of more
than 3.5 billion years of evolution. The theory of evolution argues that all species on Earth
originated from common ancestors.
Evidence for Evolution
Several factors have led scientists to accept the theory of evolution. The main factors are
described here.
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Fossil record. One of the most convincing forms of evidence is the fossil record. Fossils
are the remains of past life. Fossils are often located in sedimentary rocks, which form
during compression of settling mud, debris, and sand. The order of layers of sedimentary
rock is consistent with the proposed sequence in which life on Earth evolved. The
simplest organisms are located at the bottom layer, while top layers contain increasingly
complex and modern organisms, a pattern that suggests evolution. The process of carbon
dating has been used to confirm how old the fossils are and that fossils found in the lower
layers of sedimentary rock are indeed older than the ones found in the higher layers. This
helps scientists to chart evolutionary history based on time. And new fossils are turning
up all the time; for example, the fossil called Tiktaalik, which was found in 2006, is
believed to mark the transition from fish to land animals.
Biogeography. Another form of evidence comes from the fact that species tend to
resemble neighboring species in different habitats more than they resemble species that
are in similar habitats but far away.
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Comparative anatomy. Comparative anatomy provides us with another line of evidence.
It refers to the fact that the limb bones of different species, for example, are similar.
Species that closely resemble one another are considered to be more closely related than
species that do not resemble one another. For example, a horse and a donkey are
considered to be more closely related than are a horse and a frog. Biological
classifications (kingdom, phylum, class, order, family, genus, and species) are based on
how organisms are related. Organisms are classified into a hierarchy of groups and
subgroups based on similarities that reflect their evolutionary relationships. The same
underlying anatomical structures of groups of bones, nerves, muscles, and organs are
found in all animals, even when the function of these underlying structures differ.
Embryology. Embryology provides another form of evidence for evolution. Embryos go
through the developmental stages of their ancestors to some degree. The early embryos of
fish, amphibians, reptiles, birds, and mammals all have common features, such as tails.
Comparative molecular biology. Comparative molecular biology confirms the lines of
descent suggested by comparative anatomy and fossil record. The relatedness of two
different species can be found by comparing their DNA.
Darwin also proposed that evolution occurs gradually, through mutations and natural selection.
He argued that some genes or combinations of genes give an individual a survival or
reproductive advantage, increasing the chance that these useful combinations of genes will make
it to future generations. Whether a given trait is advantageous depends on the environment of the
organism. We can witness the changes in populations of living organisms: antibiotic-resistant
bacteria, super lice that are resistant to chemical treatments, and the increased frequency of darkcolored moths versus the lightly colored variety (Biston betularia) after the industrial revolution
in Britain. These are all examples of evidence of natural selection.
Natural selection is only one of several mechanisms by which gene frequency in a population
changes. Other factors include mating patterns and breeding between populations.