Ch 4 Part 4: modifications of mendelian ratios (SEPT 22nd)

What do genes express in response to

Genes are expressed in response to both the genetic environment and physical environment of the organism

Define Penetrance

what is the expression like

Frequency that identical genotypes are expressed

-expression is binary: it’s either expressed or not expressed

Define Expressivity

what is the expression like

Degree to which genes are expressed for known identical genotypes

-expression by degree

Define incomplete penetrance

Individuals that carry a specific disease-causing mutation will not always develop symptoms of disease

What is an example of incomplete penetrance…

how many show the trait and what type of allele is it?

what causes low penetrance

Brachydactyly in humans.

-50%-80% of people with dominant allele show trait

-genes that give susceptibility for cancer have low penetrance

What is an example of expressivity…

what is the penetrance, what type of allele is it in?

symptom

how is it manifested

Osteogenesis imperfecta

-autosomal dominant. has 100% penetrance

-failure to make functional collagen

-manifested in differing degrees

Compare complete and incomplete penetrance

Complete penetrance: Identical known genotypes yield 100% expected phenotype

Incomplete penetrance: Identical known genotypes yield less than 100% expected phenotype

Compare constant expressivity and variable expressivity

Constant: Identical known genotypes with no expressivity effect yield 100% expected phenotype

Variable: Idenitcal known genotypes with an expressivity effect yield a range of phenotypes

Describe incomplete penetrance with variable expressivity

Identical known genotypes produce a broad range of phenotypes. due to varying degrees of gene activation and expression

Define sex limitation

Autosomal genes may be expressed in one sex only

Examples of sex limitation and which sex

-Females dont get testicular cancer

-Males dont get uterine cancer

-milk production in mammals

-horn production in sheep

-distribution of facial and bodily hair in humans

Define sex influence

Differences in expressivity or penetrance between sexes

What is an example of sex influence

male pattern baldness

What occurs with baldness in humans…

what type of chromosome is it

what differs between men and women

what happens in men

what happens in women

-It is Autosomal

-Relationship between genotype and phenotype differs between men and women

-males: b allele is dominant, resulting in male pattern baldness

-females: b allele is recessive, pattern of baldness is disorganized

What does hair loss further vary in

penetrance and expressivity

Describe how temperature is an environmental effect

Temperature: Mutations may result in temperature sensitivity of ensymes

How does temperature affect siamese cats

In siamese cats, expression of black pigmentation in fur is temperature dependent

-a tyrosinase mutation causes failure to function at high temperature, inability to produce melanin

-tyrosinase will function at cooler extremities

Describe how position is an environmental effect… 

what does this mean with heterochromatin

Position: Location of a gene in the chromosome may influence the level of expression

-heterochromatin (tightly packed, no gene expression): DNA with high repeat content, low levels of expression

How does position affect fruit fly eye pigmentation

inserting repeats near genes that code for red pigment mutations, will result in varied expression of red pigment

Describe how age is an environmental effect

Aging results in changes in internal environment

-little is known regarding the mechanism by which aging induces changes in expression

What are some examples connected to age being an environmental effect

at what age is it present in

-male pattern baldness (20-30 years)

-Duchenne muscular dystrophy (5-6 years)

(a fatal disorder in which muscles begin to weaken in early childhood)

Define Genetic anticipation

Age of onset increases/changes over generations, not an environmental effect

Define myotonic dystrophy, an example of genetic anticipation

mutations in one of two genes that promote communication between muscle cells and neurons

- slip-strand mutations produce repeats that elongate the coding sequence in serial

-increased repeats increase the probability of further slip-strand error

-when DNA is repeated serially, replicate DNA can “loop out” and still avoid error repair function of polymerases

How does myotonic dystrophy affect later generations due to genetic anticipation

-where the condition becomes more severe and appears at an earlier age in successive generations.

-Anticipation is due to the DMPK gene having an unstable CTG trinucleotide repeat that expands in length during meiosis.

-The increased repeat count in offspring results in more severe symptoms.

Myotonic dystrophy generational example, include generation number, repeat number and phenotypic effects

Gen 1: few repeat number, cataracts late in life

Gen 2: more repeat number, muscle weakness in adults

Gen 3: even more repeat number, cognitive disability in children

Gen 4: many repeat number, developmental failure in embryogenesis