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.
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. Tall Dwarf x F1 tall
Next, Mendel crossed some of these F 1 plants among themselves. Of these offspring (the F 2 generation), about 3/4 of the plants were tall and 1/4 were dwarf. tall tall x F 2 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 F 1 plants looked as though they had inherited the trait of just one of their two parents, but in the F 2 generation both traits always appeared -- and always in a 3 to 1 ratio.
The trait which was expressed in the F 1 generation was always about 3 times as numerous in the F 2 generation as was the other one which was hidden in the F 1 '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).
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.
Rules of Classical Genetics Traits (genes) are passed from parent to offspring –mechanism unknown Two genes for each trait –One from each parent There are dominant and recessive genes –Dominant expressed
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.
Transcription of DNA Transcription –Information transport –Uses RNA Process –Unzip DNA –RNA binds to exposed bases –RNA moves out of nucleus (mRNA)
The Synthesis of Proteins tRNA –Reads message –Structure Amino acid 3 bases Process –mRNA moves to ribosome –rRNA aligns mRNA and tRNA –tRNA matches codon on mRNA –Amino acid chain forms Basis for protein
Protein synthesis cont. One gene codes for one protein Protein drives chemical process in cell DNA –Introns –Exons All living things on Earth use the same genetic code
Mutations and DNA Repair Mutations –Change in DNA of parent –Causes Nuclear radiation X-rays UV light DNA Repair –10,000 ‘hits’ per day –Cells repair damage
Why Are Genes Expressed? Gene control –Turning genes on and off –Each cell contains same genes –Not all cells have same function –Certain genes activated Scientists currently studying how
Viruses Virus –Not alive –No metabolism –Cannot reproduce on own Structure –Short DNA or RNA –Protein coating How it works –Taken into cell –Takes over cell –Produces more copies –Kills cell
HIV Human Immunodeficiency Virus (HIV) –Contains RNA –Codes back to DNA –DNA incorporated into cell –Makes new viruses –Cell dies Complex –Two protein coats Outer coat fits T cell receptors Inner coat encloses RNA
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.