today: genomic imprinting and epigenetics. haploid diploid x 23 in humans x 23 in humans x 23 in...
TRANSCRIPT
haploid
diploid
X 23in humans X 23
in humans
X 23in humans
Inheritance = The interaction between genes inherited from Mom and Dad.
Sex-linked traits: Genes on the X chromosome
No one affected, female carriers
50% of males affected, 0 % female affected
50% males affected, 50% females affected
A= normal ; a= colorblind
similar to Fig 4.13
At an early stage of embryonic development
The epithelial cells derived from this
embryonic cell will produce a patch of
white fur
While those from this will produce a patch of black fur
Fig 7.4
Promotes compaction
Prevents compaction
Mammalian X-inactivation involves the interaction of 2 overlapping genes.
The Barr body is replicated and both
copies remain compacted
Barr body compaction is heritable within an individual
• A few genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals– Pseudoautosomal genes
(Dosage compensation in this case is unnecessary because these genes are located both on the X and Y)
– Up to a 25% of X genes in humans may escape full inactivation
• The mechanism is not understood
Epigenetics: http://www.pbs.org/wgbh/nova/sciencenow/3411/02.html
Lamarck was right? Sort of…
Image from: http://www.sparknotes.com/biology/evolution/lamarck/section2.rhtml
Genomic Imprinting
• Genomic imprinting is a phenomenon in which expression of a gene depends on whether it is inherited from the male or the female parent
• Imprinted genes follow a non-Mendelian pattern of inheritance
– Depending on how the genes are “marked”, the offspring expresses either the maternally-inherited or the paternally-inherited allele **Not both
A hypothetical example of imprinting
A=curly hair
a=straight hair
B=beady eyes
b=normal
*=methylation
A* in males
B* in females
aB*
aB* A*
bA*b
A hypothetical example of imprinting
A=curly hair
a=straight hair
B=beady eyes
b=normal
*=methylation
A* in males
B* in females
A*abB*
A*abB*
aB*
aB* A*
bA*b
A hypothetical example of imprinting
A=curly hair
a=straight hair
B=beady eyes
b=normal
*=methylation
A* in males
B* in females
A*abB*
A*abB*
A*abB
AabB*
aB*
aB* A*
bA*b
A hypothetical example of imprinting
A=curly hair
a=straight hair
B=beady eyes
b=normal
*=methylation
A* in males
B* in females
A*abB*
A*abB*
A*abB
AabB*
A*b, A*B,ab, aB
Ab, AB*,ab, aB*
aB*
aB* A*
bA*b
similar to Fig 7.10
Thus genomic imprinting is permanent in the somatic cells of an animal
–However, the marking of alleles can be altered from generation to generation
• Genomic imprinting must involve a marking process
• At the molecular level, the imprinting is known to involve differentially methylated regions–They are methylated either in the oocyte or
sperm• Not both
Imprinting and DNA Methylation
• For most genes, methylation results in inhibition of gene expression
–However, this is not always the case
Haploid female gametes transmit an unmethylated gene Haploid male gametes transmit
a methylated gene
Fig 7.11Changes in methylation during gamete development alter the imprint
Imprinting plays a role in the inheritance of some human diseases: Prader-Willi syndrome (PWS) and Angelman syndrome (AS)
–PWS is characterized by: reduced motor function, obesity, mental deficiencies
–AS is characterized by: hyperactivity, unusual seizures, repetitive muscle movements, mental deficiencies
Usually, PWS and AS involve a small deletion in chromosome 15
–If it is inherited from the mother, it leads to AS–If it is inherited from the father, it leads to PWS
• AS results from the lack of expression of UBE3A (encodes a protein called EA-6P that transfers small ubiquitin molecules to certain proteins to target their degradation)
– The gene is paternally imprinted (silenced)
• PWS results (most likely) from the lack of expression of SNRNP (encodes a small nuclear ribonucleoprotein that controls gene splicing necessary for the synthesis of critical proteins in the brain)
– The gene is maternally imprinted (silenced)