1. Course: Genetics Faculty of Graduate Studies An-Najah National University NON-MENDELIAN INHERITANCE
Dr. Heba Al-Fares

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
Indeed, linkage which we considered in the last two lectures follows non-Mendelian inheritance

3. INTRODUCTION
Additional patterns of inheritance that deviate from a Mendelian pattern include:
Maternal effect and epigenetic inheritance
Involve genes in the nucleus
Extranuclear inheritance
Involves genes in organelles other than the nucleus
Mitochondria
Chloroplasts

4. 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
Surprisingly, the genotypes of the father and offspring themselves do not affect the phenotype of the offspring
This phenomenon is due to the accumulation of gene products that the mother provides to her developing eggs
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

5. 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

6. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. A nucleoid can contain more than one chromosome
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
Note:
A nucleoid can contain more than one chromosome
An organelle can contain more than one nucleoid
Chloroplasts tend to have more nucleoids per organelle than mitochondria
7-55

15. There is a 400-fold variation in the size of mitochondrial genomes
Besides variation in copy number, the sizes of organellar genomes also vary greatly among different species
There is a 400-fold variation in the size of mitochondrial genomes
There is also a substantial variation in size of chloroplast genomes 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

16. 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

17. 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

18. 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

19. Figure 7.16
7-63
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

20. 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

21. 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

22. Figure 7.17
7-66
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

23. 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

24. 7-77

25. 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

26. 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