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