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Genetics 24231 Faculty of Agriculture. Instructor: Dr. Jihad Abdallah Topic 10: Non-Mendelian inheritance. NON-MENDELIAN INHERITANCE. Many genes do not follow a Mendelian inheritance pattern e.g., Closely linked genes do not follow Mendel’s law of independent assortment

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Genetics 24231Faculty of Agriculture

Instructor: Dr. Jihad Abdallah

Topic 10: Non-Mendelian inheritance


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NON-MENDELIAN INHERITANCE

  • Many genes do not follow a Mendelian inheritance pattern

    • e.g., Closely linked genes do not follow Mendel’s law of independent assortment

    • This chapter will discuss additional and more 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

    • For maternal effect genes, the genotypes of the father and the offspring do not affect the phenotype of offspring

  • Explained by the accumulation of gene products the mother provides 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)

  • First to study an example of maternal effect

  • Involved morphological features of 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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Dextral

female

Sinistral

male

Sinistral

female

Dextral

male

All sinistral

All dextral


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  • Sturtevant(1923) offspring for maternal effect genes

  • Sturtevant(1923) proposed that Boycott’s results could be explained by a maternal effect gene

    • Conclusions drawn from F2 and F3 generations

    • Dextral (D) is dominant to sinistral (d)

    • Phenotype of offspring is determined by genotype of mother


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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


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EPIGENETIC INHERITANCE affect early development stages of the embryo

  • Modification occurs to a nuclear gene or chromosome that alters gene expression.

  • Occur 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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DOSAGE COMPENSATION affect early development stages of the embryo

  • Males and females of many species have different numbers of certain sex chromosomes (e.g., X 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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  • Apricot eye color in affect early development stages of the embryoDrosophila

    • Conferred by an X-linked gene

    • Homozygous females resemble males (two copies of the allele in a female produce a phenotype similar to one copy in a male)

    • Females heterozygous for the apricot allele have paler eye color


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  • Dosage compensation affect early development stages of the embryodoes not occur for all eye color alleles in Drosophila

    • e.g., Eosin eye color

      • Conferred by an X-linked gene

      • Homozygous eosin females have darker eye color than hemizygous eosin males

        • Dark eosin and light eosin

      • Females heterozygous for the eosin allele and the white allele have light eosin eye color

      • Two copies of the allele in a female produce a phenotype different than one copy in a male


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Murray Barr and Ewart Bertram (1949) affect early development stages of the embryo

  • 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 affect early development stages of the embryo

    • DNA in inactivated X chromosomes becomes highly compacted

      • A Barr body is formed

    • Most genes cannot be expressed


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  • XX females affect early development stages of the embryo 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 affect early development stages of the embryo

  • Occurs during gamete formation (before fertilization)

  • Involves a single gene or chromosome

  • Governs whether offspring express maternally- or paternally-derived gene


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Genomic imprinting affect early development stages of the embryo

  • 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 affect early development stages of the embryo

    • 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 affect early development stages of the embryoIgf-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

  • Different results in reciprocal crosses generally indicate sex-linked traits but in this case, it indicates genomic imprinting of autosomal alleles


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  • The imprint of the affect early development stages of the embryoIgf-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 affect early development stages of the embryo

    • 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 affect early development stages of the embryo

    • 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 affect early development stages of the embryo

  • Most genes are found in the cell’s nucleus

  • Some genes are found outside of the nucleus

    • Some organelles possess genetic material

    • Resulting phenotypes display non-Mendelian inheritance patterns

      • “Extranuclear inheritance”

      • “Cytoplasmic inheritance”


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  • Mitochondria and chloroplasts possess DNA affect early development stages of the embryo

    • 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 species

    • 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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  • Pigmentation in displays non-Mendelian inheritanceMirabilis 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 displays non-Mendelian inheritanceMirabilis 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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