Clear and orderly summary of Chapter 5 of the book "Genetics: Analysis and Principles, 6th Edition by Robert Brooker". Together with all my other summaries of Genetics I got an 8,5 for this course.
Mendelian inheritance patterns that directly influence outcome of offspring’s trait
and obey Mendel’s laws
Mendelian inheritance 4 rules
1. Expression of genes in offspring directly influence traits
2. Except of rare mutations, genes passed unaltered from generation to
generation
3. Genes obey Mendel’s law of segregation
4. Crossing involving 2 or more genes, genes obey Mendel’s law of independent
assortment
5.1 Maternal effect
Rule 1 of Mendelian is broken
Maternal effect; genotype of the mother directly determines phenotype of the
offspring. Watch out; genotype of father does NOT affect phenotype of offspring.
So, NO Punnett square
Genotype of the mother determines phenotype of offspring
Reciprocal cross is in contradict of Mendelian pattern of inheritance
Female gametes receive gene product form mother
This non-Mendelin inheritance pattern can be explained by
the process of oogenesis in female animals.
Surrounding maternal cells (nurse cells) provide oocyte
with nutrients
Normal haploid oocyte receives D or d allele from nurse
cells but NOT both.
Now haploid oocyte receives D and d allele from nurse
cells. This gene product persists (aanhouden) for a time
after egg fertilized and embryonic development has begun.
So, the gene product of nurse cells, which reflect genotype
of mother influence early developmental stages of embryo
Snail coiling;
- Female with DD offspring D right-handed body plan
- Female with Dd offspring D right-handed body plan
- Female with dd offspring d left-handed body plan
The coiling can be traced to the orientation of the mitotic spindle
Watch out; sperm genotype is irrelevant, because sperm’s gene will occur too late
Maternal effect genes paly role in
- Cell division
- Cleavage pattern
- Body axis orientation
, 5.2 Epigenetic inheritance; Dosage compensation
Rule 2 of Mendelian is broken
Epigenetic; study that leads to changes in gene expression that can be passed from
cell to cell and are reversible, but do not involve a change in DNA sequence.
It is not permanent over course of many generations; the changes can be fixed
during individual’s lifetime.
Epigenetic effects result of DNA and chromosomal modifications that occur during
oogenesis, spermatogenesis or early stage of embryogenesis.
Dosage compensation = epigenetic change
Dosage compensation; level of expression of genes on sex chromosomes is similar
in both sexes. It occurs in species in which males and females differ in their sex
chromosomes.
Occurs via different mechanisms
- X-chromosome inactivation (XCI); female equalize expression of X-linked
genes by turning off one of their 2 X-chromosomes in somatic cells.
- Doubling expression of X-linked genes; males double the X-linked gene
fruit flies
Male birds; ZZ Female birds; ZW
Dosage compensation does NOT occur in birds
X-chromosome inactivation (XCI) lyon hypothesis
The process of X-chromosome inactivation in females compensate for the single X
chromosomes found in males.
One of X-chromosomes is inactivated randomly during early stage of embryonic in
each somatic cell and become Barr body.
Barr body; structure in somatic cells of female that is highly condensed
(geconcentreerd) X-chromosomes
This process can lead to variegated (afwiksselend) phenotype
When cell division occurs at inactivated X-chromosomes both
copies remain highly compacted and inactivated. So, X-
chromosome inactivation is maintained when somatic cells
divide
X-chromosome inactivation controlled by X-inactivation center
Xic
3 phases (see chapter 16)
1. Initiation
2. Spreading phase
3. Maintenance; occurs in adult female
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