Chapter 4 - Genetics: The Science of Heredity

Section 1 - Mendel's Work


  Heredity is the passing of physical characteristics from parents to offspring. Gregor Mendel was curious about the different forms of characteristics, or traits, of pea plants. Mendel's work was the foundation of genetics, the scientific study of heredity.

  A new organism begins to form when egg and sperm join in the process called fertilization. Before fertilization can happen in pea plants, pollen must reach the pistil of a pea flower through pollination. Pea plants are usually self-pollinating, meaning pollen from a flower lands on the pistil of the same flower. Mendel developed a method by which he cross-pollinated, or "crossed,” pea plants.

  Mendel crossed two pea plants that differed in height. He crossed purebred tall plants with purebred short plants. These parent plants, the P generation, were purebred because they always produced offspring with the same trait as the parent. In all of Mendel's crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the "lost" form of the trait always reappeared in about one fourth of the plants. From his results, Mendel reasoned that individual factors, one from each parent, control the inheritance of traits. Today, scientists call the factors that control traits genes. The different forms of a gene are called alleles.

  An organism's traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive. A dominant allele is one whose trait always shows up in the organism when the allele is present. A recessive allele is hidden whenever the dominant allele is present. A trait controlled by a recessive allele will only show up if the organism does not have the dominant allele.

  In Mendel's cross, the purebred tall plant has two alleles for tall stems. The purebred short plant has two alleles for short stems. The F1 plants are all hybrids: they have two different alleles for the trait-one allele for tall stems and one for short stems. Geneticists use a capital letter to represent a dominant allele and a lowercase version of the same letter for the recessive allele.

  Mendel's discovery was not recognized during his lifetime. In 1900, three different scientists rediscovered Mendel's work. Because of his work, Mendel is often called the Father of Genetics.




Section 2 - Probability and Heredity


  Probability is a number that describes how likely it is that an event will occur. The principles of probability predict what is likely to occur, not necessarily what will occur. For example, in a coin toss, the coin will land either heads up or tails up. Each of these two events is equally likely to happen. In other words, there is a 1 in 2 chance that a tossed coin will land heads up, and a 1 in 2 chance that it will land tails up. A 1 in 2 chance can be expressed as a fraction,1/2, or as a percent, 50 percent. The result of one coin toss does not affect the result of the next toss. Each event is independent of another.

  When Gregor Mendel analyzed the results of his crosses in peas, he carefully counted all the offspring. Over time, he realized that he could apply the principles of probability to his crosses. Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses.

  A tool that applies the laws of probability to genetics is a Punnett square. A Punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross. Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome. In a Punnett square, all the possible alleles from one parent are written across the top. All the possible alleles from the other parent are written down the left side. The combined alleles in the boxes of the Punnett square represent all the possible combinations in the offspring. In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

  Two useful terms that geneticists use to describe organisms are genotype and phenotype. An organism's phenotype is its physical appearance, or visible traits. An organism's genotype is its genetic makeup, or allele combinations. When an organism has two identical alleles for a trait, the organism is said to be homozygous for that trait. An organism that has two different alleles for a trait is said to be heterozygous for that trait.

  For all of the traits in peas that Mendel studied, one allele was dominant while the other was recessive. This is not always the case. In an inheritance pattern called codominance, the alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring. Codominant alleles are written as capital letters with superscripts to show that neither is recessive.




Section 3 - The Cell and Inheritance


  In the early 1900s, scientists were working to identify the cell structures that carried Mendel's hereditary factors, or genes. In 1903, Walter Sutton observed that sex cells in grasshoppers had half the number of chromosomes as the body cells. He also noticed that each grasshopper offspring had exactly the same number of chromosomes in its body cells as each of the parents. He reasoned that the chromosomes in body cells actually occurred in pairs, with one chromosome in each pair coming from the male and the other coming from the female.

  From his observations, Sutton concluded that genes are located on chromosomes. He proposed the chromosome theory of inheritance. According to the chromosome theory of inheritance, genes are carried from parents to their offspring on chromosomes.

  Organisms produce sex cells during meiosis. Meiosis is the process by which the number of chromosomes is reduced by half to form sex cells- sperm and eggs. During meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism. When they combine, each sex cell contributes half the number of chromosomes to produce offspring with the correct number of chromosomes.

  Punnett squares show the results of meiosis. When chromosome pairs separate, so do the alleles carried on the chromosomes. One allele from each pair goes to each sex cell.

  Chromosomes are made up of many genes joined together like beads on a string. Each chromosome contains a large number of genes, each gene controlling a particular trait. Each chromosome pair has the same genes. The genes are lined up in the same order on both chromosomes. However, the alleles for some of the genes might differ from each other, making the organism heterozygous for some traits. If the alleles are the same, the organism is homozygous for those traits.




Section 4 - The DNA Connection


  Today, scientists know that genes control the production of proteins in the cells of an organism. Proteins determine the size, shape, and other traits of organisms. Recall that chromosomes are composed mostly of DNA. A DNA molecule is made up of four nitrogen bases-adenine (A), thymine (T), guanine (G), and cytosine (C). The order of the nitrogen bases along a gene forms a genetic code that specifies what type of protein will be produced. In the genetic code, a group of three DNA bases codes for one specific amino acid.

  During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein. Protein synthesis occurs on the ribosomes in the cytoplasm of the cell. DNA, however, is located in the cell nucleus. Before protein synthesis occurs, a genetic "messenger,” called ribonucleic acid or RNA, is made based on a code in the DNA. RNA is similar to DNA, except RNA has only one strand and it has uracil instead of thymine.

  In the first step of protein synthesis, the DNA molecule "unzips” and directs the production of messenger RNA. There are several types of RNA involved in protein synthesis. Messenger RNA copies the coded message from the DNA in the nucleus, and carries it to the ribosomes in the cytoplasm. Transfer RNA carries amino acids and adds them to the growing protein.

  Sometimes changes called mutations occur in a gene or chromosome. Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism's trait, or phenotype, may be different from what it normally would have been. If a mutation occurs in a body cell, the mutation affects only the cell that carries it. However, if a mutation occurs in a sex cell, the mutation can be passed on to an offspring and affect the offspring's phenotype. Some mutations are the result of small changes in an organism's hereditary material. Others occur when chromosomes don't separate correctly during meiosis.

  Some of the changes brought about by mutations are harmful to an organism. A few mutations, however, are helpful, and still others are neither harmful nor helpful. A mutation is harmful if it reduces an organism's chance for survival and reproduction. Whether or not a mutation is harmful depends partly on the organism's environment. For example, a white lemur may not survive in the wild, but the mutation has no effect on its ability to survive in a zoo.



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