HW: Introducing Variation

Read the following article and answer the questions. If needed, use headphones and listen to the text using Text to Speech. 

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1. Look at the flowers shown on this page. Even though they all look different, they are in fact the same type of flower. These flowers are gerbera daisies, and as you can see, they come in many varieties. But 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, you can say that 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 their offspring 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 contains an exact copy of that organism’s DNA. All of the DNA within each cell is divided into chromosomes. For sexually reproducing organisms, cells contain two copies of each chromosome. One chromosome comes from the mother, and one chromosome comes from the father. Each chromosome contains 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. Using our flowers as an example, 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. All of this genetic information found in the DNA of organisms is in the form of a code. This allows the information carried in DNA to be copied and transferred from parents to offspring. During the process of meiosis in sexually reproducing organisms, specialized cells are formed. These cells are the gametes, or sex cells, referred to as sperm cells (father) and egg cells (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. When eukaryotes reproduce sexually, 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 and one set of chromosomes from the father’s sperm. This creates a unique set of chromosomes for each offspring. 1 Introducing Variation Genetic Variation Variation of Traits

4. Each individual within a species has its own unique code. Those unique codes determine the varying traits that can be seen from one individual organism to the next. The combination of alleles, one from the father and one from the mother, results in a unique genetic combination. It is this unique combination of alleles in each individual that creates species diversity. This variation in the DNA code is found in the genes on chromosomes. It is this variation which allows for all of those varieties of the same type of flower.

5. Meiosis increases genetic variation in organisms that undergo sexual reproduction. Sexual reproduction creates greater genetic variety in two ways. First, an offspring inherits DNA from both of its parents. This causes new random combinations of alleles, resulting in a variety of traits that differ from the mother and the father. Genes are randomly assorted when they are passed to offspring. Even two siblings from the same set of parents will have different combinations of genes and traits. Only identical twins have exactly same DNA.

6. Meiosis also contributes to genetic variation through crossing over. During one phase of meiosis, pairs of chromosomes that contain the same genes will line up at the center of the cell. When this happens, sections of the chromosomes can cross over and 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 that can be passed on to offspring.

7. Sexual reproduction is beneficial to 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 and to one another. This type of reproduction is mostly associated with prokaryotes and other single celled organisms. Simple animals such as the hydra and sponge may reproduce sexually or asexually at various stages of their lives. Many plants can reproduce asexually by sending out shoots. This is generally called vegetative reproduction.

8. Genetic variation is reduced with asexual reproduction. There are, however, benefits to the parent organism. Animals that are immobile (cannot move), such as sponges, would have great difficulty finding a mate. Asexual reproduction allows them to produce offspring without having to travel. Another advantage is that in asexual reproduction the parent expends much less energy compared to sexual reproduction. Organisms can produce many offspring without using a lot of their energy or time. Finally, in a stable environment, asexual reproduction produces offspring with the necessary genetic traits to survive and thrive in their environment.