liveonearth (liveonearth) wrote,

Genetics (week 4): Inheritance, Autosomal Dominant, Huntington's

start wtih parental genotypes, sort out possible gamete types
if parents Aa and Aa expect offspring in AA, Aa, Aa, aa
genotypic ratio of 1:2:1
phenotypic ratio is 3A:1a (phenotype = genotype + environment)

3 classic monkey wrenches
variable expressivity: phenotype expression mb mild, mod, sev
penetrance: dominant gene may not be expressed, penetrance score in percentages
pleiotropy: one gene may affect multiple traits

Mendel's 1876ish research ignored then rediscovered in 1905
he got lucky, peas are diploid like humans
"if he'd picked cannabis he'd have got nothing done" because it's triploid (marij)
"trait determiners" = his name for genes
things that follow his rules now called "simple genetics"
later: linkage, multifactorial inheritance, these violate Mendel's rules

incl any & all relevant info re: genetic relationships
square: male; circle: female; diamond: gender not indicated
filled: affected
number inside: merged siblings same gender
strike through: dead
still birth: SB underneath
P--> : your patient
--> : focal point of pedigree
triangle: abortion (empty: spontaneous, filled: affected, strike: terminated)
horiz line between pts: relationship line, mb broken with strikes for ended relationship
vert line below: line of descent
horiz btw sibs: sibship line
customary to put male first in couple, first birth first among sibs
line with square btw 2 circles: multiple partners
child from circle with no partner: hidden partner
"you'll do anything to throw people away", lots of people are not represented just to make space
double line btw couple: consanguinity (usu 1st cousins)
line connecting branch: identical twins, no line connecting branch: dizygotic twins
brackets around square/circle: adopted (dotted line connecting adopted to someone nearby on pedigree)
generations roman numeralized, then each in a generation numbered
words like aunt, grandparent, not descriptive enough
pedigrees get huge quickly
building one: start with pt, recrod sibs, then lines of descent to children, parents and all their cnxns
then lines of descent from them
*recommend that we do this for our own families and try to track some characteristic--eye color, alcoholism
usu have to redraw multiple times to get it right

sample Q's:

mc PKU hydroxylase gene at 12q24.1 location and affected are homozygous recessive means: most prevalent genetic variant for enz that converts phenyalanine to tyrosine is on xsome 12, long arm, region 2 band 4.1 and pts with 2 copies of gene express phenylketonuria
q is long arm

mutations come from: radiation (ionizing, ie: cosmic rays, xrays, radioactive), chemicals (polycyclic aromatic hyrocarbons, smoked food, nitrites, alkylating and methylating agents

how can single base pair sub in beta globin gene result in rheumatism?
point mutat-->single aa sub-->abn HGB-->sickling, clumped RBCs-->mm and jt damage-->rheumatic pain dt ischemia

base pair sub might not result in phenotype chance dt:
genetic code degenerative, mutation located in intron, change is in non-critical part of prot

predicted genotypic ratio of offspring from couple when one parent is homozygous dominant and one is heterozygous? draw punnett square if needed, ratio is 1:1

depending on amount of phenylalanine that PKU pt exposted to, sx range from severe to just hypo-pigment, name for this? variable expressivity.

not all pts with dominant gene for polydactyly exhibit trait, this is penetrance. assume it's 80% and one parents is affected heterozygote with 50:50 chance of passing on mutation, what is odds of child having polydactyly? 50% x 80% = 40%

gene for albinism, gene for CF, gene for huntinton's all cause diverse and multiple sx: this is pleiotropy.

draw a pedigree using this data: You are the consultand, you are 1st born child in sibship with 2 younger sisters, your parents are alive, your father had a brother (your paternal uncle) who died of Alz, your mother has a sis (your maternal aunt) and she has a husband who together have two sons (your 1st cousins) and this first born first cousin and his spouds have a new set of identical twin daughters. There is no other info about the family.

the gene is dominant and is on an autosome (not a sex xsome)
Aa and AA both are affected
AA is rare because it's so bad, most affected pts have Aa
most affected persons are HETEROZYGOUS
every affected person has an affected parent: vertical pattern on pedigree
normal children mating with normals will have normal children
if reduced penetrance not all affected pts will have affected parent, occ "skips a generation"
sometimes an affected person will appear with no hx in pedigree: new mutation
dzs: anonychia, cold urticaria, HUNTINTON'S, OSTEOGENESIS IMPERFECTA

OMIM 143100
autosomal dominant
Woody Guthrie
sx: chorea = progressive involuntary twitching, personality change, dementia, death
onset average age ~40yo, fatal within 10 years of onset
incidence: 1/20,000ish, rare in Africans, Asians
etio: mutant gene undergoes trinucleotide repeat/expansion mutation (CAG <40 --> >100)
triplicate repeat dz
appears earlier in each generation due to additional repeats, over 100 repeats onset in 20's
1980's study Wexler and Gusella
dx: genetic testing now possible, double edged sword: incurable
pedigree: quasi-vertical look because sometimes people die before onset, ages are listed
people who marry into affected pedigree highly unlikely to carry the gene, it's rare
kids of affected parent have 50 50 chance of having it
grandkid of affected grandparent has 25% chance if parental status not known
Tags: genetics, marijuana, nd4, pathology

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