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Life Science: GED Test Prep (page 2)

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Updated on Jul 5, 2011

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.

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