Genetics 10201232
Faculty of Agriculture &
Veterinary Medicine
Instructor: Dr. Jihad Abdallah
Topic 10: Non-Mendelian inheritance
1
• Many genes do not follow Mendelian inheritance
– linked genes do not follow Mendel’s law of
independent assortment
– Other non-Mendelian inheritance patterns:
• Maternal effect
• Epigenetic inheritance
• Extranuclear inheritance
2
Maternal Effect
– Inheritance pattern for certain nuclear genes in
which the genotype of the mother directly
determines the phenotype of her offspring
the genotypes of the father and the offspring do
not affect the phenotype of offspring
– Explained by the accumulation of gene products
provided by the mother to her developing eggs
3
The genotype of the mother determines the
phenotype of the offspring for maternal effect
genes
A. E. Boycott (1920s)
• Was the first to study an example of maternal effect
in water snail (Limnea peregra)
– Shell and internal organs can be either righthanded (Dextral) or left-handed (sinistral)
• Determined by cleavage pattern
of egg after fertilization
– Dextral orientation is more common and dominant
4
• Boycott began with two different true-breeding
strains
– One dextral, one sinistral
• Dextral ♀ x sinistral ♂  dextral offspring
• Reciprocal cross  sinistral offspring
• Contradict a Mendelian pattern of inheritance
Dextral
female
Sinistral
male
All dextral
Sinistral
female
Dextral
male
All sinistral
5
Female gametes receive gene products from
the mother that affect early development
stages of the embryo
• Oogenesis in female animals
– Oocyte is formed
– Nourished by surrounding diploid maternal nurse
cells
• Receives gene products from nurse cells
• Genotype of nurse cells determines gene products in
oocyte
6
7
Epigenetic Inheritance
– Modification which occurs to a nuclear gene or
chromosome that alters gene expression.
– Occurs during spermatogenesis, oogenesis, and
early stages of embryogenesis
– Gene expression is altered
• May be fixed during an individual’s lifetime
– Expression is not permanently changed over
multiple generations
• DNA sequence is not altered
• When the individual makes gametes, the genes may
become activated and remain operative in the offspring
which receives it.
8
•
Two types of epigenetic inheritance will be
discussed:
1. Dosage compensation
2. Genomic imprinting
9
Dosage Compensation
– Males and females of many species have
different numbers of sex chromosomes
– But the level of expression of many genes
on sex chromosomes is similar in both
sexes
– In mammals, it is initiated during early stages
of development
10
11
Murray Barr and Ewart Bertram (1949)
• Identified a highly condensed structure in
interphase nuclei of somatic cells of
female cats
– This structure was absent in male cats
– “Barr body”
– Later identified as a highly condensed X
chromosome
12
• X chromosome inactivation
– DNA in inactivated X chromosomes becomes
highly compacted
• A Barr body is formed
– Most genes cannot be expressed
13
• XX females  1 Barr body
• XY males  0 Barr bodies
• XO females  0 Barr bodies
(Turner
syndrome)
• XXX females  2 Barr bodies (Triple X
syndrome)
• XXY males  1 Barr body
(Kleinfelter
syndrome)
14
Genomic Imprinting
• Occurs during gamete formation (before
fertilization)
• Involves a single gene or chromosome
• Governs whether offspring express the
maternally- or the paternally-derived gene
15
Genomic Imprinting
• Genomic imprinting involves the physical
marking of a segment of DNA
– Mark is retained and recognized throughout
the life of the organism inheriting the marked
DNA
– Resulting phenotypes display non-Mendelian
inheritance patterns
– Offspring expresses one allele, not both
– “Monoallelic expression”
16
• Genomic imprinting in mice
– The Igf-2 gene encodes an insulin-like growth
factor
• Functional allele required for normal size
• Igf-2m allele encodes a non-functional protein
– Imprinting results in the expression of the
paternal allele only
• Paternal allele is transcribed
• Maternal allele is not transcribed (transcriptionally
silent)
17
• The Igf-2 gene encodes an insulin-like
growth factor
• Functional allele required for normal size
• Igf-2m allele encodes a non-functional protein
– Igf-2m Igf-2m ♀ x Igf-2 Igf-2 ♂
Normal offspring
– Igf-2m Igf-2m ♂ x Igf-2 Igf-2 ♀
Dwarf offspring
18
– The imprint of the Igf-2 gene is erased during
gametogenesis
– A new imprint is then established
• Oocytes possess an imprinted gene that is
silenced
• Sperm possess a gene that is not silenced
– The phenotypes of offspring are determined
by the paternally derived allele
19
• Genomic imprinting
– Involves differentially methylated regions
(DMRs) located near imprinted genes
• Maternal or paternal copy is methylated,
not both
– Methylation generally inhibits expression
• Can enhance binding of transcriptioninhibiting proteins and/or inhibit binding of
transcription-enhancing proteins
20
21
– Methylation occurs during gametogenesis
• Methylated in oocyte or sperm, not both
– Imprinting is maintained in the somatic cells
of the offspring
– Imprinting is erased during gametogenesis in
these offspring
• New imprinting established
22
23
Extranuclear Inheritance
• Most genes are found in the nucleus
• Some genes are found outside of the
nucleus (mitochondria and chloroplasts)
– Resulting phenotypes display non-Mendelian
inheritance patterns
• “Extranuclear inheritance”
• “Cytoplasmic inheritance”
24
• Mitochondria and chloroplasts possess
DNA
– Circular chromosomes resemble smaller
versions of bacterial chromosomes
– Located in the nucleoid region of the organelles
• Multiple nucleoids often present
• Each can contain multiple copies
of the chromosome
25
26
• Mitochondrial genome size varies greatly
among different species
– 400-fold variation in mitochondrial
chromosome size
• Mitochondrial genomes of animals tend to be fairly
small
• Mitochondrial genomes of fungi, algae, and protists
tend to be intermediate in size
• Mitochondrial genomes of plants tend to be fairly
large
27
• Human mitochondrial DNA is called mtDNA
– Circular chromosome 17,000 base pairs in
length
• Less than 1% of a typical bacterial chromosome
– Carries relatively few genes
• Genes encoding rRNA and tRNA
• 13 genes encoding proteins
functioning in ATP generation
via oxidative phosphorylation
28
• Chloroplast genomes tend to be larger
than mitochondrial genomes
– Correspondingly greater number of genes
– ~100,000 – 200,000 bp in length
– Ten times larger than the mitochondrial
genome of animal cells
• The inheritance pattern of extranuclear
genetic material displays non-Mendelian
inheritance
– Mitochondria and plastids do not segregate
into gametes as do nuclear chromosomes
29
• Pigmentation in Mirabilis jalapa
– The four-o’clock plant
– Pigmentation is determined by chloroplast
genes
• Green phenotype is the wild-type condition
– Green pigment is formed
• White phenotype is due to a mutation in a
chloroplast gene
– Synthesis of green pigment is diminished
• Cells containing both types of chloroplasts
“Heteroplasmy” display green coloration because
the normal chloroplasts produce the green pigment
30
• Pigmentation in Mirabilis jalapa
– Pigmentation in the offspring depends solely
on the maternal parent
• “Maternal inheritance”
• Chloroplasts are inherited only through the
cytoplasm of the egg
31
32
33
• Symbiosis involves a close relationship
between two species where at least one
member benefits
– Endosymbiosis involves such a relationship
where one organism lives inside the other
• Mitochondria and chloroplasts were once
free-living bacteria
– Engulfed and retained by early eukaryotes
(Endosymbiosis)
34
35