chapter 12 the chromosomal basis of inheritance. timeline 1866- mendel's paper 1875- mitosis...
TRANSCRIPT
Timeline
• 1866- Mendel's Paper
• 1875- Mitosis worked out
• 1890's- Meiosis worked out
• 1902- Sutton, Boveri et. al. connect chromosomes to Meiosis.
Sutton
• Developed the “Chromosome Theory of Inheritance”.
• Mendelian factors or alleles are located on chromosomes.
• Chromosomes segregate and show independent assortment.
Morgan
• Chose to use fruit flies as a test organism in genetics.
• Allowed the first tracing of traits to specific chromosomes.
Genetic Symbols• Mendel - use of uppercase or
lowercase letters.T = tallt = short
• Morgan: symbol from the mutant phenotype.+ = wild phenotype
Examples
• Recessive mutation:• w = white eyes• w+ = red eyes
• Dominant Mutation• Cy = Curly wings• Cy+ = Normal wings
The F1 offspring:
• All had red eyes.
• This suggests that white eyes is a genetic _________?
• Recessive.
F1 X F1 = F2
• Morgan expected the F2 to have a 3:1 ratio of red:white
• He got this ratio, however, all of the white eyed flies were MALE.
• Therefore, the eye color trait appeared to be linked to sex.
Morgan discovered:
• Sex linked traits.
• Genetic traits whose expression are dependent on the sex of the individual.
Morgan Discovered
• There are many genes, but only a few chromosomes.
• Therefore, each chromosome must carry a number of genes together as a “package”.
Linked Genes• Traits that are located on the same
chromosome.• Result:
• Failure of Mendel's Law of Independent Assortment.
• Ratios mimic monohybrid crosses.
Example
b+b vg+vg X bb vgvg
(b+ linked to vg+)
(b linked to vg)
If unlinked: 1:1:1:1 ratio.
If linked: ratio will be altered.
Crossing-Over
• Breaks up linkages and creates new ones.
• Recombinant offspring formed that doesn't match the parental types.
Linkage Strength
• Degree of strength related to how close the traits are on the chromosome.• Weak - farther apart
• Strong - closer together
Genetic Maps• Constructed from crossing-over
frequencies.
• 1 map unit = 1% recombination frequency.
Sex Linkage in Biology• Several systems are known:1. Mammals – XY and XX2. Diploid insects – X and XX3. Birds – ZZ and ZW4. Social insects – haploid and diploid
Chromosomal Basis of Sex in Humans
• X chromosome - medium sized chromosome with a large number of traits.
• Y chromosome - much smaller chromosome with only a few traits.
Human Chromosome Sex
• Males - XYFemales - XX
• Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.
SRY• Sex-determining Region
Y chromosome gene.• If present - male • If absent - female• SRY codes for a cell receptor.
Sex Linkage
• Inheritance of traits on the sex chromosomes.
• X- Linkage (common)
• Y- Linkage (very rare if exists at all)
Males• Hemizygous - 1 copy of X
chromosome.
• Show ALL X traits (dominant or recessive).
• More likely to show X recessive gene problems than females.
X-linked Disorders
• Color blindness
• Duchenne's Muscular Dystrophy
• Hemophilia (types a and b)
• Immune system defects
X-linked Patterns• Trait is usually passed from a
carrier mother to 1/2 of sons.
• Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait.
Comment
• Watch how questions with sex linkage are phrased:
• Chance of children?
• Chance of males?
Y-linkage• Hairy ear pinnae.
• Comment - new techniques have found a number of Y-linked markers that can be shown to run in the males of a family.• Ex: Jewish priests
Sex Influenced Traits
• Traits whose expression differs because of the hormones of the sex.
• These are NOT on the sex chromosomes.
• Ex. – beards, mammary gland development, baldness
Baldness• Testosterone – the trait act as a
dominant.• No testosterone – the trait act as a
recessive.• Males – have gene = bald• Females – must be homozygous to
have thin hair.
Barr Body
• Inactive X chromosome observed in the nucleus.
• Way of determining genetic sex without doing a karyotype.
Lyon Hypothesis• Which X inactivated is random.• Inactivation happens early in
embryo development by adding CH3 groups to the DNA.
• Result - body cells are a mosaic of X types.
Number Alterations
• Aneuploidy - too many or too few chromosomes, but not a whole “set” change.
• Polyploidy - changes in whole “sets” of chromosomes.
Nondisjunction
• When chromosomes fail to separate during meiosis
• Result – cells have too many or too few chromosomes which is known as aneuploidy
Meiosis I vs Meiosis II
• Meiosis I – all 4 cells are abnormal
• Meiosis II – only 2 cells are abnormal
Aneuploidy
• Caused by nondisjunction, the failure of a pair of chromosomes to separate during meiosis.
Turner Syndrome
• 2N - 1 or 45 chromosomesGenotype: X_ or X0.
• Phenotype: female, but very poor secondary sexual development.
Characteristics• Short stature.• Extra skin on neck.• Broad chest.• Usually sterile• Normal mental development except
for some spatial problems.
Question
• Why are Turner Individuals usually sterile?
• Odd chromosome number.
• Two X chromosomes need for ovary development.
Kleinfelter Syndrome• 2N + 1
• Genotype: XXY
• Phenotype: male, but sexual development may be poor. Often taller than average, mental development fine, usually sterile.
Meta female
• 2N + 1 or 2N + 2
• Genotype: XXX or XXXX
• Phenotype: female, but sexual development poor. Mental impairment common.
Trisomy events
• Trisomy 21: Down's Syndrome
• Trisomy 13: Patau Syndrome
• Both have various physical and mental changes.
Question?
• Why is trisomy more common than monosomy?
• Fetus can survive an extra copy of a chromosome, but being hemizygous is usually fatal.
Question?
• Why is trisomy 21 more common in older mothers?
• Maternal age increases risk of nondisjunction.
Question?
• In plants, even # polyploids are often fertile, why odd # polyploids are sterile. Why?
• Odd number of chromosomes can’t be split during meiosis to make spores.
Result
• Loss of genetic information.
• Position effects: a gene's expression is influenced by its location to other genes.
Cri Du Chat Syndrome
• Part of p arm of #5 missing.
• Good survival, but low birth weight and slow gain.
• Severe mental impairment.
• Small sized heads common.
Philadelphia Chromosome
• An abnormal chromosome produced by an exchange of portions of chromosomes 9 and 22.
• Causes chronic myeloid leukemia.
Parental Imprinting of Genes
• Gene expression and inheritance depends on which parent passed on the gene.
• Usually caused by different methylations of the DNA.
Example:
• Prader-Willi Syndrome and Angelman Syndrome
• Both lack a small gene region from chromosome 15.• Male imprint: Prader-Willi
Female imprint: Angelman
Cause:
• Imprints are "erased" in gamete producing cells and re-coded by the body according to its sex.
• Gametes are methylated to code as “male “ or “female”.
Result
• Phenotypes don't follow Mendelian Inheritance patterns because the sex of the parent does matter.
Extranuclear Inheritance
• Inheritance of genes not located on the nuclear DNA.
• DNA in organelles.• Mitochondria
• Chloroplasts
Result
• Mendelian inheritance patterns fail.
• Maternal Inheritance of traits where the trait is passed directly through the egg to the offspring.
Chloroplasts
• Gives non-green areas in leaves, called variegation.
• Several different types known.
• Very common in ornamental plants.
Mitochondria
• Myoclonic Epilepsy
• Ragged Red-fiber Disease
• Leber’s Optic Neuropathy
• All are associated with ATP generation problems and affect organs with high ATP demands.
Comment
• Cells can have a mixture of normal and abnormal organelles.
• Result - degree of expression of the maternal inherited trait can vary widely.