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Peter J. Russell
CHAPTER 10
Mendelian Genetics
edited by Yue-Wen Wang Ph. D.
Dept. of Agronomy,台大農藝系
NTU
遺傳學 601 20000
Chapter 10 slide 1
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.
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Chapter 10 slide 2
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
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Chapter 10 slide 3
Fig. 10.1 Influences on the physical manifestation (phenotype) of the genetic blueprint
(genotype)
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 4
Mendel’s Experimental Design
1.
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.
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Chapter 10 slide 5
Fig. 10.3 Procedure for crossing pea plants
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 6
Fig. 10.4 Seven character pairs in the garden pea that Mendel studied in his breeding
experiments
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 7
Monohybrid Crosses and Mendel’s Principle of
Segregation
1. 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).
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Chapter 10 slide 8
Fig. 10.5 Results of one of Mendel’s breeding crosses
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 9
Fig. 10.6 The F2 progeny of the cross shown in Figure 10.5
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 10
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).
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Chapter 10 slide 11
Fig. 10.7 Dominant and recessive alleles of a gene for seed shape in peas
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 12
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.
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Chapter 10 slide 13
Fig. 10.8a Mendel’s first law, principle of segregation of Mendelian factors: Production
of the F1 generation
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 14
Fig. 10.8b Mendel’s first law, principle of segregation of Mendelian factors:
Production of the F2 generation
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 15
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
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Chapter 10 slide 16
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.
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Chapter 10 slide 17
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
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 18
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 truebreeding, 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.
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Chapter 10 slide 19
Fig. 10.10 Determining the genotypes of the F2 smooth progeny of Figure 10.8 by
selfing the plants grown from the smooth seeds
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 20
Fig. 10.11 Determination of F2 smooth seeds’ genotype by testcrossing to a wrinkledseed plant
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
台大農藝系 遺傳學 601 20000
Chapter 10 slide 21
Dihybrid Crosses and Mendel’s Principle of
Independent Assortment
The 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.
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Chapter 10 slide 22
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)
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Chapter 10 slide 23
Fig. 10.12a Principle of independent assortment: Production of the F1 generation
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Chapter 10 slide 24
Fig. 10.12b Derivation of F2 genotypes and 9:3:3:1 phenotypic ratio by use of the Punnett
square
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Chapter 10 slide 25
Fig. 10.13 Using the branch diagram approach to calculate the F2 phenotypic ratio of
the cross in Figure 10.12
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Chapter 10 slide 26
iActivity: Tribble Traits
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Chapter 10 slide 27
Trihybrid Crosses
1. 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
, because
the number
of 10 slide 28
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Chapter
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
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Chapter 10 slide 29
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Chapter 10 slide 30
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.
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Chapter 10 slide 31
Statistical Analysis of Genetic Data: The ChiSquare Test
1. 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遺傳學
rejected.
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601 20000
Chapter 10 slide 32
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Chapter 10 slide 33
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Chapter 10 slide 34
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.
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Chapter 10 slide 35
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.
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Chapter 10 slide 36
Fig. 10.16 Symbols used in human pedigree analysis
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Chapter 10 slide 37
Fig. 10.17 Example of a human pedigree
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Chapter 10 slide 38
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.
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Chapter 10 slide 39
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
台大農藝系 遺傳學 601 20000
Chapter 10 slide 40
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 wildtype.
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Chapter 10 slide 41
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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Chapter 10 slide 42