31212 Objective Make predictions about genotypes and phenotypes

3/12/12 • Objective: Make predictions about genotypes and phenotypes in offspring • Warm-Up: What is the relationship between genes and traits?

Inheritance/Heredity • Heredity, or inheritance, is the study of how traits are transmitted from parents to offspring • For many traits, patterns of genetic inheritance provide only part of the story—the environment and separate regulatory genes affect how traits are expressed • Nonetheless, there are basic principles of heredity that can make predictions about how traits move through generations • Remember that expression is the process that links genes with traits

Vocabulary • Allele: a specific version of a gene ▫ Ex: There are two alleles for hemoglobin—one for healthy hemoglobin, and one for mutated (sickle cell) hemoglobin ▫ At the intro level, alleles are generally represented by a single letter (H or h) ▫ There may be several alleles for a single gene • Genotype: the alleles present in an individual; an organism’s genetic make-up • Phenotype: the observed trait; the expression of the genotype

Vocabulary • Homozygous: Adjective used to describe an individual who has two copies of the same allele for a particular gene ▫ Ex: AA, aa ▫ Homozygotes are also called purebreds (though not when discussing humans!) • Heterozygous: Adjective used to describe an individual who has two different alleles for a particular gene ▫ Ex: Aa ▫ Heterozygotes are also called carriers

Vocabulary • Recessive: a trait is recessive if there must be two copies of its allele present in order for it to be expressed • Dominant: a trait is dominant if it is expressed when only one copy of its allele is present • By convention, capital letters are used for dominant alleles and lowercase letters are used for recessive alleles • Dominant does not always mean “better” or “healthy”

Vocabulary • P generation: the parental generation in a given scenario/experiment • F 1 generation: the first generation of offspring in a given scenario/experiment • F 2 generation: the second generation of offspring (i. e. , “grandchildren”) in a given scenario/experiment • Self-fertilization: Some plants and hermaphroditic organisms can sometimes fertilize themselves—this means that you are crossing an individual with itself

• Genotypic ratio: The ratio of genotypes produced by a cross; when calculated with a Punnet square, this ratio is theoretical (“expected”) and may differ from the experimental (“actual” or “observed”) genotypic ratio • Phenotypic ratio: The ratio of phenotypes produced by a cross; when calculated with a Punnet square, this ratio is theoretical (“expected”) and may differ from the experimental (“actual” or “observed”) phenotypic ratio

Practice • Cystic fibrosis is a recessive disorder. If two people who are heterozygous for cystic fibrosis have a child, what is the probability that the child will be a carrier for cystic fibrosis? • What is the probability that the child will not have cystic fibrosis? • Find the genotypic and phenotypic ratios of this cross.

Old Heredity Hypotheses • The knowledge that characteristics are passed from parents to offspring is very old ▫ This knowledge is what made the domestication of animals and plants possible thousands of years ago • Scientific understanding of how inheritance works didn’t develop until within the last 150 years • There were two especially common ideas about heredity that scientists held prior to the work of Gregor Mendel

Old Ideas • Acquired Characteristics (Lamarck) ▫ Organisms modify their traits during life ▫ They pass on these modified traits to their offspring �Ex: A giraffe stretches its neck reaching for leaves. Its offspring have long necks. • What do you think about this hypothesis?

Old Ideas • Blending Inheritance ▫ Mother’s traits and father’s traits “blend” in the offspring ▫ Ex: a black sheep and white sheep would have gray offspring • Mendel’s experiments specifically tested this hypothesis

Practice • In roses, red flowers are dominant and white flowers are recessive. A purebred rosebush is crossed with a purebred white rosebush. • What would the Blending Hypothesis predict that the offspring would look like? • What do you think the offspring would actually look like?