Genetics 10201232 Faculty of Agriculture & Veterinary … 10.pdfGenetics 10201232 Faculty of...

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Genetics 10201232

Faculty of Agriculture &

Veterinary Medicine

Instructor: Dr. Jihad Abdallah

Topic 10: Non-Mendelian inheritance

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• 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

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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

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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 right-

handed (Dextral) or left-handed (sinistral)

• Determined by cleavage pattern

of egg after fertilization

– Dextral orientation is more common and dominant

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• 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

Sinistral

female

Dextral

male

All dextral All sinistral

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• 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

Female gametes receive gene products from

the mother that affect early development

stages of the embryo

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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.

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• Two types of epigenetic inheritance will be

discussed:

1. Dosage compensation

2. Genomic imprinting

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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

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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

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• X chromosome inactivation

– DNA in inactivated X chromosomes becomes

highly compacted

• A Barr body is formed

– Most genes cannot be expressed

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• 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)

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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

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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”

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• 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)

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• 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

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– 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

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• 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 transcription-

inhibiting proteins and/or inhibit binding of

transcription-enhancing proteins

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– 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

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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”

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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)

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