Week 7 - Cormier Flashcards
Why are single-gene traits often called Mendelian?
They appear in roughly fixed proportions.
~ 4000 human diseases have Mendelian patterns of inheritance
What is a segment of DNA at a specific location called?
locus
What is the sequence of DNA that contains a particular locus?
Gene
What are alternative variants of a gene called?
Alleles
What do variant alleles show?
Polymorphism
What are variant alleles referred to as?
polymorphic alleles
or
polymorphisms
What do some polymorphisms affect?
disease susceptibility
(vs. wildtype)
What does wild-type allele mean?
the single prevailing allel present in the majority of individuals
What do mutations refer to?
New genetic changes in a family and/or to disease-causing mutant alleles.
What does genotype refer to?
An entire set of alleles in a genome or the set of alleles at a specific locus.
What does phenotype refer to?
The observable expression of a genotype as a morphological, clinical, cellular or biochemical trait.
What does homozygous mean?
An individual’s two alleles are functionally identical at a specific locus.
(something you can measure, not just different sequence)
What does heterozygous mean?
The two alleles are functionally different at a specific locus.
(something you can measure, not just different sequence)
What does the term “kindred” refer to in a pedigree?
The extended family depicted in the pedigree.
What is the proband in a pedigree?
The first affected person who is brought to clinical attention.
-all other family members are analyzed in relation to the proband
(there can be multiple probands)
What does the term consanguineous mean?
Couples who have one or more ancestors in common.
What does fitness mean or refer to?
A genetics term that refers to the measure of the impact of a condition or genotype on reproduction and is defined by the number of offspring of affected individuals who survive to reproductive age, compared with an appropriate control group.
What are the four basic patterns of single-gene inheritance?
- Autosomal dominant
- Autosomal recessive
- X-linked dominant
- X-linked recessive
What two factors affect pedigree patterns?
- Penetrance
- Expressivity
What is penetrance? Reduced penetrance?
The probability that a mutant gene will have any phenotypic expression.
Reduced penetrance: when the percentage of individuals demonstrating some disease phenotype is less than 100% the mutant gene.
What is expressivity? Variable expressivity?
The severity of expression of the phenotype among individuals with the same disease causing genotype.
Variable expressivity: when the severity of the disease differs in people who have the smae genotype.
What is neurofibromatosis (NF1) an example of?
Variable expressivity
(due to different mutations in the NF1 gene)
also autosomal dominant disease
What is allelic heterogeneity?
Many loci contain multiple mutant alleles in a population.
(different mutations in same gene)
ex. CFTR & PKU
What is locus heterogeneity?
Many disease phenotypes can be caused by mutations in distinctly different genes.
(different genes with mutations causing common/similar phenotypes)
ex. Retinitis pigmentosa
What is phenotypic heterogeneity?
Different mutations in the same gene cause completely different diseases.
ex. Hirschsprung disease
What does sex influenced mean?
One sex has a significantly high frequency of developing an autosomal recessive disorder.
ex. Hemochromatosis
What does sex-limited mean?
Some autosomal dominant disorder can show a sex ratio that differs from 1:1, and in some cases the trait is seen only in one sex.
ex. Male-limited precocious puberty
(difficult to distinguish from x-linked disorders becaus the trait is transmitted through unaffected carrier females)
What are common characteristics of autosomal dominant inheritance?
- trait appears in every generation
- each affected person has an affected parent
- offspring of an affected parent have a 50% risk of inheriting the trait
- males/females are equally likely to transmit the trait to children of either sex
- exceptions when sex-limted/sex-influenced
How are X-linked dominant and recessive patterns distinguished?
By the phenotype of heterozygous females:
if the trait is consistently expressed in carriers then it is dominant
(if not, then it is recessive)
What are common characteristics of X-linked recessive disorders?
- Much high incidence in males than females
- heterozygote females are usually unaffected
- some may show phenotype depending on pattern of X-inactivation
- Mutant allele is transmitted from an affected male through all of his daughters
- any of his daughter’s sons have a 50% chance of inheriting it
- Mutant allele is NOT transmitted directly from father to son
What are common characteristics of X-linked dominant inheritance?
- Affected males with normal mates have no affected sons but all affected daughters
- Both male and female offspring of affected females have 50% chance of inheriting the trait
- Heterozygous affected females develop a milder phenotype than heterozygous males
- often male lethality
- Pattern of inheritance from an affected female is indistinguishable from autosomal dominance
What are common characteristics of autosomal recessive diseases?
- Occurs only in mutant homozygotes or compound heterozygotes
- affected individuals have two mutant and no normal alleles
- Unaffected heterozygote parentss are called carriers
- Skips generations
What are some of the factors that can complicate pedigree patterns?
- Reduced penetrance
- Reduced expressivity
- Genotypes that may not survive to birth
- Lacking family histories
- False paternity
- Occurence of new mutations
- Small families
- Genetic heterogeneity (allelic, locus, phenotypic)
- Other genes and environmental factors that affect expression
What is mutational mosaicism?
- A mutation occuring during cell proliferation (such as early development) that leads to only a proportion of cells carrying the mutation.
- can occur in germline or somatic cells
- ex. X-inactivation
What is the effect of maternal inheritance of disorders caused by mutations in the mitochondrial genome?
- Mitochondrial disorders can demonstrate a wide range of severity
- segregation of mitochondria during cellular replication is random
- A mutation in mitochondrial DNA may or may not be transmitted to a daughter cell
- The number of mutant mitochondrial in daughter cells vary from one cell to the next
- zygotes inherit their mitochondria only from the egg
All disease have what common component?
All disease have a genetic component.
(according to Dr. Cormier)
What kind of medical era are we entering (according to Dr. Cormier)?
Era of individualized medicine.
Promise of an era of regenerative medicine.
What affects disease susceptibility?
host genetic makeup
What are the five types of genetic polymorphisms?
- insertions
- deletion
- tandem repeat
- single nucleotide polymorphisms
- restriction fragment length polymorphisms
What is Restriction Fragment Length Polymorphism?
- allelic variant that abolishes or generates a restriction endonuclease recognition site or changes the size of an RFLP
What is restriction fragment lenth polymorphism (RFLP) used for?
- use to distinguish between 2 chromosomes
- usually just a biomarker & not a cause of a dysfunctional gene
- analyzed by Southern blotting or PCR
What is variable number of tandem repeats (VNTR)?
- location in a genome where a short nucleotide sequence is organized as a tandem repeat
a. k.a. simple sequence length polymorphisms (SSLPs)
What are variable number of tandem repeats (VNTR) or simple sequence length polymorphisms (SSLPs) used for?
- personal or parental identification
- often polymorphic in size between chromosomes & individuals
- analyzed mainly by PCR