Download Genetics Lecture Part 2

Ch15 Lecture
I. Chromosomal Theory of inheritance: mendelian genes have specific loci along chromosomes which
undergo segregation and independent assortment
Mendel picked peas because of the quantity of varied traits
Ex. Be familiar with Morgan’s experiment with Drosophila
Prolific breeders every two weeks
only 4 pairs of chromosomes
Wild type vs. mutant
For flies the gene symbol is for the mutant Ex. Eye color w (white) w+ (red)
Red is dominant to white and sex linked
II. Sex linked genes exhibit unique patterns of inheritance
Different chromosomal systems for sex discrimination
A. Inheritance of sex linked genes (on X or Y)
a. Color blindness
b. Hemophilia: missing one or more blood clotting protein
c. Duchene muscular dystrophy : weakening of the muscles and loss of coordination
due to the absence of a key muscle protein: dystrophin
B. X inactivation
a. Barr bodies
i. Which X chromomes condenses is random in the embryonic tissue
ii. Meaning women that are heterozygotes for a sex linked trait have cells
expressing different genes = MOSAIC
iii. Inactivation = METHYLATION and XIST gene ( X inactive specific
transcript) active on the Barr body
C. Gene linkage
a. Genetic recombination: cross over
b. Gene mapping
i. Linkage map: a genetic map based on recombination frequencies
a. 1 map unit – 1% recombination frequency
b. If the recombination frequencies = 50 % really not linked
Linkage map is not real units but gives the order of genes
III. Alterations of chromosome number or structure cause some genetic disorders
A. Abnormal Chromosome Number
a. Meiotic nondisjunction (can occur on mitosis: so a tissue may have this )
a. Aneuploidy (wrong numberof chromosomes)
b. Monosomic (2n-1)
c. Trisomic (2n+1)
d. Polyploidy ( more than 2 complete sets of chromosomes)
i. Triploidy (3n)
1. fertilization of a 2n egg
ii. Tetraploidy
1. 2n zygote not dividing after S phase = 4n
***Polyploidy common in plants and responsible for diversity /evolution of plants
Bananas (3n) wheat (6n)
Q: what is the significance that there are more organisms with polyploidy versus
aneuploids?
More normal appearance if there is a complete extra set versus one or two
B. Alterations of Chromosome Structure: errors in meiosis or damaging agents : radiation
1. deletion
Non reciprocal cross over
2. Duplication
3. inversion
4. translocation
Human Disorders Due to Chromosomal Alterations:
Down Syndrome : trisomy 21
0.4% if mother is under 30 and .92% if over 40
Aneuploidy of sex chromosomes:
Y has relatively little genes
X you really only need one
Klinefelter Syndrome: XXY males 1 in 2,000 have sterile
small male sex organs
XYY have no real difference form males
XXX are normal phenotype
XO (only only viable monosomy in humans)
Disorders caused by Structurally Altered Chromosomes
Deletions even in heterozygotes even has severe problems
1. cri du chat (deletion if chromosome 5) die in infancy usually
Translocations: certain cancers
1. chronic myelogenous leukemia (CML): reciprocal translocation
occurs between a large piece of chromosome 22 and a small piece
on tip 9 = much shorter chromosome 22 (Philadelphia
chromosome) in white blood cells
IV. Some inheritance patterns are exceptions to the standard chromosome theory
Normally occurring exceptions : sex of parent contributing an allele is a factor
1. Genomic imprinting: genes inside nucleus
a. 2 to 3 dozen traits in mammals that depend on which parent passed the trait
b. Occurs during gamete formation = results in silencing one of the alleles
c. Imprinted differently in sperm and eggs
d. A zygote only expresses one allele (from mom or dad)
e. Imprints are erased during the next generation of gamete formation and newly
imprinted according to the sex of the imprint
f. methylation
Insulin like growth factor 2 (Igf2): although required for normal fetal
growth only the paternal a (-CH3 added to cystine) allele is expressed
2. Inheritance of organelle genes: gene outside nucleus
a. Extranuclear or cytoplasmic genes
i. Mitochondria, chloroplasts and plant plastids = plasmids
ii. Egg holds organelles = maternal
iii. Mitochondrial genes makeup ETC and ATP synthase
1. defects would cause less available ATP
2. nervous system and muscles mostly affected
a. mitochondrial myopathy (weakness, intolerance of
exercise and muscle deterioration)
b. Lebers hereditary optic neuropathy (blindness)
c. Maternal: diabetes, heart disease and Alzheimers ?
d. Mitochondria accumulate lots of mutations = AGING
process
Processes that do not seem to follow the laws of genetics are caused by the fact that there are
more than one gene on each chromosome and errors in replication : nondisjunction. Cross over,
breakage etc…
1. sex linkage
2. epistasis
PV92 Alu