Ch10 Mendelian Genetics

台大農藝系遺傳學 601 20000 Chapter 10 slide 1

CHAPTER 10Mendelian Genetics

Peter J. Russell

edited by Yue-Wen Wang Ph. D.Dept. of Agronomy, NTU


Introduction

1. Gregor Mendel (1822–1884) laid the foundation for our current understanding of heredity.

2. Mendel did not know about chromosomes or genes, which were discovered after his lifetime.


Genotype and Phenotype

1. Hereditary traits are under control of genes (Mendel called them particulate factors).

2. Genotype is the genetic makeup of an organism, a description of the genes it contains.

3. Phenotype is the characteristics that can be observed in an organism.

4. Phenotype is determined by interaction of genes and environment. Genes provide potential, but environment determines whether that potential is realized

台大農藝系遺傳學 601 20000 Chapter 10 slide 4Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Fig. 10.1 Influences on the physical manifestation (phenotype) of the genetic blueprint

(genotype)


Mendel’s Experimental Design1. Mendel began his work in 1854 with the garden pea Pisum sativum, by crossbreeding plants with

different characteristics. He reported his theory explaining transmission of traits to the next generation in 1865, but its significance was not realized until several decades later.

2. His success resulted from focusing on well-defined traits one at a time, quantifying the offspring and analyzing the results mathematically.

3. Garden peas are excellent for this type of research, because they grow easily, produce large numbers of seeds quickly and routinely self-fertilize. Experimental cross-fertilization is also readily accomplished in peas.

4. Mendel first grew strains of peas using self-fertilization to be certain that the traits of interest were unchanged in subsequent generations (true-breeding or pure-breeding strains).

5. Then he looked at inheritance of traits selected because they have only two distinct possibilities for phenotype. The traits he studied are listed, and the dominant phenotype is indicated by an asterisk:

a. Flower/seed coat color (one gene controls both): *grey/purple vs. white/white.

b. Seed color: *yellow vs. green.

c. Seed shape: *green vs. yellow.

d. Pod color: *green vs. yellow.

e. Pod shape: *inflated vs. pinched.

f. Stem height: *tall vs. short.

g. Flower position: *axial vs. terminal.


Fig. 10.3 Procedure for crossing pea plants


Fig. 10.4 Seven character pairs in the garden pea that Mendel studied in his breeding

experiments


Monohybrid Crosses and Mendel’s Principle of Segregation1. Terminology used in breeding experiments:

a. Parental generation is the P generation.

b. Progeny of P generation is the first filial generation, designated F1.

c. When F1 interbreed, the second filial generation, F2, is produced.

d. Subsequent interbreeding produces F3, F4 and F5 generations.

2. A monohybrid cross involves true-breeding strains that differ in a single trait.

3. To determine whether both parents contribute equally to the phenotype of a particular trait in offspring, a set of reciprocal crosses is performed. By convention, the female parent is given first.

4. In Mendelian genetics, offspring of a monohybrid cross will exactly resemble only one of the parents. This is the principle of uniformity in F1.

5. Traits that disappear in the F1 reappear in the F2. The F2 generation will have a ratio of about one individual with the “reappearing” phenotype to three individuals with the phenotype that was present in the F1. Mendel reasoned that information to create the trait was present in the F1 in the form of “particulate factors,” which we now call genes (Figure 10.6).


Fig. 10.5 Results of one of Mendel’s breeding crosses


Fig. 10.6 The F2 progeny of the cross shown in Figure 10.5


6. Each particulate factor exists in alternative forms (now called alleles) that control a specific trait. True-breeding strains contain identical factors. The F1 contain one of each, but since the trait is just like one of the parents rather than a mix, one (dominant) allele has masked expression of the other (recessive) one.

7. By convention, letters may be used to designate alleles, with the dominant a capital letter (S), and the recessive in lower case (s).

8. Individuals with identical alleles (e.g., genotypes SS and ss) are called homozygous for that gene, because all their gametes will have the same allele for this trait. Individuals with different alleles (e.g., Ss) are heterozygous, because 1⁄2 of their gametes will contain one allele, and 1⁄2 the other (Figure 10.7).


Fig. 10.7 Dominant and recessive alleles of a gene for seed shape in peas


9. Diagrams of a smooth x wrinkled cross appear in Figure 10.8. The Punnett square is a diagram showing all possible combinations of the gametes produced by each parent. Note that it accounts for the 3:1 ratio in the F2 generation.

10. When Mendel had conducted experiments for the seven different traits in garden peas, he made these conclusions:

a. Results of reciprocal crosses are always the same.

b. The F1 resembled only one of the parents.

c. The trait missing in the F1 reappeared in about 1⁄4 of the F2 individuals.


Fig. 10.8a Mendel’s first law, principle of segregation of Mendelian factors: Production

of the F1 generation


Fig. 10.8b Mendel’s first law, principle of segregation of Mendelian factors:

Production of the F2 generation


The Principle of Segregation

Animation: Mendel's Principle of Segregation

1. The first Mendelian law, the principle of segregation, states: “Recessive characters, which are masked in the F1 from a cross between two true-breeding strains, reappear in a specific proportion in the F2.” This is because alleles segregate during anaphase I of meiosis, and progeny are then produced by random combination of the gametes.

2. A summary of terms and concepts appears in Box 10.1


Representing Crosses with a Branch Diagram

• 1. The branch diagram is an alternative approach to predicting the outcome of crosses (Box 10.2 and Figure 10.9).

• 2. Results from a branch diagram will be identical to those obtained with a Punnett square.


Fig. 10.9 Using the branch diagram approach to calculate the ratios of phenotypes in the F2 generation of the cross in Figure 10.8


Confirming the Principle of Segregation: The Use of Testcrosses

1. Mendel observed that plants with the recessive phenotype are all true-breeding. When plants with the dominant phenotype are selfed, 1⁄3 are true-breeding, and 2⁄3 produce progeny with both phenotypes.

2. A more common method to determine whether an individual with the dominant phenotype is homozygous or heterozygous is to perform a testcross by crossing the individual with one that is homozygous recessive.


Fig. 10.10 Determining the genotypes of the F2 smooth progeny of Figure 10.8 by

selfing the plants grown from the smooth seeds


Fig. 10.11 Determination of F2 smooth seeds’ genotype by testcrossing to a wrinkled-

seed plant


Dihybrid Crosses and Mendel’s Principle of Independent AssortmentThe Principle of Independent Assortment

Animation: Mendel's Principle of Independent Assortment

1. After Mendel analyzed crosses involving two pairs of traits (dihybrid crosses), he formulated his second law, the principle of independent assortment, which says that the factors for different traits assort independently of one another. This allows for new combinations of the traits in the offspring.

2. A dihybrid cross will produce four possible phenotypic classes, in a 9:3:3:1 ratio.


Branch Diagram of Dihybrid Crosses

1. Dihybrid crosses can be represented with either a Punnett square or a branch diagram. With practice, the student should be able to calculate results of crosses simply by applying Mendelian principles (Figure 10.13).

2. Individuals of unknown genotype that show the dominant phenotype can be represented by a dominant allele and a dash (e.g., S-) to indicate that the second allele is unknown.

3. If alleles assort independently, all possible phenotypes will be represented in the F2, in a ratio of 9:3:3:1. If the F1 are test crossed, all types of offspring in a ratio of 1:1:1:1 will be produced (Figure 10.12).

4. In the F2 of a dihybrid cross there will be four phenotypic classes, and nine genotypic classes (Table 10.2)


Fig. 10.12a Principle of independent assortment: Production of the F1 generation


Fig. 10.12b Derivation of F2 genotypes and 9:3:3:1 phenotypic ratio by use of the Punnett square


Fig. 10.13 Using the branch diagram approach to calculate the F2 phenotypic ratio of the cross in Figure 10.12


iActivity: Tribble Traits


Trihybrid Crosses1. Crosses involving three independently assorting character pairs are called

trihybrid. There are 64 possible combinations of the eight different gamete types contributed by each parent, creating 27 different genotypes. There will be eight different phenotypes, in a predicted ratio of 27:9:9:9:3:3:3:1.

2. Some useful generalizations about Mendelian genetics, with the following assumptions: (1) The parents are two different true-breeding strains for the gene(s) under study; (2) The F1 self-fertilize or interbreed. Under these assumptions:

a. The F1 will be heterozygous for each gene involved in the cross.

b. When the F1 interbreed, the F2 will contain 3⁄4 dominant phenotype and 1⁄4 recessive phenotype individuals, with genotype frequencies of 1⁄4 for AA, 1⁄2 for Aa, and 1⁄4 for aa.

c. There are two phenotypic classes in the F2 of a monohybrid cross, four in a dihybrid cross and eight in a trihybrid cross. General rule is that there are 2n phenotypic classes in the F2, where n is the number of independently assorting heterozygous gene pairs.

d. There are three genotypic classes in the F2 of a monohybrid cross, while there are nine in a dihybrid cross and 27 in a trihybrid cross. General rule is that there are 3n phenotypic classes in the F2, where n is the number of independently assorting heterozygous gene pairs.

e. The phenotypic rule (2n) is also used to predict the number of genotypic classes produced in the test cross of a multiply heterozygous F1, because the number of genotypic classes will match the number of phenotypic ones.


Fig. 10.14 Branch diagram derivation of the relative frequencies of the eight phenotypic classes in the F2 of a trihybrid cross



The “Rediscovery” of Mendel’s Principles

1. Mendel’s work was published in 1866, but received little attention from the scientific community until about 1900.

2. In 1902, William Bateson, experimenting with fowl, showed that Mendelian principles apply in animals. He coined the terms genetics, zygote, F1, F2, and allelomorph (which was shortened to allele). W.L. Johannsen named Mendelian factors genes in 1909.


Statistical Analysis of Genetic Data: The Chi-Square Test1. One of the advantages of the Mendelian approach is that it is

quantitative and may be tested mathematically. A hypothesis is presented as a null hypothesis, proposing that the observed data are the same as the predicted results.

2. A chi-square (χ2) test checks for goodness-of fit between the expected and observed results, to determine whether differences are likely to be due to chance alone.

3. See Tables 10.2 and 10.4 for an example using the chi-square test to analyze a hypothesis.

4. Chi-square analysis cannot tell us that a hypothesis is correct or incorrect, only whether the observed result is a good fit with predictions of the hypothesis.

a. If differences between the results and the prediction are unlikely to be due to chance alone, the hypothesis will be rejected and another one tried.

b. Typically if the probability of obtaining the observed χ2 values is greater than 5% (P> 0.05) the hypothesis is not rejected.




Mendelian Genetics in Humans

1. W. Farabee in 1905 was the first to demonstrate Mendelian principles in humans, showing that brachydactyly is inherited as a simple dominant trait.


Pedigree Analysis

1. The study of the phenotypic records of a family over several generations is pedigree analysis. The individual upon whom the study focuses is the propositus (male) or proposita (female).

2. The symbols of pedigree analysis are summarized in Figure 10.16, and Figure 10.17 shows a sample pedigree. Note that generations are numbered with Roman numerals while individuals are numbered with Arabic numerals.


Fig. 10.16 Symbols used in human pedigree analysis


Fig. 10.17 Example of a human pedigree


Examples of Human Genetic Traits

1. Recessive traits are well documented in humans, and are usually the result of a mutation causing loss or modification of a gene product. Albinism (Figure 10.18) is an example.

2. Deleterious recessive alleles persist in the population because heterozygous individuals carry the allele without developing the phenotype, and so are not at a selective disadvantage.

3. Characteristics of recessive inheritance of a relatively rare trait:

a. Parents of most affected individuals have normal phenotypes but are heterozygous. If the allele is rare the trait will “skip” generations.

b. Mating of heterozygotes will produce 3⁄4 normal progeny and 1⁄4 with the recessive phenotype.

c. If both parents have the recessive trait, all their progeny will usually also have the trait.


Fig. 10.18 Albinism (a) Two individuals with albinism: blue musicians Johnny (left) and Edgar Winter (right) (b) A pedigree showing the transmission of the autosomal recessive trait of albinism


4. Dominant traits are also well documented in humans. A mutation may produce a dominant phenotype by causing a function to be gained due to an altered gene product capable of a new activity. Examples:

a. Woolly hair (Figure 10.19).

b. Achondroplasia.

c. Brachydactyly.

d. Marfan syndrome.

5. Dominant alleles produce a distinct phenotype when in a heterozygote whose other allele is wild-type. Due to the rarity of dominant mutant alleles causing recognizable traits, homozygous dominant individuals are very unusual. Most pairings are between a heterozygote and a homozygous recessive (wild-type) individual.

6. Characteristics of dominant inheritance of a relatively rare trait:

a. Affected individuals have at least one affected parent.

b. The trait is present in every generation.

c. Offspring of an affected heterozygote will be 1⁄2 affected and 1⁄2 wild-type.


Fig. 10.19 Woolly hair (a) Members of a Norwegian family, some of whom exhinbit the trait of woolly hair (b) Part of a pedigree showing the transmission of the autosomal dominant trait of woolly hair

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Ch10 Mendelian Genetics