Clear and orderly summary of Chapter 4 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.
Simple Mendelian inheritance; dominant/recessive relationships that produces
observed ratios in the offspring.
4.1 Overview of simple inheritance patterns
Several inheritance patterns involving single genes differ from those observed by
Mendel.
Genetics want to understand both inheritance patterns and underlying molecular
mechanisms.
Variety of inheritance patterns – explained by variety of different molecular
mechanism; single gene inheritance;
- Simple dominant/recessive inheritance
- Incomplete penetrance
- Incomplete dominance
- Overdominance
- Codominance
- X-linked inheritance
- Sex-influenced inheritance
- Sex-limited inheritance
- Lethal alleles
4.2 Dominant and recessive alleles
Recessive mutant alleles
Wild-type alleles; prevalent (heersend) in a population
Genetic polymorphism; more than one wild-type allele may occur. (i.e. in flowers both
red and yellow are prevalent
Mutant allele; allele that has been created by altering a wild-type allele
Mutations are more likely to disrupt gene function.
Mutant alleles are defect in ability to express a functional protein. They are rare in
population, inherited in recessive fashion
Why mutant alleles inherited recessively?
1. 50% of the normal protein is enough to accomplish the protein’s cellular
function
2. The heterozygote may actually produce more than 50% of the function protein
Dominant mutant alleles
How can mutant allel be dominant over wild-type allele?
- Gain-of-function mutation; change gene or protein so it gains a new or
abnormal function. (i.e. overespressed or expressed in wrong cell type)
- Dominant negative mutation; produced altered gene product that acts
antagonistically (tegenstellend) to the normal gene
- Haploinsufficiency; dominant mutant allele is loss-of-function allele. Person
with heterozygote exhibits an abnormal or disease phenotype. Allele that
doesn’t produce enough gene product to bring (rode + witte bloem wordt in F1
roze bloem) incomplete dominance
, Incomplete penetrance
Incomplete penetrance; situation in which an allele that is expected to cause a
particular phenotype does not. So, allele that is expected to be expressed is not
expressed. Dominant in heterozygote.
Expressivity; the degree in which the trait (eigenschap) is expressed. Traits may vary
in their expressivity
Incomplete penetrance AND expressivity important!!!
Terminology about different mutations
1. Loss of function (null mutation) (hypomorph)
2. Gain of function (neomorph) (hypermorph)
3. Suppressors mutation – compensate for other mutations mutation here
doesn’t work anymore but second mutation works
4. Enhancer mutations – enhance phenotype of mutation
4.3 Environmental effect on gene expression
Two main factors that determine organism’s traits;
- Genes
- Environment
Environment has effect on phenotype
The outcome of traits is influenced by the environment
Temperature sensitive allele; allele in which resulting phenotype depends on the
environmental temperature
Norm of reaction; effect of environment on organisms’ trait
4.4 Incomplete dominance, overdominance and codominance
Incomplete dominance; heterozygote exhibit phenotype that is intermediate between
corresponding homozygotes (rode + witte bloem = roze bloem in F1)
In this case 50% of the function protein is not enough to give the red color
1:2:1 phenotype NOT 3:1
Trait dominant or incomplete dominant depend on how closely we examine the trait
in the individual. The more closely we look the more likely we discovered that
heterozygote is not quite the same as wild-type homozygote
Overdominance (heterozygote advantage); phenomenon in which a heterozygote
has greater reproductive success than homozygotes
ss sickled red blood cells. Ss NOT sickled red blood cells
At molecular level, overdominance is due to two alleles that
produce slightly different proteins. How can these produce a
favorable phenotype in heterozygotes?
1. Disease resistance (not a real molecular explanation)
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