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

  1. Inheritance of sex linked genes (on X or Y)
  2. Color blindness
  3. Hemophilia: missing one or more blood clotting protein
  4. Duchene muscular dystrophy : weakening of the muscles and loss of coordination due to the absence of a key muscle protein: dystrophin
  1. X inactivation
  2. Barr bodies
  3. Which X chromomes condenses is random in the embryonic tissue
  4. Meaning women that are heterozygotes for a sex linked trait have cells expressing different genes = MOSAIC
  5. Inactivation = METHYLATION and XIST gene ( X inactive specific transcript) active on the Barr body
  1. Gene linkage
  2. Genetic recombination: cross over
  3. Gene mapping
  4. Linkage map: a genetic map based on recombination frequencies
  5. 1 map unit – 1% recombination frequency
  6. 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

  1. Meiotic nondisjunction (can occur on mitosis: so a tissue may have this )
  2. Aneuploidy (wrong numberof chromosomes)
  3. Monosomic (2n-1)
  4. Trisomic (2n+1)
  5. Polyploidy ( more than 2 complete sets of chromosomes)
  6. Triploidy (3n)
  7. fertilization of a 2n egg
  8. Tetraploidy
  9. 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
  2. 2 to 3 dozen traits in mammals that depend on which parent passed the trait
  3. Occurs during gamete formation = results in silencing one of the alleles
  4. Imprinted differently in sperm and eggs
  5. A zygote only expresses one allele (from mom or dad)
  6. Imprints are erased during the next generation of gamete formation and newly imprinted according to the sex of the imprint
  7. methylation

Insulin like growth factor 2 (Igf2): although required for normal fetal growth only the paternal a (-CH3 added to cystine) allele is expressed

  1. Inheritance of organelle genes: gene outside nucleus
  2. Extranuclear or cytoplasmic genes
  3. Mitochondria, chloroplasts and plant plastids = plasmids
  4. Egg holds organelles = maternal
  5. Mitochondrial genes makeup ETC and ATP synthase
  6. defects would cause less available ATP
  7. nervous system and muscles mostly affected
  8. mitochondrial myopathy (weakness, intolerance of exercise and muscle deterioration)
  9. Lebers hereditary optic neuropathy (blindness)
  10. Maternal: diabetes, heart disease and Alzheimers ?
  11. 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