1. Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 7
NON-MENDELIAN INHERITANCE
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2. 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
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3. 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
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4. 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
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5. The first example of a maternal effect gene was discovered in the 1920s by 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
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6. 7-6 Reciprocal cross Figure 7.1
 reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. All parent organisms must be true breeding to properly carry out such an experiment. In one cross, a male expressing the trait of interest will be crossed with a female not expressing the trait. In the other, a female expressing the trait of interest will be crossed with a male not expressing the trait.
Reciprocal cross
A 3:1 phenotypic ratio would be predicted by a Mendelian pattern of inheritance
Figure 7.1
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7. Alfred Sturtevant later explained the inconsistency 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
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8. CO 7

9. Fig. 7.1(TE Art) Parental generation DD dd dd DD F1 generation Dd Dd
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Fig. 7.1(TE Art)
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

11. 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
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12. 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
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13. D gene products cause egg cleavage that promotes a right-handed body plan
Figure 7.2
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14. 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
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15. 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
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16. 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
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17. 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
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18. 7-17

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

20. 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)
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21. 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
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22. 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
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23. 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
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24. In general, mitochondrial genomes are
Fairly small in animals
Intermediate in size in fungi, algae and protists
Fairly large in plants
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25. 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
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