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Genetics: Analysis and Principles Robert J. Brooker CHAPTER 7 NON-MENDELIAN INHERITANCE Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

INTRODUCTION Mendelian inheritance patterns involve genes that
Directly influence the outcome of an organism’s traits and Obey Mendel’s laws Most genes in eukaryotic species follow a Mendelian pattern of inheritance However, there are many that don’t Linkage can be considered as non-Mendelian inheritance 7-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

INTRODUCTION Patterns of inheritance that deviate from a Mendelian pattern: Maternal effect and epigenetic inheritance Involve genes in the nucleus Extranuclear inheritance Involves genes in organelles other than the nucleus Mitochondria Chloroplasts 7-3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7.1 MATERNAL EFFECT Maternal effect refers to an inheritance pattern for certain nuclear genes in which the genotype of the mother directly determines the phenotype of her offspring This phenomenon is due to the accumulation of gene products that the mother provides to her developing eggs 7-4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The first example of a maternal effect gene was discovered in the 1920s by A. E. Boycott
He was studying morphological features of the water snail, Limnaea peregra In this species, the shell and internal organs can be arranged in one of two directions Right-handed (dextral) Left-handed (sinistral) The dextral orientation is more common and dominant The snail’s body plan curvature depends on the cleavage pattern of the egg immediately after fertilization Figure 7.1 describes Boycott’s experiment 7-5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7-6 Reciprocal cross Figure 7.1
A 3:1 phenotypic ratio would be predicted by a Mendelian pattern of inheritance Figure 7.1 7-6

Alfred Sturtevant later explained the incongruity with Mendelian inheritance
Snail coiling is due to a maternal effect gene that exists as dextral (D) and sinistral (d) alelles The phenotype of the offspring depended solely on the genotype of the mother His conclusions were drawn from the inheritance patterns of the F2 and F3 generations 7-7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Parental generation DD dd dd DD F1 generation Dd Dd All dextral
Fig. 7.1(TE Art) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parental generation DD dd dd DD F1 generation Dd Dd All dextral All sinistral F2 generation Males and females 1 DD 2 Dd 1 dd All dextral Cross to each other F3 generation Males and females 1 sinistral 3 dextral

DD or Dd mothers produce dextral offspring
Note that the phenotype of each generation depends on the maternal genotype of the previous generation DD or Dd mothers produce dextral offspring dd mothers produce sinistral offspring The phenotype of the progeny is determined by the mother’s genotype NOT phenotype The genotypes of the father and offspring do not affect the phenotype of the offspring 7-9 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The gene products are a reflection of the genotype of the mother
They are transported to the cytoplasm of the oocyte where they persist for a significant time after the egg has been fertilized Thus influencing the early developmental stages of the embryo Figure 7.2 7-11

D gene products cause egg cleavage that promotes a right-handed body plan
Figure 7.2 7-12

Even if the egg is fertilized by sperm carrying the D allele
d gene products cause egg cleavage that promotes a left-handed body plan Even if the egg is fertilized by sperm carrying the D allele The sperm’s genotype is irrelevant because the expression of the sperm’s gene would be too late Figure 7.2 7-13

Maternal effect genes encode RNA or proteins that play important roles in the early steps of embryogenesis For example Cell division Cleavage pattern In Drosophila, geneticists have identified several dozen maternal effect genes These have profound effects on the early stages of development 7-14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7.2 EPIGENETIC INHERITANCE
Epigenetic inheritance refers to a pattern in which a modification occurs to a nuclear gene or chromosome that alters gene expression However, the expression is not permanently changed over the course of many generations Epigenetic changes are caused by DNA and chromosomal modifications These can occur during oogenesis, spermatogenesis or early embryonic development 7-15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Dosage Compensation The purpose of dosage compensation is to offset differences in the number of active sex chromosomes Dosage compensation has been studied extensively in mammals, Drosophila and Caenorhabditis elegans Depending on the species, dosage compensation occurs via different mechanisms Refer to Table 7.1 7-16 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Barr body is a highly condensed X chromosome
Fig. 7.3 Barr body is a highly condensed X chromosome

The mechanism of X inactivation, also known as the Lyon hypothesis, is schematically illustrated in Figure 7.4 The example involves a white and black variegated coat color found in certain strains of mice A female mouse has inherited two X chromosomes One from its mother that carries an allele conferring white coat color (Xb) One from its father that carries an allele conferring black coat color (XB) 7-20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

While those from this will produce a patch of black fur
The epithelial cells derived from this embryonic cell will produce a patch of white fur At an early stage of embryonic development While those from this will produce a patch of black fur Figure 7.4 7-21

During X chromosome inactivation, the DNA becomes highly compacted
Most genes on the inactivated X cannot be expressed When this inactivated X is replicated during cell division Both copies remain highly compacted and inactive In a similar fashion, X inactivation is passed along to all future somatic cells Another example of variegated coat color Is found in calico cats Refer to Figure 7.3b 7-22 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The Lyon Hypothesis Put to the Test
In 1963, Ronald Davidson, Harold Nitowsky and Barton Childs set out to test the Lyon hypothesis at the cellular level To do so they analyzed the expression of a human X-linked gene The gene encodes glucose-6-phosphate dehydrogenase (G-6-PD), an enzyme used in sugar metabolism 7-23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Biochemists had found that individuals vary with regards to the G-6-PD enzyme
This variation can be detected when the enzyme is subjected to agarose gel electrophoresis One G-6-PD allele encodes an enzyme that migrates very quickly The “fast” enzyme Another allele encodes an enzyme that migrates slowly The “slow” enzyme The two types of enzymes have minor differences in their structures These do not significantly affect G-6-PD function 7-24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 7.5 illustrates the mobility of G-6-PD proteins from various individuals
Thus heterozygous adult females produce both types of enzymes Hemizygous males produce either the fast or the slow type 7-25 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The Hypothesis According to the Lyon hypothesis, an adult female who is heterozygous for the fast and slow G-6-PD alleles should express only one of the two alleles in any particular somatic cell and its descendants, but not both 7-26 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 7.6 7-27

The heterozygous woman produced both types of G-6-PD enzymes
Interpreting the Data All nine clones expressed one of the two types of G-6-PD enzyme, not both These epithelial cells were used to generate the nine clones (as described in steps 2 to 4) Clones 2, 3, 5, 6, 9 & 10 expressed only the slow type Clones 4, 7 & 8 expressed only the fast type The heterozygous woman produced both types of G-6-PD enzymes 7-29 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Interpreting the Data These results are consistent with the hypothesis that X inactivation has already occurred in any given epithelial cell AND This pattern of inactivation is passed to all of the cell’s progeny 7-30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Genomic Imprinting Genomic imprinting is a phenomenon in which expression of a gene depends on whether it is inherited from the male or the female parent Imprinted genes follow a non-Mendelian pattern of inheritance Depending on how the genes are “marked”, the offspring expresses either the maternally-inherited or the paternally-inherited allele Not both This is termed monoallelic expression 7-39 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Let’s consider the following example in mice:
The Igf-2 gene encodes a growth hormone called insulin-like growth factor 2 A functional Igf-2 gene is necessary for a normal size Imprinting results in the expression of the paternal but not the maternal allele The paternal allele is transcribed into RNA The maternal allele is not transcribed Igf-2m is a mutant allele that yields a defective protein This may cause a mouse to be dwarf depending on whether it inherits the mutant allele from its father or mother 7-40 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Fig. 7.9

Offspring are heterozygous (Igf-2 Igf-2m)
Cross: Homozygous normal male (Igf-2 Igf-2) X homozygous mutant female (Igf-2m Igf-2m) Offspring are heterozygous (Igf-2 Igf-2m) They have inherited an Igf-2 allele from their father This allele is expressed yielding a functional protein The mouse grows to a normal size Reciprocal cross: Homozygous mutant male (Igf-2m Igf-2m) X homozygous normal female (Igf-2 Igf-2) They have inherited an Igf-2m allele from their father This allele is expressed yielding a defective protein The mouse has a dwarf phenotype 7-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

These stages are described in Figure 7.10
At the cellular level, imprinting is an epigenetic process that can be divided into three stages Establishment of the imprint during gametogenesis Maintenance of the imprint during embryogenesis and in the adult somatic cells Erasure and reestablishment of the imprint in the germ cells These stages are described in Figure 7.10 The example also considers the imprinting of the Igf-2 gene 7-42 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Both male and female mice express the Igf-2 in their somatic cells
Erasure and Reestablishment During gametogenesis, the imprint is erased; it is reestablished depending on the sex of the animal Male mouse transmits transcriptionally active alleles Female mouse transmits transcriptionally inactive alleles m Transcribed into mRNA in the somatic cells of offspring; But yields defective proteins Figure 7.10 7-43

Genomic imprinting is permanent in somatic cells
However, the marking of alleles can be altered from generation to generation It may involve A single gene A part of a chromosome An entire chromosome Even all the chromosomes from one parent Imprinting is the reason that parthenogenesis ("virgin birth") does not occur in mammals. Two complete female genomes cannot produce viable young because of the imprinted genes. 7-44 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Genomic Imprinting Failure to inherit several nonimprinted genes on the father's chromosome #15 causes a human congenital disorder called Prader-Willi syndrome. Failure to inherit one nonimprinted gene (UBE3A) on the mother's chromosome #15 causes Angelman syndrome. Failure of imprinting in somatic cells may lead to cancer. The cancerous cells of a malignancy called Wilms´ tumor and many cases of colon cancer have both copies of the IGF2 gene expressed (where only one, the father's, should be). Reduced methylation — and hence increased expression — of proto-oncogenes can lead to cancer, while increased methylation — and hence decreased expression — of tumor suppressor genes can also do so.

Imprinting and DNA Methylation
Genomic imprinting must involve a marking process At the molecular level, the imprinting of several genes is known to involve differentially methylated regions (DMRs) These are located near the imprinted genes They are methylated either in the oocyte or sperm Not both They contain binding sites for one or more transcriptional factors 7-45 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

However, sometimes methylation activates specific genes
For most genes, methylation at a DMR results in inhibition of gene expression Methylation could Enhance the binding of proteins that inhibit transcription and/or Inhibit the binding of proteins that enhance transcription Because of this, imprinting is usually described as a process that silences gene expression by preventing transcription However, sometimes methylation activates specific genes 7-46 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Let’s consider two imprinted genes in humans, H19 and Igf-2
They lie close to each other on human chromosome 11 Appear to be controlled by the same DMR This DMR Is ~ 2000 bp Contains binding sites for proteins that regulate the transcription of both genes Is highly methylated on the paternally inherited chromosome Methylation silences the H19 gene and activates the Igf-2 gene 7-47 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7-48 Figure 7.11a Only binds to unmethylated DMR H19 gene activated
Igf-2 gene silenced Only binds to methylated DMR H19 gene silenced Igf-2 gene activated Figure 7.11a 7-48 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Example of methylation patterns in somatic cells and gametes of male and female offspring
Paternal chromosome Maternal chromosome Female Offspring Male Offspring Haploid female gametes transmit an unmethylated DMR Haploid male gametes transmit a methylated DMR 7-49

To date, imprinting has been identified in dozens of mammalian genes
However, the biological significance of genomic imprinting is still a matter of speculation 7-50 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Most commonly, PWS and AS involve a small deletion in chromosome 15
Imprinting does play a role in the inheritance of certain human diseases such as Prader-Willi syndrome (PWS) and Angelman syndrome (AS) PWS is characterized by Reduced motor function Obesity Mental deficiencies AS is characterized by Hyperactivity Unusual seizures Repetitive symmetrical muscle movements Most commonly, PWS and AS involve a small deletion in chromosome 15 If it is inherited from the mother, it leads to AS If it is inherited from the father, it leads to PWS 7-51 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

AS results from the lack of expression of a single gene, UBE3A
Researchers have discovered that this region contains closely linked but distinct genes These are maternally or paternally imprinted AS results from the lack of expression of a single gene, UBE3A The gene is paternally imprinted (silenced) PWS results (most likely) from the lack of expression of a single gene, designated SNRNP The gene is maternally imprinted (silenced) 7-52 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Prader-Willi syndrome
Fig. 7.12(TE Art) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 15 15 Deleted region that includes both the AS and the PW genes PW gene AS gene PW gene silenced in egg PW AS gene silenced in sperm AS PW AS Fertilized egg Fertilized egg PW PW AS AS Angelman syndrome The offspring does not carry an active copy of the AS gene. Prader-Willi syndrome The offspring does not carry an active copy of the PW gene Silenced allele Expressed allele

7.3 EXTRANUCLEAR INHERITANCE
Extranuclear inheritance refers to inheritance patterns involving genetic material outside the nucleus The two most important examples: mitochondria and chloroplasts These organelles are found in the cytoplasm Extranuclear inheritance = cytoplasmic inheritance 7-54 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The genetic material of mitochondria and chloroplasts is located in a region called the nucleoid
Refer to Figure 7.13 The genome is composed of a single circular chromosome containing double-stranded DNA 7-55

In general, mitochondrial genomes are
Fairly small in animals Intermediate in size in fungi, algae and protists Fairly large in plants 7-57 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Nuclear encoded genes in red

The main function of mitochondria is oxidative phosphorylation
A process used to generate ATP (adenosine triphosphate) ATP is used as an energy source to drive cellular reactions The genetic material in mitochondria is referred to as mtDNA The human mtDNA consists of only 17,000 bp (Figure 7.14) It carries relatively few genes rRNA and tRNA genes 13 genes that function in oxidative phosphorylation Note: Most mitochondrial proteins are encoded by genes in the nucleus These proteins are made in the cytoplasm, then transported into the mitochondria 7-58 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Necessary for synthesis of proteins inside the mitochondrion
Function in oxidative phosphorylation Figure 7.14 7-59

The main function of chloroplasts is photosynthesis
The genetic material in chloroplasts is referred to as cpDNA It is typically about 10 times larger than the mitochondrial genome of animal cells The cpDNA of tobacco plant consists of 156,000 bp It carries between 110 and 120 different genes rRNA and tRNA genes Many genes that are required for photosynthesis As with mitochondria, many chloroplast proteins are encoded by genes in the nucleus These proteins contain chloroplast-targeting signals that direct them from the cytoplasm into the chloroplast 7-60 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7-61 Figure 7.15 A genetic map of the tobacco chloroplast genome
Genes designated ORF (open reading frame) encode polypeptides with unknown functions Figure A genetic map of the tobacco chloroplast genome 7-61

Maternal Inheritance in the Four-o’clock Plant
Carl Correns discovered that pigmentation in Mirabilis jalapa (the four o’clock plant) shows a non-Mendelian pattern of inheritance Leaves could be green, white or variegated (with both green and white sectors) Correns determined that the pigmentation of the offspring depended solely on the maternal parent and not at all on the paternal parent This is termed maternal inheritance Refer to Figure 7.16 7-62 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Green phenotype is the wild-type White phenotype is the mutant
In this example, maternal inheritance occurs because the chloroplasts are transmitted only through the cytoplasm of the egg The pollen grains do not transmit chloroplasts to the offspring The phenotype of leaves can be explained by the types of chloroplasts found in leaf cells Green phenotype is the wild-type Due to normal chloroplasts that can make green pigment White phenotype is the mutant Due to a mutation that prevents the synthesis of the green pigment A cell can contain both types of chloroplasts A condition termed heteroplasmy In this case, the leaf would be green 7-64 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Consider a fertilized egg that inherited two types of chloroplast
Figure provides a cellular explanation for the variegated phenotype in Mirabilis jalapa Consider a fertilized egg that inherited two types of chloroplast Green and white As the plant grows, the chloroplasts are irregularly distributed to daughter cells Sometimes, a cell may receive only white chloroplasts Such a cell will continue to divide and produce a white sector Cells that contain only green chloroplasts or a combination of green and white will ultimately produce green sectors 7-65 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Maternal (cytoplasmic) inheritance

The Petite Trait in Yeast
Mutations that yield defective mitochondria are expected to make cells grow much more slowly Boris Ephrussi and his colleagues identified Saccharomyces cerevisiae mutants that have such a phenotype These were called petites because they formed small colonies on agar plates Wild-type strains formed larger colonies 7-67 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Biochemical and physiological evidence indicated that petite mutants had defective mitochondria
Genetic analyses showed that petite mutants are inherited in different ways Two main types of mutants were identified 1. Segregational mutants Have mutations in genes located in the nucleus Segregate in a Mendelian manner in meiosis Refer to Figure 7.18a 2. Vegetative mutants Have mutations in genes located in the mitochondrial genome Show a non-Mendelian pattern of inheritance Refer to Figure 7.18b 7-68 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Yeast come in two mating types: a and a
Thus, Euphressi was able to cross yeast belonging to two different strains Segregational mutant Nuclear and mendelian Each resulting tetrad shows a 2:2 ratio of wild-type to petite Zygote then meiosis This result is typical of Mendelian inheritance Figure 7.18 7-69 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Euphressi discovered two types of vegetative petites
Neutral and Suppressive Zygote then meiosis Zygote then meiosis Each resulting tetrad shows a 4:0 ratio of wild-type to petite These results contradict the normal 2:2 ratio expected for the segregation of Mendelian traits Each resulting tetrad shows a 0:4 ratio of wild-type to petite Figure 7.18 7-70 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Blue and red indicate mendelian segregation of nuclear genes

Researchers later found that
Neutral petites lack most of their mitochondrial DNA Suppressive petites lack only small segments of mtDNA When two yeast cells are mated, offspring inherit mitochondria from both parents 7-71 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 7.18 Progeny have both “wild type” and “suppressive petite” mitochondria So how come only petite colonies are produced? Two possibilities i. Suppressive petite mitochondria could replicate faster than wild-type mitochondria ii. Recombination between wild-type and petite mtDNA may ultimately produce defects in the wild-type mitochondria Progeny have both “wild type” and “neutral petite” mitochondria They display a normal phenotype because of the wild type mitochondria 7-72 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Inheritance of Chloroplasts in Algae
The unicellular alga Chlamydomonas reinhardtii is a model organism It contains a single chloroplast Occupies ~ 40% of the cell’s volume Most strains are sensitive to the antibiotic streptomycin (sms) In 1954, Ruth Sager identified a mutant that was resistant to streptomycin (smr) 7-73 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Like yeast, Chlamydomonas can be found in two mating types
mt+ and mt– Mating type is due to nuclear inheritance It segregates in a 1:1 manner Sager and her colleagues discovered that “resistance to streptomycin” was not inherited in a Mendelian manner smr was inherited from the mt+ parent but not from the mt– parent In subsequent studies, they mapped several genes, including the smr gene, to the chloroplast chromosome 7-74 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7-75 Figure 7.19 Because the mt+ strain was smr
Because the mt+ strain was sms Figure 7.19 7-75

The Pattern of Inheritance of Organelles
The pattern of inheritance of mitochondria and chloroplasts varies among different species Heterogamous species Produce two kinds of gametes Female gamete  Large Provides most of the cytoplasm of the zygote Male gamete  Small Provides little more than a nucleus In these species, organelles are typically inherited from the mother 7-76 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The Pattern of Inheritance of Organelles
Species with maternal inheritance may, on occasion, exhibit paternal leakage The paternal parent provides mitochondria through the sperm In the mouse, for example, 1-4 paternal mitochondria are inherited for every 100,000 maternal mitochondria per generation of offspring 7-78 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Human Mitochondrial Diseases
Human mtDNA is transmitted from mother to offspring via the cytoplasm of the egg Several human mitochondrial diseases have been discovered These are typically chronic degenerative disorders affecting the brain, heart, muscles, kidneys and endocrine glands Example: Leber’s hereditary optic neuropathy (LHON) Affects the optic nerve May lead to progressive loss of vision in one or both eyes LHON is caused by mutations in several different mitochondrial genes 7-79 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

MATERNAL INHERITANCE OF MITOCHONDRIAL DNA MUTATIONS

The Endosymbiosis Theory
The endosymbiosis theory describes the evolutionary origin of mitochondria and chloroplasts These organelles originated when bacteria took up residence within a primordial eukaryotic cell During evolution, the characteristic of the intracellular bacterial cell gradually changed to that of the organelle The endosymbiotic origin of organelles is supported by several observations These include Organelles have circular chromosomes (like bacteria) Organelle genes are more similar to bacterial genes than to those found within the nucleus 7-80 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The eukaryotic cell was now able to undergo phtosynthesis
The eukaryotic cell was now able to synthesize greater amounts of ATP Figure 7.20 7-81 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The Endosymbiosis Theory
During the evolution of eukaryotic species, most genes originally found in the bacterial genome have been lost or transferred to the nucleus Modern day mitochondria and chloroplasts have lost most of the genes still found in present-day cyanobacteria and purple bacteria The gene transfer has primarily been unidirectional From the organelles to the nucleus In addition, gene transfer can occur between organelles Between two mitochondria, two chloroplasts or a mitochondrion and a chloroplast The biological benefits of gene transfer remain unclear 7-82 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

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