Download Chapter 6: Extranuclear Inheritance, Imprinting, and Maternal Effect

Chapter 6: Extranuclear Inheritance, Imprinting, and Maternal Effect
Student Learning Objectives
Upon completion of this chapter you should be able to:
1.
2.
3.
4.
5.
Know the structure and endosymbiotic origin of mitochondria and chloroplasts.
Understand the various patterns of inheritance associated with extranuclear genomes.
Understand the process of epigenetic inheritance.
Know the molecular mechanism of genomic imprinting in mammals.
Understand how maternal effect influences the phenotype of the offspring and the
molecular basis of this pattern of inheritance.
6.1 Extranuclear Inheritance: Chloroplasts
Overview
Not all of the genetic material in the eukaryotic cell is located in the nucleus. The
chloroplast and mitochondria contain their own DNA. Because these two organelles are located in
the cytoplasm, the inheritance of organellar DNA is called cytoplasmic inheritance or
extranuclear inheritance. This section first examines the nature of the chloroplast genome. Note
that the genetic material of the chloroplast is found in an area called the nucleoid (Figure 6.1).
The chromosome is termed cpDNA, and is typically circular (Figure 6.2).
The section then explores the patterns of transmission of chloroplasts from parent to
offspring. Two examples are covered in detail: leaf-color in the four o’clock plant, Mirabilis
jalapa (Figure 6.3), and streptomycin-resistance in the unicellular alga, Chlamydomonas
reinhardtii (Figure 6.5).
Outline of Key Terms
Extranuclear inheritance
Cytoplasmic inheritance
Chloroplast DNA (cpDNA)
Nucleoid
Nuclear genes
Maternal inheritance
Heteroplasmy
Heterogamous species
Focal Points


Maternal inheritance (Figure 6.3)
Chloroplast inheritance (Figure 6.5)
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Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1.
2.
3.
4.
5.
6.
A species that makes two morphologically different types of gametes is said to be _______.
The chloroplast genome is designated _______.
A cell that contains two different types of chloroplasts is called ___________.
In the alga Chlamydomonas, chloroplasts are inherited from the __________ parent.
In the four o’clock plant, chloroplasts are inherited from the __________ parent.
In the four o’clock plant, a leaf that contains sectors of white tissue and sectors of green tissue
is said to have a _________ phenotype.
6.2 Extranuclear Inheritance: Mitochondria
Overview
As with chloroplasts, mitochondria have their own genetic material, and their pattern of
transmission is non-Mendelian. In this section, we will examine the nature of the mitochondrial
genome, and how mitochondria are transmitted from parents to offspring. Note that the genetic
material of the mitochondrion is also found in an area called the nucleoid (Figure 6.6). The
chromosome is termed mtDNA and, like cpDNA, it is circular (Figure 6.7), although usually
much smaller.
While the transmission patterns of mitochondria may vary, most species show a maternal
inheritance pattern (Refer to Table 6.4). This is certainly the case in mammals, including humans.
Moreover, many human diseases are caused by mutations in mitochondrial DNA. These are
usually chronic degenerative disorders that affect cells requiring high levels of ATP, such as
nerve and muscle cells (Refer to Table 6.5).
Outline of Key Terms
Mitochondrial DNA (mtDNA)
Paternal leakage
Focal Points


Genetic map of human mitochondrial DNA (Figure 6.7)
Transmission of mitochondria (pages 118-119)
Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1.
2.
3.
4.
The mitochondrial genome is designated _________.
The mitochondrial genome is found in an area called the _________.
In humans, mitochondria typically show a ________ pattern of transmission.
In humans, a rare event occurs where mitochondria are transmitted by the sperm. This
phenomenon is called ________.
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6.3 Theory of Endosymbiosis
Overview
Why would chloroplasts and mitochondria contain their own DNA? Well the reason can
be traced back to the evolutionary origin of these organelles, which is thought to involve a
symbiotic relationship. This section explores the theory of endosymbiosis, which was first
proposed for chloroplasts in 1883, and for mitochondria in 1922. In short, the theory posits that
these two organelles are remnants of prokaryotic cells that took residence in primitive eukaryotic
cells. It is also important to note that during the evolution of eukaryotic species, most genes that
were originally found in these prokaryotic cells have been lost or transferred to the nucleus.
Outline of Key Terms
Endosymbiosis
Endosymbiosis theory
Focal Points


The endosymbiotic origin of chloroplasts and mitochondria (Figure 6.8)
Transfer of genes from organelles to nucleus (page 120)
Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1.
2.
3.
4.
Mitochondria evolved from an endosymbiotic relationship involving __________.
Chloroplasts evolved from an endosymbiotic relationship involving __________.
Animals and _________ contain mitochondria but not chloroplasts.
Plants and _________ contain mitochondria and chloroplasts.
6.4 Epigenetics: Imprinting
Overview
This section examines epigenetic inheritance – a pattern in which a gene or chromosome
is modified so as to alter gene expression. An example is genomic imprinting, which is
characterized by monoallelic expression. Simply put, genomic imprinting allows the expression
of one parent’s genes, regardless of dominance. The system presented in this chapter involves the
Igf-2 gene in mice. Study Figures 6.9 and 6.10 carefully since genomic imprinting can often be a
difficult topic. The molecular marking process that causes imprinting involves DNA methylation
(Figure 6.12). Methylation is a powerful tool in genomic regulation, and you are going to see
much more of it in upcoming chapters. Finally, the section discusses how imprinting plays a role
in some human diseases, including Prader-Willi syndrome (PWS) and Angelman syndrome (AS).
(Refer to Table 6.6 and Figure 6.3)
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Outline of Key Terms
Epigenetic inheritance
Genomic imprinting
Monoallelic expression
DNA methylation
Imprinting control region (ICR)
Focal Points


The process of genomic imprinting (Figures 6.9 and 6.10)
The role of genomic imprinting in development of PWS and AS (Figure 6.13)
Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1. In _________ inheritance, a modification occurs to a nuclear gene or chromosome that alters
gene expression.
2. In mouse, the gene Igf2 encodes a protein _________ called insulin-like growth factor 2.
3. The Igf2 gene is imprinted such that only the _________ allele is transcribed in somatic cells.
4. For most imprinted genes, _________ causes an inhibition of gene expression.
5. In humans, PWS and AS are caused by deletions in chromosome _________.
6.5 Maternal Effect
Overview
As we have already seen, not all genes follow a Mendelian pattern of inheritance. In this
last section of the chapter you are introduced to yet another example: the concept of maternal
effect. Simply stated, maternal effect means that the gene products of the mother have a stronger
influence on the zygote during embryonic development than the genotype of the zygote.
The section starts by introducing the principles of maternal effect using the water snail
Lymnaea peregra as a model system. You should examine Figure 6.14 very carefully, noting that
it is the genotype of the mother that determines the phenotype of the offspring. Recall that
phenotypes are determined by gene products. In this case, the gene products released by maternal
nurse cells during development of the embryo play the prime influence on the phenotype. This
mechanism is illustrated in detail in Figure 6.15.
Outline of Key Terms
Maternal effect
Nuclear genes
Reciprocal cross
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Focal Points


Experimental evidence of maternal effect (Figure 6.14). Note the relationship between
genotypes and phenotypes.
The mechanism of maternal effect (Figure 6.15)
Exercises and Problems
The following pertains to the water snail, Lymnaea peregra. Using the information provided in
the text, complete the following table.
Mother
Genotype
Phenotype
DD
Dextral
(2)
Dextral
Dd
Sinistral
(6)
Sinistral
Father
Genotype
Phenotype
dd
Sinistral
DD
Dextral
Dd
Sinistral
dd
Sinistral
Offspring
Genotype
Phenotype
Dd
(1)
(3)
Sinistral
(4)
(5)
dd
(7)
Chapter Quiz
1. In four o’clock plants, a cross between a white-leaved maternal parent and a green-leaved
paternal parent yields which of the following offspring?
a. All white-leaved.
b. All green-leaved.
c. All have variegated leaves.
d. 1/3 white-leaved: 1/3 green-leaved: 1/3 with variegated leaves.
e. 1/2 white-leaved: 1/2 green-leaved
2. Which of the following does not occur during genomic imprinting?
a. The imprint is established in the gametes.
b. The imprint is maintained in the offspring.
c. The imprint is erased in the gametes of the offspring.
d. A new imprint is established in the gametes of the offspring.
e. All of the above occur.
3. Studies of Chlamydomonas were involved in the discovery of which of the following?
a. genomic imprinting
b. epigenetic inheritance
c. maternal effect
d. chloroplast inheritance
e. mitochondrial inheritance
4. In which of the following does the genotype of the mother control the offspring’s phenotype?
a. maternal inheritance
b. maternal effect
c. X inactivation
d. gene dosage
e. none of the above
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5. The endosymbiosis theory attempts to explain which of the following?
a. The development of Barr bodies in XX cells.
b. The influence of the mother’s genotype on the offspring’s phenotype.
c. The process of DNA methylation.
d. The presence of mitochondrial and chloroplast genomes in eukaryotic cells.
6. With regards to inheritance of traits, DNA methylation is associated with which of the
following?
a. maternal inheritance
b. maternal effect
c. genomic imprinting
d. X chromosome inactivation
e. none of the above
7. mtDNA and cpDNA is associated with the study of ______ inheritance.
a. extranuclear
b. epigenetic
c. Mendelian
d. nuclear
8. A cross between two Igf2Igf2– mice is expected to produce which of the following offspring?
a. All normal
b. All dwarf
c. 1 normal : 1 dwarf
d. 3 normal : 1 dwarf
9. A woman affected with a mitochondrial disorder will likely transmit the disease to
a. all of her daughters, but none of her sons.
b. all of her sons, but none of her daughters.
c. all of her children.
d. none of her children.
10. A male born with Prader-Willi syndrome carries a deletion in chromosome 15. Which parent
transmitted the deletion to him?
a. the father
b. the mother
c. either his father or mother
Answer Key for Study Guide Questions
This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers
in parentheses ( ) represent possible alternate answers to a problem, while answers marked with
an asterisk (*) indicate that the response to the question may vary.
6.1
1. heterogamous
2. cpDNA
3. heteroplasmic
4. mt+
5. maternal (female)
6. variegated
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6.2
1. mtDNA
2. nucleoid
3. maternal inheritance
4. paternal leakage
6.3
1. purple bacteria
2. cyanobacteria
3. fungi (protozoa)
4. algae
6.4
1. epigenetic
2. growth hormone
3. paternal (father’s)
4. DNA methylation
5. 15
6.5
1. Dextral
2. dd
3. Dd
4. ¼ DD, ½ Dd, ¼ dd
5. Dextral
6. dd
7. Sinistral
Quiz
1. a
2. e
3. d
4. b
5. d
6. c
7. a
8. c
9. c
10. a
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