Unit 2 Lesson 4 Heredity Copyright Houghton Mifflin

Unit 2 Lesson 4 Heredity Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity Give Peas a Chance What is heredity? • Traits, such as hair color, result from the information stored in genetic material. • Heredity is the passing of genetic material from parents to offspring. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • Gregor Mendel was an Austrian monk. In the 1800 s, Mendel performed the first major experiments in heredity. • Mendel studied seven characteristics of pea plants. • A characteristic is a feature that has different forms in a population. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • The seven different characteristics Mendel studied were plant height, flower and position, seed shape, seed color, pod shape, pod color, and flower color. • Each characteristic had two different forms. These different forms are called traits. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • Mendel studied each characteristic separately, always starting with plants that were truebreeding. • True-breeding plants always produce offspring with the same trait if allowed to self-pollinate naturally. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • Mendel crossed plants that were true-breeding for producing yellow seed pods with plants that were true-breeding for green seed pods. • All of the plants from the first generation produced green seed pods. • Mendel called the green seed pod the dominant trait, and the yellow seed pod the recessive trait. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • Next, Mendel let the first generation plants selfpollinate. • Out of the generation that resulted, called the second generation, about three-fourths had green seed pods and one-fourth had yellow pods. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • The recessive trait had seemed to disappear in the first generation, but it reappeared in the second generation. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What did Gregor Mendel discover about heredity? • Mendel hypothesized that each plant must have two heritable “factors” for each trait, one from each parent. • Some traits, such as yellow color, could only be observed if a plant had two of the same factors. • A plant with two different factors would show the dominant factor but be able to pass on both factors to its offspring. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity It’s in Your Genes! How are traits inherited? • Mendel’s ideas can be further explained by our modern understanding of DNA. • What Mendel called “factors” are actually segments of DNA known as genes. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • Genes are segments of DNA. They give instructions for producing a certain characteristic. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • The offspring has two versions of the same gene for every characteristic—one from each parent. • Different versions of a gene are known as alleles. • Dominant alleles are shown with a capital letter, and recessive alleles are shown with a lowercase version of the same letter. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • An organism with one dominant and one recessive allele for a gene is heterozygous for that gene. • An organism with two of the same alleles for a gene is homozygous for that gene. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • The combination of alleles that you inherited from your parents is your genotype. • Your observable traits make up your phenotype. • The phenotypes of some traits follow patterns similar to the ones Mendel discovered. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • The dominant allele contributes to the phenotype if one or two copies are present in the genotype. • The recessive allele contributes to the phenotype only when two copies of it are present. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • If one chromosome in the pair contains a dominant allele and the other contains a recessive allele, the dominant allele determines the phenotype. • This is called complete dominance. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • Some characteristics are a result of several genes acting together. • Sometimes, one gene influences more than one trait. • For example, many genetic disorders, such as sickle cell anemia, are linked to a single gene but affect many traits. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity How are traits inherited? • Sometimes, the environment can influence an organism’s phenotype. • Some traits are acquired only from one’s environment and are not inherited. • For example, your ability to read and write is an acquired trait. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity Bending the Rules What are the exceptions to complete dominance? • Some traits do not follow the pattern of complete dominance. • For traits that show incomplete dominance and codominance, one trait is not completely dominant over another. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What are the exceptions to complete dominance? • In incomplete dominance, each allele in a heterozygous individual influences the phenotype. • The result of incomplete dominance is a phenotype that is a blend of the phenotypes of the parents. • An example of this in humans is hair. A person with one allele for straight hair and one allele for curly hair will have wavy hair. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What are the exceptions to complete dominance? • For a trait that shows codominance, both of the alleles in a heterozygous individual contribute to the phenotype. • Heterozygous individuals have both of the traits associated with their two alleles. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 4 Heredity What are the exceptions to complete dominance? • Human blood type is an example of codominance. • Three alleles, called A, B, and O, play a role in determining blood type. • A person with an A allele and a B allele has type AB blood. Copyright © Houghton Mifflin Harcourt Publishing Company