Chapter 6 Heredity The Big Idea Heredity is the passing of the instructions for traits from one generation to the next.

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1 Chapter 6 Heredity The Big Idea Heredity is the passing of the instructions for traits from one generation to the next. Section 1 Mendel and His Peas Key Concept The work of Gregor Mendel explains the rules of heredity and is the foundation of modern genetics. Before Mendel The passing of traits from parents to offspring is called heredity. People once believed that traits from both parents are simply mixed together in the offspring. This is called blending inheritance. While offspring often look like a combination of both parents, the traits are not actually blended together. But the rules of heredity were not discovered until a scientist named Gregor Mendel did some experiments with pea plants. Gregor Mendel s Work The work of Gregor Mendel explains the rules of heredity and is the foundation of modern genetics. Mendel s experiments showed that blending inheritance is incorrect for many traits. His work changed the way people think about inheritance. As a monk in Austria, Mendel chose to put most of his time into research. Self-Pollinating Peas Mendel chose to study the garden pea. They worked well for his experiments because they grow quickly and there are many variations. Pea plants were ideal also because they can self-pollinate. This means sperm from a flower can fertilize the eggs of a flower on the same plant. Self-pollinated plants can be true-breeding, which means the offspring always have the same characteristics as the parent. Cross-Pollinating Peas In cross-pollination, sperm from one plant fertilize the eggs on a different plant. Many plants can be fertilized only through cross- 1

2 pollination. In cross-pollination, sperm are carried in pollen by insects, birds, bats, or other organisms. In some plants, the pollen is carried by the wind. Pea plants can also be fertilized through cross-pollination. Mendel s Work with Pea Plants Mendel used only true-breeding plants to see what happened if plants with different traits for the same characteristic were crossed. To ensure cross-fertilization, he removed the pollen-producing anthers from one plant. He then used pollen from a different plant to fertilize the anther-less plant. This way he could control which parents crossed. Mendel studied one characteristic at a time. A characteristic is a feature that has different forms in a population, such as hair color. The different forms of a characteristic are called traits, such as brown hair, red hair, or black hair. For his experiments, Mendel chose the characteristics of pea plants, including flower color, seed shape, pod color, and plant height. Mendel s First Experiments In his crosses, Mendel found the offspring always had the characteristics of one parent, but not the other. The offspring of such a cross are called first-generation plants. In the cross between purple flowered plants and white-flowered plants, the offspring all had purple flowers. Mendel chose to call the trait that appeared in the first generation the dominant trait. He chose to call the trait that disappeared in the first generation the recessive trait. Mendel s Second Experiments In his second set of experiments, Mendel allowed the first generation plants to self-pollinate. In each case, most of the offspring had the dominant trait, but some of 2

3 them had the recessive trait. The offspring of the first generation plants are called the second generation. Mendel counted the number of plants that had the dominant trait and the number of plants that had the recessive trait for each characteristic. Mendel then computed the ratio of dominant to recessive plants. A ratio is an expression of a relationship between two numbers. Mendel found a ratio of about three dominant traits for every one recessive trait. (The ratio can be written as 3:1 or 3/1). Mendel s Conclusions Mendel realized that his results could only be explained if each plant had two sets of instructions for each characteristic. He concluded that each parent gives one set of instructions to the offspring, and that the dominant instructions determine the trait. Mendel published his findings in 1865, but 30 years passed before the significance of his work was realized. Thinking About Different Perspectives Imagine that you are Gregor Mendel. What are your thoughts about inheritance before, during, and after you do your experiments with pea pods? Section 2 Traits and Inheritance Key Concept Sudden motions along breaks in Earth s crust can release energy in the form of seismic waves. A Great Idea Mendel knew that each parent passed on instructions for each trait. Scientists now call these instructions for an inherited trait genes. Offspring end up with two versions of each gene for every characteristic one from each parent. Different versions of the same gene are called alleles. For the offspring to show a recessive trait, it must inherit the recessive allele from both parents. Offspring show the dominant trait if they inherit 3

4 the dominant allele form even just one parent. Phenotype Genotype The appearance of an organism is called its phenotype. The particular alleles of an organism are called its genotype. Dominant alleles are shown with capital letters. Recessive alleles are shown with lowercase letters. If purple flowers are dominant, and the letter P stands for the alleles for flower color, than the genotype of a purple flowered plant could be PP or Pp. The genotype of a plant with a white phenotype (white flowers) has to be pp. Organisms with two of the same alleles (PP or pp) are homozygous for that trait. Organisms with one of each allele (Pp) are heterozygous for that trait. The genotype of a true breeding purple pea is PP. A true breeding white plant is pp. All of their offspring would be Pp, and would be purple. Punnett Squares A Punnett square is used to predict the possible genotypes of offspring in a particular cross. Once you know the possible genotypes, you can also determine the phenotypes (appearance) of the offspring. A Punnett square can help explain the results of Mendel s second set of experiments. A cross of Pp with Pp gives the following results: PP, Pp, pp, and pp. All except pp will have purple flowers, because all except pp have the dominant allele. What Are the Chances? The mathematical chance that something will happen is known as probability. Probability is usually written as a percentage or a fraction. When you toss a coin, there is a 50% chance of getting heads. 4

5 In a parent with two different alleles, such as Pp, there is a 50% chance of offspring getting P and a 50% chance of getting p. Calculating Probabilities To find the probability of several events happening, multiply the probability of the first event by the probability of the second event. The probability of tossing heads twice in a row is 1/2 X 1/2, or 1/4. The probability of getting white flowers (pp) in the cross Pp X Pp is 1/2 X 1/2, or 1/4 (25%). Practicing Calculations What is the probability of getting white flowers (pp) in the cross Pp X pp? More About Traits In Mendel s experiments, each trait was controlled by one gene. But sometimes the inheritance of traits is more complicated. A gene may influence more than one trait, or a trait may be influenced by more than one gene. For example, your eye color is the result of several genes acting together. The environment, both internal and external, can also affect the phenotype, or appearance, of an organism. For example, height is influenced by nutrition. In some organisms, coloration can be determined by diet. In humans, skin color is determined by several genes. It is also affected by exposure to the sun. Genetic Variation Scientists estimate than humans have about 30,000 genes. While humans may have the same genes, each individual has a unique set of alleles. The differences in the sets of alleles between individuals in a population is called genetic variation. 5

6 Genes affect more than your appearance. Genes affect the internal functioning of your body as well. Every hormone, enzyme, protein, and other chemical that your body makes is influenced by genes. Section 3 Meiosis Key Concept Meiosis and sexual reproduction allow for the combination of genetic material from two different cells. Chromosome Numbers Meiosis and sexual reproduction increase the number of possible genetic combinations in a population. There is a carefully controlled process that ensures sexual reproduction leads to the proper number of chromosomes in the offspring. Each species has a normal number of chromosomes in their body cells. Human body cells have 46 chromosomes, corn cells have 20, and dogs cells have 78. Each chromosome in a body cell has a matching pair with the same genes but different alleles. Chromosomes that carry the same sets of genes are called homologous chromosomes. Chromosomes in Reproduction Cells that have homologous pairs of chromosomes are called diploid. Body cells are diploid. Before sexual reproduction can occur, an organism must make sex cells. The male sex cells are sperm, and the female sex cells are eggs. Sex cells do not have homologous pairs and are called haploid. Human sex cells have 23 chromosomes. Sex cells are haploid so that when they combine during fertilization, they produce an individual with the normal number of chromosomes. Human body cells have 46 chromosomes. Drawing Draw a diagram of a cell with four chromosomes. Use different colors, shading, or symbols to show that the cell contains two pairs of homologous chromosomes. 6

7 Meiosis Sex cells are made during a process called meiosis. Meiosis is a copying process that produces cells that have half the usual number of chromosomes. When a cell undergoes meiosis, it divides twice. This produces four cells, each with half the number of chromosomes as the parent cell. Meiosis and Mendel Meiosis can help explain the results first observed by Mendel. If the true breeding plant for wrinkled seeds provided the sperm, all of the sperm cells would have the same recessive allele, r. If the true breeding plant for smooth seeds provided the eggs, all of the eggs would have the dominant allele R. The only genotype possible in the offspring in this cross is Rr. The only phenotype possible is smooth seeds, since all of the offspring would have the dominant allele. Meiosis and Inheritance So in sexual reproduction, half of the offspring s genetic material comes from the mother s egg, and half comes from the father s sperm. Mitochondria, which have their own DNA, come from the mother s egg. So all of the mitochondria in your body can be traced back to your mother. Wrap- Up Think about the methods you have used to study the concepts in this chapter. Which types of s are the most helpful to you? What types of concepts do they help you study? Think about review methods you can use when you are studying. 7