Classical and Modern Genetics. Chapter 23. Great Idea: All living things use the same genetic code to guide the chemical reactions in every cell. Chapter Outline. Classical Genetics DNA and the Birth of Molecular Genetics The Genetic Code. Classical Genetics.
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Classical and Modern Genetics
All living things use the same genetic code to guide the chemical reactions in every cell.
Chapter 23- Part 1 Classical Genetics
Genetics got it’s start as the study of inheritance.Charles Darwin proposed that favorable traits could be passed from generation to generation resulting in natural selection.
However, Darwin did not know how these traits were passed on.
Chromosomes from the Indian muntjak
It remained for the Austrian monk Gregor Mendel, in 1865, to carry out the definitive experiments.
Mendel crossed tall and dwarf pea plants: all offspring were tall.
Next, Mendel crossed some of these F1 plants among themselves. Of these offspring (the F2 generation), about 3/4 of the plants were tall and 1/4 were dwarf.
F2 tall tall tall dwarf
Mendel tested 6 other traits of pea plants:
traits for seed shape (wrinkled or smooth)seed color (yellow or green), etc.
In each case, all of the F1 plants looked as though they had inherited the trait of just one of their two parents, but in the F2 generation both traits always appeared -- and always in a 3 to 1 ratio.
The trait which was expressed in the F1 generation was always about 3 times as numerous in the F2 generation as was the other one which was hidden in the F1's.
Homozygous = same
Heterzygous = different
When both alleles for a trait are identical, say that the organism is homozygous for that trait. When the 2 alleles are different, is heterozygous.
TT = Homozygous Tt = Heterozygous tall tall
Tall is dominant over dwarf; dwarf is said to be a recessive trait (i.e. can only be expressed when there are two copies of it).
Homozygous = same Heterzygous =different
Tall Dwarf Tall
TT tt Tt
Mendel's original cross produced only tall offspring:
However, in the second generation the rules of probability dictate that 1/4 of the plants will be tt = dwarf and 3/4 will have at least one T and hence be tall.
Mendel Studied Many Traits in Pea Plants--
Seed shape- smooth or wrinkled
Seed color- green or yellow
Pod shape- smooth or bumpy
Pod color- green or yellow
Flower location- at leaf or tip of branch
Many traits are passed on by genes.
The genes encode the information for proteins.
The genes are segments of DNA.
Mendel found that two factors determine traits.
These are alternate forms of genes- one from each parent.
These are now called alleles.
Alleles: two different forms of the gene.
For many hereditary traits, genes exist in two or more different forms called alleles. On each pair of chromosomes, there is one allele for a particular gene on each. ex. A, B, O blood groups. In humans there are 3 alleles: A, B, and O.
Genotype AO BO AB OO
Phenotype A B AB O
Genotype- genetic composition
Phenotype- physical characteristics
Genotype AO BO AB OO Phenotype A B AB O
Ex. ABO blood groups. A and B are codominant and O is recessive.
DNA and the Birth of Molecular Genetics
The Genetic Code
The polymerase chain reaction (PCR) copies a sequence of DNA.
(a) A strand of DNA is mixed in solution with DNA precursors (nucleotides), a primer that targets a specific piece of DNA, and an enzyme (polymerase) that helps to assemble DNA. The mix is heated to 200°F to separate DNA strands.
(b) When cooled to 140°F, primers attach to the DNA strands.
(c) At 160°F nucleotides begin to attach to the DNA strands.
(d) At the end you have two copies of the desired DNA.
DNA fingerprinting requires
breaking DNA into short
fragments, tagging those
fragments with radioactive
tracers, and then mixing the
fragments in a gel.
In an electric field, smaller fragments move farther along the gel, and the distribution of fragments can be recorded on a photographic film
(b). Because each person’s DNA sequence is unique, each DNA fingerprint is distinctive.
The steps in DNA fingerprinting