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Genetics, Heredity, and Chromosomes HandoutHeredity is the passing of traits from parents to offspring. For example, you and your brother might have blue eyes but both of your parents have brown eyes. How does this happen? More than 150 years ago, Gregor Mendel, an Austrian monk, performed experiments that helped answer many questions about heredity. Mendel’s research into the questions of heredity gave scientists a basic understanding of genetics. Genetics is the study of how traits are passed from parents to offspring. Because of his research, Mendel is known as the “Father of Genetics.”During the 1850s, Mendel studied genetics by doing controlled breeding experiments with pea plants. To observe how a trait was inherited (passed down,) Mendel controlled which plants pollinated other plants. Pollination occurs when pollen lands on the pistil of a flower. Sperm cells from the pollen then fertilize egg cells in the pistil. Self-pollination occurs when pollen from one plant lands on the pistil of a flower on the same plant. Cross-pollination occurs when pollen from one plant reaches the pistil of a flower on a different plant. Mendel allowed one group of flowers to self-pollinate. With another group, he cross pollinated the plants himself. By cross-pollinating plants himself, Mendel was able to select which plants pollinated other plants. Mendel cross-pollinated hundreds of plants for each set of traits he wanted to learn more about. The traits included flower color, seed color, and seed shape. By testing large numbers of plants, Mendel was able to predict which crosses would produce which traits. After analyzing the results of his experiments, Mendel concluded that two factors control each inherited trait. He also proposed that when organisms reproduce, the sperm and the egg each contribute one factor for each trait. Mendel also hypothesized that some factors are dominant, blocking out other factors. A genetic factor that blocks another genetic factor is called a dominant allele. A genetic factor that is blocked by the presence of a dominant factor is called a recessive allele. A recessive trait is seen only when two recessive alleles are present in offspring. Inside each cell is a nucleus that has threadlike structures called chromosomes. Chromosomes contain genetic information that controls traits. Each cell in an offspring contains chromosomes from both parents. These chromosomes exist in pairs (homologous pairs)-one chromosome from each parent. Each chromosome can have information about hundreds or thousands of traits. A gene is a section on a chromosome that has genetic information for one trait. For example, a gene of a pea plant might have information about flower color. An offspring inherits two genes for each trait, one from each parent. The different forms of a gene are called alleles. Geneticists call how a trait appears, or is expressed, the trait’s phenotype. A person’s eye color is an example of a phenotype. The trait of eye color can be expressed as blue, green, or other colors. Mendel concluded that two alleles control the expression of phenotype of each trait. The two alleles that control the phenotype of a trait are called the traits’ genotype. You cannot see an organism’s genotype. But you can make guesses about a genotype based on its phenotype. Scientists use symbols to represent the alleles in a genotype. Uppercase letters represent dominant alleles, and lowercase letters represent recessive alleles. The dominant allele, if present, is written first. When the two alleles of a gene are the same, its genotype is homozygous. Both RR and rr are homozygous genotypes. The RR genotype has two dominant alleles. The rr genotype has two recessive alleles. If the two alleles of a gene are different, its genotype is heterozygous. Rr is a heterozygous genotype. Phenotype and GenotypePhenotype (observed physical traits)Genotype (alleles of a gene)Round seedsHomozygous Dominant (RR)Heterozygous (Rr)Wrinkled seedsHomozygous Recessive (rr)Reading Comprehension Questions What is genetics? ______________________________________________________________________________________________________________________________________________________Why did Mendel perform cross-pollination experiments?______________________________________________________________________________________________________________________________________________________________________________________________________________________What did Mendel conclude about inherited traits? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________How do dominant and recessive factors interact?__________________________________________________________________________________________________________________________________________________________________________________________________________________________In a pea plant, an allele for a tall stem is dominant to an allele for a short stem. You see a pea plant with a short stem. What can you conclude about the genotype of this plant? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________How do alleles determine the expression of traits?____________________________________________________________________________________________________________________________________________________________________________________________________________________________Identify the genotype of a plant that produces a wrinkled pea. ________________________Explain the difference in a genotype and a phenotype. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ................
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