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Non Mendelian Inheritance Patterns

Non Mendelian Inheritance Patterns

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Non Mendelian Inheritance Patterns

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  1. Non Mendelian Inheritance Patterns

  2. NON-MENDELIAN INHERITANCE • Mendelian inheritance patterns • Involve genes directly influencing traits • Obey Mendel’s laws • Law of segregation • Law of independent assortment • Include • Dominant / recessive relationships • Gene interactions • Phenotype-influencing roles of sex and environment • Most genes of eukaryotes follow a Mendelian inheritance pattern

  3. NON-MENDELIAN INHERITANCE • Many genes do not follow a Mendelian inheritance pattern • We will discuss additional non-Mendelian inheritance patterns • Sex-linked traits • Incomplete Dominance • Co-dominance • Multiple alleles • Polygenic Inheritance • Gene linkage • Epigenetic inheritance (epistasis) • Pleiotropy • Extra-nuclear inheritance

  4. SEX LINKED TRAITS Characteristics that are inherited from genes found on the sex chromosomes • Autosomal- the allele is on one of the 22 body chromosomes. • Autosomal recessive disorders are just as common in boys as in girls. • Sex-linked- allele is on one of the two sex chromosomes (X and Y) • normally on the X chromosome as the Y chromosome is small and has very few genes. • X-linked disorders occur mostly in boys and very rarely in girls.

  5. X-linked recessive, carrier mother Unaffected father Carrier mother Unaffected Affected Carrier Unaffected son Unaffected daughter Affected son Carrier daughter SEX LINKED TRAITS • It is possible for a female to be a carrier of an X-linked trait, but not express it • Men will express all X-linked traits they inherit U.S. National Library of Medicine

  6. Various tests for color blindness. Example: Color Blindness

  7. INCOMPLETE DOMINANCE • Neither allele is dominant so there is a blending of traits when two different alleles for the same trait occur together. • Colors blend together • There is a third phenotype in heterozygote individuals red pink white

  8. CODOMINANCE • Both alleles are dominant and affect the phenotype in two different but equal ways • Traditional example is human blood type

  9. CODOMINANCE • Andalusian chickens also show this pattern of inheritance. • If you cross a black (BB) chicken • With a white (WW) chicken • You get black+white speckled (BW) chicken

  10. MULTIPLE ALLELES • Multiple alleles are when the gene has more than two versions • The versions may be expressed simultaneously, with more than one dominant and recessive allele • Take Blood type: Type A (IA) and B (IB) are dominant, and can be co-dominant • Type O (represented by i) is recessive • Notice that 3 different alleles combine to form four difference phenotypes


  12. PLEIOTROPY • A singe gene influences more than one phenotypic trait. • Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic

  13. POLYGENIC INHERITANCE • Multiple genes have an additive effect on a single character in the phenotype • Example: Skin Color or height • Usually is described by a bell-shaped curve with majority clustered in the middle

  14. GENE LINKAGE • Genes are located on the same chromosome • Alleles cannot separate according to the laws of Independent Assortment and Random Segregation • Unless…crossing over during meiosis I moves them to a different chromosome. • We measure the distance between genes by the frequency of crossing over moving one of them to a new chromosome, called gene linkage mapping • Distant genes are separated by crossing over more often than nearby genes.

  15. GENE LINKAGE • C is farther away from A than B is • We figure this out because a higher percentage of gametes are ABc than are Abc. • In fruit flies, wings and body color are linked

  16. MATERNAL EFFECT • Maternal effect • Inheritance pattern for certain nuclear genes • Genotype of mother directly determines phenotype of offspring • Genotype of father and offspring are irrelevant • Explained by the accumulation of gene products mother provides to developing eggs

  17. MATERNAL EFFECT 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- or left-handed • Dextral or sinistral, respectively • Determined by cleavage pattern of egg after fertilization • Dextral orientation is more common and dominant

  18. EPIGENETIC INHERITANCE • Epigenetic inheritance • When changes in phenotype (appearance) are caused by mechanisms other than changes in the underlying DNA sequence. • Gene expression is altered: a gene at one locus alters the effects of a gene at another locus • May be fixed during an individual’s lifetime • Modification occurs to a nuclear gene or chromosome • Expression is not permanently changed over multiple generations • DNA sequence is not altered • Example: albinism

  19. EPIGENETIC INHERITANCE • Two types of epigenetic inheritance • Dosage compensation • Offsets differences in the number of sex chromosomes • One sex chromosome is altered • Genomic imprinting • Occurs during gamete formation • Involves a single gene or chromosome • Governs whether offspring express maternally- or paternally-derived gene

  20. DOSAGE COMPENSATION • Males and females of many species have different numbers of certain sex chromosomes. Only one copy of each chromosome is expressed; the other is deactivated. • e.g., X chromosomes • The level of expression of many genes on sex chromosomes is similar in both sexes

  21. DOSAGE COMPENSATION • X chromosome inactivation • DNA in inactivated X chromosomes becomes highly compacted • A Barr body is formed • Most genes cannot be expressed

  22. DOSAGE COMPENSATION • Apricot eye color in Drosophila • Conferred by an X-linked gene • Homozygous females resemble males (only one X chromosome) • Females heterozygous for the apricot allele and a deletion have paler eye color • Two copies of the allele in a female produce a phenotype similar to one copy in a male • The difference in gene dosage is being compensated at the level of gene expression

  23. DOSAGE COMPENSATION • Sex in birds is determined by Z and W sex chromosomes • Males are ZZ, females are ZW • The Z chromosome is large • Contains most sex-linked genes • The W chromosome is a smaller microchromosome • Contains a large amount of non-coding repetitive DNA • Dosage compensation usually occurs, but not for all genes • Molecular mechanism is not understood • Highly compacted chromosomes are not seen in males • Perhaps genes on both Zs are downregulated • Perhaps genes on females Z are upregulated

  24. DOSAGE COMPENSATION • Genetic control of X inactivation • Human cells (and those of other mammals) possess the ability to count their X chromosomes • Only one is allowed to remain active • 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)

  25. DOSAGE COMPENSATION • Genomic imprinting • Methylation generally inhibits expression • Can enhance binding of transcription-inhibiting proteins and/or inhibit binding of transcription-enhancing proteins • Methylation can increase expression of some genes

  26. EXTRANUCLEAR INHERITANCE • 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”

  27. EXTRANUCLEAR INHERITANCE • 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