Download CHAPTER 1 Genetics: An Introduction

Chapter 1
Genetics: An Introduction
What is Genetics?
• Genetics is the
fraction of biology
that studies
heredity.
• Genetics is the
center of all
biology because
gene
activity underlies
all biological
processes.
• The study of Genetics involves the understanding of
biological properties that are transmitted from parents
to offspring.
• It studies genes, the molecular nature of these genes,
how these genes are transmitted from generation to
generation. It includes the study of how these genes
are expressed and how these genes’ activity is
regulated.
• An understanding of gene structure, activity and
regulation helps to understand how these genes control
all biological processes (from cell structure to
reproduction).
Classic and Modern Genetics
• The terms of “classic” and “modern genetics” are
used to separate the scientific research that has
been done in Genetics since its beginnings.
• The abbreviated timeline of events (or the
expansive timeline in your textbook) shows that
generally classic genetics involve the studies
and discoveries previous to the Central Dogma
of genetics proposed by Beadle and Tatum in
1941 (these studies encompassed mainly
transmission genetics). After 1941, we talk
about Modern Genetics (mainly Molecular
genetics).
This is onl a historical differentiation
Classic Genetics
From Mendel to the
Central Dogma of
Genetics (1866-1941).
Mendel’s published
work, Experiments in
Plant Hybridization
(1865), languished with
no discernable impact
until in 1900 three other
investigators
independently
discovered the same
genetic principles.
Mendel’s Garden Pea Pisum sativum has become the symbol
of classic genetics.
What’s wrong with Mendel?
Doesn’t he like cooked peas?
Modern Genetics
From the Central
Dogma of Genetics
(1941 to todays
Genomics).
Beadle and Tatum (one
gene-one polypeptide),
Watson and Crick
(DNA structure), Paul
Berg (genetic
engineering) are
examples of rosetta
stones findings in
Modern Genetics.
The subdisciplines of Genetics
A more accurate differentiation of the
different branches studied by genetics is
looking at the Genetics subdisciplines
¾Transmission Genetics
¾Molecular Genetics
¾Quantitative Genetics
¾Population Genetics
Transmission Genetics
Sometimes called classic
genetics deals with how
genes are transmitted from
generation to generation
and how they recombine
Molecular Genetics
It deals with the
molecular structure
and function of genes.
Analyzing the
sequence of the
nitrogen bases in a
gene (and how it may
change its expression)
is an example of
molecular genetics.
Population Genetics
POPULATION
LACTOSE
INTOLERANT
ADULTS
U.S.
European
Americans
2-19 %
Latinos
(Hispanic
Americans)
52 %
African
Americans
70-77 %
Native
Americans
95 %
Asian
Americans
95-100 %
It studies heredity of
one or a few genes in a
large group of
individuals
(population). For
example the study of
the distribution and
inheritance of lactose
intolerance in different
ethnic american
groups.
Source: Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution," current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517) and Norman Kretchner, "Lactose and
Lactase," Scientific American 277 (Oct. 1972, pp. 71-78)
Quantitative Genetics
It also studies heredity of
a trait in a large group of
individuals but the
individual trait is codified
by many genes
simultaneously. For
example the discontinous
distribution of shell color
(a polygenic trait) in the
snail Cepaea nemoralus
from a population in
England.
How do we study genetics
• We can gain
knowledge of
genetic
developments by
performing
research using the
scientific method or
hypotheticodeductive method
of investigation.
¾Basic Research: Research done to gain
an understanding of fundamental
phenomena. For example studying the
chemical composition of the cell
membrane.
¾Applied Research: Experiments done
with the idea of solving an specific
problem.
Tools used in Genetic Research
• Classic techniques of breeding such as
crosses, backcrosses and testcrossing.
• Microscopic techniques, such as SEM,
TEM, and others.
• Molecular techniques such as DNA
sequencing, PCR, cloning, etc… Use of
genetic model organisms
• Construction of Genetic maps
• Genetic Databases
Basic Concepts of Genetics 1
The biological
information
fundamental to life is
encoded in the
molecule of DNA.
DNA
(deoxyribonucleic
acid) is the genetic
material in all
prokaryotes,
eukaryotes and some
viruses. Other viruses
have RNA.
• The molecule of DNA is made of two strands (chains).
Each strand is a chain of NUCLEOTIDES.
• Each nucleotide is formed of three components: A
phosphate group (PO4-3), a pentose (5C sugar) and a
nitrogen base (A, G, C, T). The arrangement of the
nucleotides in the chain forms a double helix.
• GENES (which Mendel called factors) are specific
sequences of nucleotides.
• The full sequence of the DNA or all the genes of an
organism is called its GENOME.
Basic Concepts of Genetics 2
In the cell, the
genetic material
(DNA) is
organized in
structures called
CHROMOSOMES
Many prokaryotes
(not all) have a
single, usually
circular
chromosome.
This chromosome
is made of DNA
only.
• In eukaryotes,
the DNA is
located in the
nucleus forming
linear
chromosomes.
Each
chromosome
consists of a
single DNA
molecule
complexed
(associated)
with histone
proteins.
Scanning
micrograph of a
chromosome
Human female chromosomes shown by bright field G-banding
Human female karyotype shown by bright field G-banding of chromosomes
Basic Concepts of Genetics 3
ƒEach organisms contains 2 copies of a gene (diploid), one
maternal and one paternal. The alternative versions of the
gene are called ALLELES.
ƒAn organism having a pair of identical alleles is said to be
HOMOZYGOUS.
ƒAn organism having a pair of different alleles is said to be
HETEROZYGOUS.
ƒThe complete genetic makeup of an organism is the
GENOTYPE.
ƒThe physical expression of the genotype or the observable
properties of an individual is the PHENOTYPE.
Basic Concepts of Genetics 4
ƒMendel’s 1st Law or Principle of
segregation: The factors (alleles) segregate
independently into the gametes during
Meiosis. (i.e. An organism Aa will produce gametes A
and a.)
ƒMendel’s 2nd or Principle of independent
assortment: The two factors (alleles)
controlling one trait segregate independently
from the two factors (alleles) controlling
another trait. (i.e. Aa will segregate independently from
Bb.)
Basic Concepts of Genetics 5
Basic Concepts of Genetics 6
• The process by which a gene produces its
product is called GENE EXPRESSION.
• Beadle and Tatum hypothesis, One-geneone- polypeptide, explains how gene
expression is accomplished. Each gene is
expressed as a protein. This protein can
be the final product (such as hair protein)
or an enzyme that catalyzes the formation
of the final product.
• The expression of a DNA gene is
accomplished by the process of
TRANSCRIPTION (from DNA to mRNA)
and TRANSLATION (from mRNA to
protein), thus providing the CENTRAL
DOGMA of GENETICS:
DNA
mRNA
Protein
Basic Concepts of Genetics 7
• There are many differences between
organisms. These differences are the
result of the different genes they carry.
• These differences have
resulted from the
evolutionary process of:
¾Mutations: Change
in the genetic material.
¾Recombination: The
exchange of genetic
material between
chromosomes.
¾Selection:
Particularly favorable
gene combinations in
a given environment.