Learning objectives: Students should be able to …

1. Learning objectives: Students should be able to … • Describe how Mendel’s principles of segregation and independent assortment are a consequence of chromosome movement in meiosis. • Calculate expected frequencies of genotypes and phenotypes in monohybrid, dihybrid, and X-linked crosses. • Analyze the results of crosses and pedigrees to determine whether phenotypes are autosomal or X-linked, dominant or recessive, linked or on different chromosomes. • Explain what a dominant allele is and how this applies to incomplete dominance, codominance, gene interactions, and polygenic inheritance.

2. Chapter 13 outline: Mendel and the gene • Outline (cont) • sex linkage • crossing over • incomplete dominance • codominance • multiple allelism • pleiotropic genes • quantitative traits • Applying Mendel’s rules to humans • (controlled experiments not possible) • Mendel • how inheritance works • model organism • exp with a single trait • exp with two traits • Sutton & Boveri (1902) • meiosis to explain Mendel’s findings • chromosome theory of inheritance • Thomas Hunt Morgan • testing chromosome theory of inheritance • Nettie Stevens • sex chromosome discovery

3. Key Concepts • In many species, individuals have two alleles of each gene. The principle of segregation states that prior to the formation of eggs and sperm, the alleles of each gene separate so that each egg or sperm cell receives only one of them. • The principle of independent assortment states that alleles of different genes are transmitted to egg cells and sperm cells independently of each other.

4. Key Concepts • Genes are located on chromosomes. The principle of segregation is explained by the separation of homologous chromosomes in anaphase of meiosis I. The principle of independent assortment applies to genes found on different chromosomes and is explained by chromosomes lining up randomly in metaphase of meiosis I. • There are important exceptions and extensions to the basic patterns of inheritance that Mendel discovered.

5. Introduction • In 1865, Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas. • Early in the 20th century, Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance, which proposes that meiosis causes the patterns of inheritance that Mendel observed. • Genetics is the branch of biology that focuses on inheritance.

6. Mendel’s Experimental System • Gregor Mendel was a 19th-century monk and active member of his city’s Agricultural Society. • Mendel was interested in heredity. Heredity is the transmission of traits from parents to their offspring. A trait is any characteristic of an individual.

7. What Question Was Mendel Trying to Answer? • Mendel was addressing the basic question of why offspring resemble their parents and how transmission of traits occurs. • In his time, two hypotheses had been formulated to try to answer this question: • Blending inheritance – parental traits blend such that their offspring have intermediate traits. • Inheritance of acquired characteristics – parental traits are modified and then passed on to their offspring.

8. Garden Peas: The First Model Organism in Genetics • Genetics, the branch of biology that focuses on the inheritance of traits, uses model organisms because the conclusions drawn from them can be applied to other species. • Mendel chose the common garden pea (Pisum sativum) as his model organism because: • It is easy to grow. • Its reproductive cycle is short. • It produces large numbers of seeds. • Its matings are easy to control. • Its traits are easily recognizable.

9. How Did Mendel Arrange Matings? • Peas normally pollinate themselves, a process called self-fertilization. • Mendel could prevent this by removing the male reproductive organs containing pollen from each flower. He then used this pollen to fertilize the female reproductive organs of flowers on different plants, thus performing cross-pollination.

10. What Traits Did Mendel Study? • Mendel worked with pea varieties that differed in seven easily recognizable traits: seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length. • An individual’s observable features comprise its phenotype. Mendel’s pea population had two distinct phenotypes for each of the seven traits. • Mendel worked with pure lines that produced identical offspring when self-pollinated. He used these plants to create hybrids by mating two different pure lines that differed in one or more traits.

11. Inheritance of a Single Trait • Mendel's first experiments involved crossing pure lines that differed in just one trait. • The adults in the cross were the parental generation, the offspring are the F1 generation (for "first filial").

12. The Monohybrid Cross • Mendel’s first experimented with crossing plants that differed in only one trait. • When Mendel crossed plants with round seeds and plants with wrinkled seeds, all of the F1 offspring had round seeds. • This contradicted the hypothesis of blending inheritance. • The genetic determinant for wrinkled seeds seemed to have disappeared. • Mendel allowed the F1 progeny to self-pollinate. • The wrinkled seed trait reappeared in the next F2 generation.

16. Dominant and Recessive Traits • Mendel called the genetic determinant for wrinkled seeds recessive and the determinant for round seeds dominant. • In modern genetics, the terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants. • Mendel repeated these experiments with each of the other traits. In each case, the dominant trait was present in a 3:1 ratio over the recessive trait in the F2 generation.

18. A Reciprocal Cross • Mendel wanted to determine if gender influenced inheritance. • He performed a reciprocal cross, in which the mother's phenotype in the first cross is the father's phenotype in the second cross, and the father's phenotype in the first cross is the mother's phenotype in the second cross. • The results of the two crosses were identical. This established that it does not matter whether the genetic determinants for seed shape are located in the male or female parent.

19. Is inheritance of seed shape related to sex?

23. Particulate Inheritance • To explain these results, Mendel proposed a hypothesis called particulate inheritance, which suggests that hereditary determinants maintain their integrity from generation to generation. • This directly contradicts both the blending inheritance and inheritance of acquired characteristics hypotheses.

24. Genes, Alleles, and Genotypes • Hereditary determinants for a trait are now called genes. • Mendel also proposed that each individual has two versions of each gene. Today these different versions of a gene are called alleles. Different alleles are responsible for the variation in the traits that Mendel studied. • The alleles found in an individual are called its genotype. An individual’s genotype has a profound effect on its phenotype.

25. The Principle of Segregation • Mendel developed the principle of segregation: the two members of each gene pair must segregate—that is, separate—into different gamete cells during the formation of eggs and sperm in the parents.

26. Genetic Notational Convention • Mendel used a letter to indicate the gene for a particular trait. For example, R represented the gene for seed shape. He used uppercase (R) to show a dominant allele and lowercase (r) for a recessive allele. • Individuals have two alleles of each gene. • Individuals with two copies of the same allele (RR or rr) for a gene are said to be homozygous. • Those with different alleles (Rr) are heterozygous.

27. Crossing Pure Lines • Pure-line individuals always produce offspring with the same phenotype because they are homozygous—no other allele is present. • A mating between two pure lines that differ in one trait (RR and rr) results in offspring that all have a heterozygous genotype (Rr) and a dominant phenotype.

28. The Monohybrid Cross • A mating of two heterozygous parents results in offspring that are ¼ RR, ½ Rr, and ¼ rr, which produces a 3:1 ratio of phenotypes.

29. Testing the Model • Mendel's genetic model—a set of hypotheses that explains how a particular trait is inherited—explains the results of these crosses. • A Punnett square is now used to predict the genotypes and phenotypes of the offspring from a cross.

30. Producing a Punnett Square • Write the gamete genotypes for one parent along the top of the diagram. • Write the gamete genotypes for the other parent down the left side of the diagram. • Draw empty boxes under the row and to the right of the column of gametes. • Fill in each box with the genotypes written at the top of the corresponding column and at the left of the corresponding row. • Predict the ratios of each possible offspring genotype and phenotype by tallying the resulting genotypes in all the boxes.

31. Single Trait Cross

32. Mendel’s Experiments with Two Traits • Mendel used dihybrid crosses—matings between parents that are both heterozygous for two traits—to determine whether the principle of segregation holds true if parents differ in more than one trait. • Mendel’s experiments tested two contrasting hypotheses: • Independent assortment, in which alleles of different genes are transmitted independently of each other. • Dependent assortment, wherein the transmission of one allele depends upon the transmission of another.

33. The Principle of Independent Assortment • Mendel’s results supported the principle of independent assortment. • The Punnett square that results from a dihybrid cross predicts: • There should be 9 different offspring genotypes and 4 phenotypes. • The four possible phenotypes should be present in a ratio of 9:3:3:1. • Based on these data, Mendel accepted the hypothesis that alleles of different genes are transmitted independently of one another. This result became known as the principle of independent assortment.

38. Using a Testcross to Confirm Predictions • In a testcross, a parent that is homozygous recessive for a particular trait is mated with a parent that has the dominant phenotype but an unknown genotype. • Because the genetic contribution of the homozygous recessive parent is known, the genotype of the other parent can be inferred from the results. • Mendel used the testcross to further confirm the principle of independent assortment.

40. The Chromosome Theory of Inheritance • The chromosome theory of inheritance arose out of Sutton and Boveri’s careful observations of meiosis. It states that chromosomes are composed of Mendel’s hereditary determinants, or what we now call genes. • The physical separation of alleles during anaphase of meiosis I is responsible for Mendel’s principle of segregation.

42. The Chromosome Theory of Inheritance • The genes for different traits assort independently of one another at meiosis I because they are located on different nonhomologous chromosomes, which themselves assort independently. • This phenomenon explains Mendel’s principle of independent assortment.

44. Testing the Chromosome Theory • Early in the 20th century, Thomas Hunt Morgan adopted fruit flies (Drosophila melanogaster) as a model organism for genetic research. • Morgan’s first goal was to identify different phenotypes. • He called the most common phenotype for each trait wild-type. • He then inferred that phenotypes that differed from the wild-type resulted from a mutation, or a change in a gene. • Individuals with traits attributable to mutation are known as mutants.

45. Thomas Hunt Morgan’s Experiments • Morgan identified red eyes as the wild-type for eye color, and white eyes as a mutation. • When he mated a wild-type female fly with a mutant male fly, all of the F1 progeny had red eyes. • However, when Morgan did the reciprocal cross, the F1 females had red eyes but the F1 males had white eyes. • These experiments suggest a relationship between the sex of the progeny and the inheritance of eye color in Drosophila.

46. The Discovery of Sex Chromosomes • Nettie Stevens analyzed beetle karyotypes and found that females’ diploid cells contain 20 large chromosomes; but males’ diploid cells have 19 large and 1 small (Y) chromosomes. • Y chromosomes pair with the large X chromosome during meiosis I. • X and Y chromosomes are now called sex chromosomes—they determine the sex of the offspring. • In beetles, females have two X chromosomes while males have an X and Y. • Other species have other systems.

47. Sex Linkage and the Chromosome Theory • Sex chromosomes pair during meiosis I and then segregate during meiosis II. • This results in gametes with either an X or a Y chromosome. • Females produce all X gametes. • Males produce half X gametes and half Y gametes.

49. X-Linked Inheritance • Morgan put together his experimental results with Stevens’ observations on sex chromosomes, and proposed that the gene for white eye color in fruit flies is located on the X chromosome and that the Y chromosome does not carry an allele of this gene. • Morgan's hypothesis is called X-linked inheritance (or X-linkage). Females (XX) would then have two copies of the gene and males (XY) would have only one.