L296 Gene Dosage and Genomic Imprinting Flashcards
Bi-allelic expression vs mono-allelic expression of genes
- Biallelic expression = genes expressed from both gene copies
Monoallelic expression = genes expressed from only one gene copy
Aneuploidy vs euploidy
- Aneuploidy = abnormal # chromosomes
- Euploidy = abnormal sets of chromosomes
Most aneuploidy embryos survive: T/F?
False - most aneuploidies are incompatible with life
Trisomy vs monosomy
- Trisomy = > 2 copies of a chromosome
- Monosomy = 1 copy of a chromosome
Full-term trisomies
trisomy 13, 18, 21 and XXY, XXXY, XYY etc
Full-term monosomies
XO, no autosomal monosomies
When do we see aneuploidies affecting normal function?
When they affect dose-sensitive genes - that must have a particular range of protein expressed
Trisomy 18 =
Edwards syndrome (47,XX+18 or 47,XY+18)
If a baby with trisomy 18 survives, what does this mean?
not every cell in the body is trisomic: they are mosaics
Trisomy 21 =
Down syndrome (47,XX+21 or 47,XY+21)
Which genes contribute to the development of the down syndrome phenotype?
• Gene loci at many regions of chromosome 21 play a role in the development of the overall phenotype of Down syndrome
Autosomal recessive inheritance: how many altered copies of a gene are required to cause a phenotype?
- Two altered/non-functional copies cause phenotype
- One normal copy is sufficient for cell function
Autosomal dominant inheritance: how many altered copies of a gene are required to cause a phenotype?
- Alteration/loss of a single gene copy causes a specific phenotype
Why might having one altered gene cause a new phenotype (in autosomal dominant inheritance)?
- One altered copy of a gene might have a novel ‘gain-of-function’ to cause phenotype OR
- Having only one normal copy of a gene is not sufficient to support normal cell function (haploinsufficiency)
How might monoallelic gene expression come about?
Inactivation of one of the gene copies
Examples of mechanisms that operate to inactivate a genetic copy for monoallelic gene expression
- X chromosome inactivation in females (epigenetic)
2. Genomic imprinting (epigenetic and genetic)
Epigenetics =
= change in gene expression/repression with no change in DNA sequence, passed on through cell division
X-inactivation: at which cell stage does it occur at, and what does it leave behind?
- Occurs at 8-100 cell stage of embryo
- Barr body = condensed, inactive X chromosome remnant
Comment on the heritability of X-inactivation
Heritable: when cell divides, same chromosome is inactivated i.e. every cell from that cell in chromosomally similar
X-inactivation: does it mean that females only need one X chromosome? Why/why not?
No - some genes must be expressed from the inactivated X e.g. Turner syndrome (45, X) is a monosomy X
What is genomic imprinting?
Describes the process whereby the parental origin of a particular gene is “marked” by a reversible epigenetic mechanism
What do we say expression of genes in genomic imprinting is regulated by?
Parent-of-origin effects: expression of certain genes depends on whether they are inherited from maternal/paternal gamete
When does imprinting occur?
• Imprints are erased during gametogenesis in haploid cell, re-established in a parental-specific pattern in mature gametes and maintained during embryogenesis
Which chromosomes in particular have clusters of imprinted genes?
Chr 11 and 15