Download Intermediate Inheritance or Incomplete Dominance

Intermediate Inheritance or
Incomplete Dominance
Not all patterns of inheritance obey the principles of
Mendelian genetics.
In fact, many traits occur due to a combined
expression of alleles.
Ex.: Crossing pure red (RR) and pure white (WW)
Japanese four-’clocks results in flowers that are all
pink (RW)
To know if something is dominant, co dominant or
incomplete dominance, you must look at the
phenotype.
The ‘Big Idea’:
The structure of DNA, the molecule of heredity,
enables the molecule to copy itself.
Why was Mendel successful with the pea?
• Used pure breeding, contrasting varieties
• Studied characteristics one at a time for
many generations
• Used statistics in analyzing his results
• Obtained large numbers of offspring
• Chose pea plants which normally selffertilize
**Mendel had no knowledge of genes or
chromosomes
But what are genes made of?
• The chromosome theory of heredity states:
– Genes are small particles located on the
chromosomes
• The location of genes within the chromosomes,
however, was less certain.
• Biologist knew that chromosomes contain both
protein and DNA, but they didn’t know which one
made up the genes and directed protein synthesis.
Frederick Griffith
• In 1928, Griffith worked with bacteria called Streptococcus pneumoniae.
• There were 2 strains:
– Strain S – looked smooth because of a mucus coat – caused pneumonia and
death when injected into mice
– Strain R – looked rough (no mucus coat) – had no visible effect when injected
into mice
– Heating strain S bacteria killed them and injections of the heat-killed S bacteria
did not harm mice.
• Injected a mix of heat-killed S bacteria and living R bacteria into mice
• Hypothesized mice would not be affected by the mixture
• However – mice died of pneumonia
• What killed the mice?
• Allowed bacteria from dead mice to reproduce
• Offspring had the mucous coats
• Reasoned somehow a transforming material passed from heat-killed S bacteria to
living R changing R into S
• Concluded the transforming material was genetic material
Oswald Avery
• 1944 - Repeated Griffith’s work to determine
which molecule in the heat-killed bacterial was
most important for transformation
• Avery and other scientists discovered that DNA is
the nucleic acid that stores and transmits the
genetic information from one generation of an
organism to the next
• What is the molecular structure of genes
• How do they coordinate roles of
transmitting information generation to
generation
• What is the role of other nucleic acid,
RNA?
Edwin Chargaff
• 1952 - Showed that the percentages of
guanine and cytosine in DNA are almost
equal
• The same is true of adenine and thymine
• Known as Chargaff’s Rule
Maurice Wilkins and Rosalind Franklin
• 1952 work with X-ray diffraction led to the
discover of DNA’s structure
Hershey and Chase
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In 1952 conducted series of experiments using virus that attack bacteria
Used a virus with a DNA core surrounded by a protein coat
Labeled protein with radioactive sulfur (35S)
Virus allowed to infect bacteria
Bacteria did not show marker
Labeled DNA with 32P
Once DNA inside bacteria cell, viral DNA took over and made new virus particles.
Viral DNA offspring also showed presence of radioactive isotope of P.
Clear that DNA, NOT protein, directed synthesis of new protein and transmitted
virus features to succeeding generation.
• Convinced scientists that DNA was the genetic material.
James Watson and Francis Crick
• 1953 - Based on their knowledge of DNA’s
chemical nature and the X-ray diffraction,
formulated hypothesis that the DNA molecule is
made up of 2 chains twisted around each other in a
helical structure with pairs of nitrogenous bases
projecting toward each other
• Adenine will bond to thymine
• Guanine will bond to Cytosine
• Genes are considered to be segments of these
molecules with the sequence of bases coding for the
amino acids in protein
• Where is DNA found in the cell?
• How is it organized?
• Where are the genes that Mendel first
described a century and a half ago?
Prokaryote Cells
• lack nuclei and membrane bound organelles
• DNA located in cytoplasm
• have a single circular DNA
Eukaryotic Cells
• 1000 times the amount of DNA as prokaryotes
• DNA located in nucleus in form of a number of
chromosomes
• DNA molecules long
• chromosomes contain both DNA and proteinchromatin
• Chromatin = DNA tightly coiled around proteins
called histones
Masses of chromatin fibers
Chromatin fiber is a tightly wound supercoil
Each supercoil is made up of coils. A coil is a chain of
beadlike structures called nucleosomes