Mendelian Genetics

What Came Before? Blending Inheritance Inheritance of Acquired Characteristics

Mendel’s Model Organism Advantages of pea plants for genetic study: o There are many varieties with distinct heritable features, or traits (such as peacolor) o Mating of plants can be controlled o Quick generation time; lots of offspring o Can isolate true-breeding lines for particular traits

Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE

Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

Table 14-1

CONCEPT 1: There are alternative versions of “heritable factors”

Fig Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

CONCEPT 2: For each trait, an organism inherits two alleles, one from each parent

Fig. 14-4

CONCEPT 3: If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

Misconceptions About Dominant vs. Recessive Dominant alleles are not necessarily more common in populations than recessive alleles Dominant alleles are not necessarily “better” / adaptive Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

CONCEPT 4: Law of Segregation

Fig P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p F 1 Generation Gametes: Genetic makeup: Appearance: Purple flowers Pp P p 1/21/2 1/21/2 F 2 Generation Sperm Eggs P P PPPp p p pp 31

How can you tell the genotype of an individual with the dominant phenotype?

CONCEPT 5: Law of Independent Assortment

Fig EXPERIMENT RESULTS P Generation F 1 Generation Predictions Gametes Hypothesis of dependent assortment YYRRyyrr YR yr YyRr  Hypothesis of independent assortment or Predicted offspring of F 2 generation Sperm YR yr Yr YR yR Yr yR yr YR YYRR YyRr YYRr YyRR YYrr Yyrr yyRR yyRr yyrr Phenotypic ratio 3:1 Eggs Phenotypic ratio 9:3:3:1 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 yr 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 3/43/4 9 / 16 3 / 16 1 / 16 Phenotypic ratio approximately 9:3:3:

Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

Mendel’s Laws Translated Into Math Multiplication rule = the probability that two or more independent events will occur together is the product of their individual probabilities (AND) Addition rule = the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities (OR)

Fig Rr  Segregation of alleles into eggs Sperm R R R R R R r r r r r r 1/21/2 1/21/2 1/21/2 1/21/2 Segregation of alleles into sperm Eggs 1/41/4 1/41/4 1/41/4 1/41/4

Example: BbEe x BbEe What is the probability the offspring will be homozygous recessive for both traits? What is the probability the offspring will be homozygous recessive for at least one trait?

Example: BbEe x BbEe

PRACTICE PROBLEMS!