Download Introducing Variation

Genetic Variation
Variation of Traits
Introducing Variation
1 Look at the flowers shown on this page. They all
look different. But in fact, they are the same type of
flower. These flowers are gerbera daisies. As you
can see, they come in many varieties. How can this
be? If they are all the same type of flower, why
don’t they all look the same? The answer has to do
with DNA, genes, and the parent plants. Yes,
plants have parents. In fact, for any sexually
reproducing organism, there must be two
individuals involved. Each donates one half of their
genetic information to their offspring. One parent
gives this genetic information from a sperm cell.
The other parent gives this information from an egg
cell. Let us explore this a little further.
2 Deoxyribonucleic acid (DNA) is found in every cell of every organism. Each cell of a
multicellular organism has an exact copy of that organism’s DNA. All of the DNA within each
cell is divided into chromosomes. For sexually reproducing organisms, cells have two copies of
each chromosome. One chromosome comes from the mother. One chromosome comes from
the father. Each chromosome has segments called genes. Each chromosome carries genes for
specific traits, but in slightly different versions. These different versions of the same gene are
called alleles. Think about our flowers again. The gene for the flower color trait is found in a
specific gene on a specific chromosome. The location of this gene for flower color is the same
for every gerbera daisy. However, there may be different alleles, or versions, of this gene. It is
the different alleles that create the different colors. But how do different versions of the same
gene happen?
3 The genetic information in the DNA of organisms is in the form of a code. This allows the
information to be copied. It also must be transferred from parents to offspring. Meiosis is a
process that makes new cells in sexually reproducing organisms. Meiosis makes cells that are
different than other body cells. These cells are gametes, or sex cells. Sperm cells come from
the father. Egg cells come from the mother. Each gamete contains one half of the
chromosomes from the parent cells. The two copies of each chromosome are randomly split up.
Each gamete only has one copy of each chromosome. In sexual reproduction, the gametes
(egg and sperm) from each parent join together. As a result, the offspring inherits one set of
chromosomes from the mother’s egg. The offspring inherits the other set of chromosomes from
the father’s sperm. This creates a unique set of chromosomes for each offspring.
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Genetic Variation
Variation of Traits
4 Each individual within a species has its own unique code. Those unique codes cause the
differences in traits between individual organisms. The combination of alleles from the father
and the mother results in a unique genetic combination. This unique combination of alleles in
each individual creates species diversity. This variation in the DNA code is found in the genes
on chromosomes. This variation allows all of those varieties of the same type of flower.
5 Meiosis increases genetic variation in sexually reproducing organisms. Sexual reproduction
creates greater genetic variety in two ways. First, an offspring inherits DNA from both of its
parents. Genes are randomly assorted when they are passed to offspring. This causes new
random combinations of alleles. The offspring will have a variety of traits that are different from
the mother and the father. Even two siblings from the same parents will have different
combinations of genes and traits. Only identical twins have exactly the same DNA.
6 Meiosis also increases genetic variation through crossing over. During meiosis, pairs of
chromosomes line up at the center of the cell. Sections of chromosomes with the same genes
can get swapped. The sections can switch position from one chromosome to the other. This
process is known as crossing over. This results in a shuffling of genes on the individual
chromosomes. This provides an even greater variety of genetic combinations to pass on to
offspring.
7 Sexual reproduction helps organisms because it increases genetic variation. Asexual
reproduction has advantages as well. In asexual reproduction, a single parent produces
offspring that are genetically identical to the parent. The offspring are also genetically identical
to one another. This type of reproduction often happens with prokaryotes and other single
celled organisms. Simple animals like the sponge may reproduce sexually or asexually. Many
plants can reproduce asexually by sending out shoots. This is called vegetative reproduction.
8 Asexual reproduction reduces genetic variation of a population. There are, however, benefits to
the parent organism. Animals that cannot move, like a sponge, have difficulty finding a mate.
Asexual reproduction allows them to produce offspring without having to travel. Another
advantage is that asexual reproduction takes much less energy than sexual reproduction.
Organisms can have many offspring without using a lot of their energy or time. Finally, if the
environment is stable, the parent likely has the right genetic traits for their environment. Asexual
reproduction produces offspring with those successful traits.
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Genetic Variation
Variation of Traits
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2
An organism’s characteristics, or traits, are due to genes. Different versions of genes
create different traits. What are these different versions of genes called?
A
DNA
B
Chromosomes
C
Alleles
D
Gametes
Which of the following is not created during the process of meiosis in eukaryotic cells?
A
An exact copy of the parent cell.
B
Sperm and egg cells.
C
Cells with half of the parent’s chromosomes.
D
Cell that contain the parent’s DNA.
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Genetic Variation
Variation of Traits
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4
What determines each organism’s unique traits? In other words, what creates the
variation that we see in organisms?
A
The combination of alleles within each organism.
B
The alleles donated from the mother and the father.
C
The variation in the DNA code.
D
All of the above.
There are several factors that lead to genetic variation in organisms. Which of the
following is NOT one of those factors?
A
Producing offspring that are genetically identical to the parents.
B
The random combination of alleles from both parents.
C
The process of crossing over during meiosis.
D
The formation of sperm and egg cells during meiosis.
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Genetic Variation
Variation of Traits
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Some organisms reproduce sexually. Other organisms reproduce asexually. What
are some benefits of asexual reproduction?
A
It benefits organisms that are immobile.
B
It gives offspring the genetic traits to survive in a particular environment.
C
It uses less energy.
D
All of the above.
Why is sexual reproduction so important to many organisms? Pick the best answer from
the choices below.
A
It creates identical copies of organisms.
B
It creates offspring from parents.
C
It increases genetic variation.
D
It creates egg and sperm cells.
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