non mendelian inheritance patterns
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Non Mendelian Inheritance Patterns. 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 - PowerPoint PPT PresentationTRANSCRIPT
Non Mendelian Inheritance Patterns
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Mendelian inheritance patternsInvolve genes directly influencing traitsObey Mendel’s laws
Law of segregationLaw of independent assortment
IncludeDominant / recessive relationshipsGene interactionsPhenotype-influencing roles of sex and environment
Most genes of eukaryotes follow a Mendelian inheritance pattern
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NON-MENDELIAN INHERITANCE
Many genes do not follow a Mendelian inheritance patternWe will discuss additional non-Mendelian inheritance
patternsSex-linked traitsIncomplete DominanceCo-dominanceMultiple allelesPolygenic InheritanceGene linkageEpigenetic inheritance (epistasis)PleiotropyExtra-nuclear inheritance
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NON-MENDELIAN INHERITANCE
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.
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SEX LINKED TRAITS
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SEX LINKED TRAITSIt 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
X-linked recessive, carrier mother
Unaffectedson
Unaffecteddaughter
Affectedson
Carrierdaughter
Unaffectedfather
Carriermother
Carrier
Affected
Unaffected
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Example: Color Blindness
Various tests for color blindness.
Neither allele is dominant so there is a blending of traits when two different alleles for the same trait occur together.
Colors blend togetherThere is a third phenotype
in heterozygote individuals
INCOMPLETE DOMINANCE
red whitepink
Both alleles are dominant and affect the phenotype in two different but equal ways
Traditional example is human blood type
CODOMINANCE
CODOMINANCEAndalusian chickens also show this pattern of inheritance.If you cross a black (BB) chickenWith a white (WW) chickenYou get black+white speckled (BW) chicken
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
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MULTIPLE ALLELES
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MULTIPLE ALLELES
Dad = A/O and
Mom = B/O
Mom
B O
DadA A/B A/O
O O/B O/O
A singe gene influences more than one phenotypic trait.
Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic
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PLEIOTROPY
Multiple genes have an additive effect on a single character in the phenotypeExample: Skin Color
or heightUsually is described
by a bell-shaped curve with majority clustered in the middle
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POLYGENIC INHERITANCE
Genes are located on the same chromosomeAlleles cannot separate according to the laws of
Independent Assortment and Random SegregationUnless…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.
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
GENE LINKAGE
Maternal effectInheritance pattern for certain nuclear genesGenotype of mother directly determines phenotype of
offspringGenotype of father and offspring are irrelevant
Explained by the accumulation of gene products mother provides to developing eggs
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MATERNAL EFFECT
A. E. Boycott (1920s)First to study an example of maternal effectInvolved morphological features of water snail
Limnea peregraShell and internal organs can be either right- or left-handed
Dextral or sinistral, respectivelyDetermined by cleavage pattern
of egg after fertilizationDextral orientation is more
common and dominant
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MATERNAL EFFECT
Epigenetic inheritanceWhen 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 lifetimeModification occurs to a nuclear gene or chromosomeExpression is not permanently changed over multiple
generationsDNA sequence is not altered
Example: albinism
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EPIGENETIC INHERITANCE
Two types of epigenetic inheritanceDosage compensation
Offsets differences in the number of sex chromosomesOne sex chromosome is altered
Genomic imprintingOccurs during gamete formationInvolves a single gene or chromosomeGoverns whether offspring express maternally- or paternally-
derived gene
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EPIGENETIC INHERITANCE
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 chromosomesThe level of expression
of many genes on sex chromosomes is similar in both sexes
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DOSAGE COMPENSATION
X chromosome inactivationDNA in inactivated X chromosomes becomes highly
compactedA Barr body is formed
Most genes cannot be expressed
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DOSAGE COMPENSATION
Apricot eye color in DrosophilaConferred by an X-linked geneHomozygous females resemble males (only one X
chromosome)Females heterozygous for the apricot allele and a deletion
have paler eye colorTwo copies of the allele in a female produce a phenotype
similar to one copy in a maleThe difference in gene dosage is being compensated at the
level of gene expression
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DOSAGE COMPENSATION
Sex in birds is determined by Z and W sex chromosomesMales are ZZ, females are ZWThe Z chromosome is large
Contains most sex-linked genesThe W chromosome is a smaller microchromosome
Contains a large amount of non-coding repetitive DNADosage compensation usually occurs, but not for all genes
Molecular mechanism is not understoodHighly compacted chromosomes are not seen in malesPerhaps genes on both Zs are downregulatedPerhaps genes on females Z are upregulated
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DOSAGE COMPENSATION
Genetic control of X inactivationHuman cells (and those of other mammals) possess the
ability to count their X chromosomesOnly one is allowed to remain active
XX females 1 Barr bodyXY males 0 Barr bodiesXO 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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DOSAGE COMPENSATION
Genomic imprintingMethylation 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
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DOSAGE COMPENSATION
Most genes are found in the cell’s nucleusSome genes are found outside of the nucleus
Some organelles possess genetic materialResulting phenotypes display non-Mendelian inheritance
patterns“Extranuclear inheritance”“Cytoplasmic inheritance”
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EXTRANUCLEAR INHERITANCE
Mitochondria and chloroplasts possess DNACircular chromosomes resemble smaller versions of
bacterial chromosomesLocated in the nucleoid region of the organelles
Multiple nucleoids often presentEach can contain multiple copies
of the chromosome
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EXTRANUCLEAR INHERITANCE