Today…
Genome 351, 4 April 2013, Lecture 1
»
Outline of course
»
Pedigrees (example: cystic fibrosis)
» Mendel’s experiments with pea plants
»
Proteins
»
Cells
Cystic fibrosis
-Inherited disease that affects the lungs and digestive
system
-Affects ~30,000 children and adults in the United
States (~70,000 worldwide).
-A defective gene and its corresponding protein product
cause the body to produce unusually thick, sticky mucus
that:
* clogs the lungs and leads to life-threatening lung
infections; and
* obstructs the pancreas and stops natural enzymes
from helping the body break down and absorb food.
A simple pedigree
Two unaffected individuals have three children, the
youngest of whom has cystic fibrosis (CF)
= cystic
fibrosis
= Normal
A larger family
Two unaffected individuals have eight children, two
of whom have cystic fibrosis
Another generation
Building pedigrees
= Unaffected male
= Affected male
= Unaffected female
= Affected female
= Deceased male
= Unknown sex
Horizontal line = mating
Vertical line = offspring
= Identical twins
Building pedigrees (cont’d)
Building pedigrees (cont’d)
I
II
=
III
Some early theories on heredity
• Blending of traits
• Vital spark (paternal or maternal)
• Sperm carries preformed individual
(homunculus)
Gregor Mendel (1822–1884) introduces a
more systematic approach
Reasons why Mendel was successful:
• Choice of a good model organism—garden pea
-
relatively short generation time—one per year
lots of progeny per cross
self-pollination and out-crossing possible
true-breeding strains readily available from local
merchant
• Choice of clear character differences to track
- Yellow vs. green seed pods, round vs. wrinkled
seeds, purple vs. white flowers, etc.
• Careful mathematical analysis of the results
- allowed him to develop and test specific models
Mendel’s experiments
Establish true-breeding
strains, each of which exhibit
clear character differences
crosses within the true-breeding
population yield progeny that show
the same trait as the parent
x
x
Make crosses between
different true-breeding
strains
Identify and count the
progeny traits (phenotypes)
Make crosses between the
progeny…
x
??
…are the progeny traits (phenotypes)
like one parent or the other? How
many of each class are there?
Results of Mendel’s experiments:
Generation
I:
True-breeding
green pea pod
strain
x
True-breeding
yellow pea pod
strain
Predictions of:
Blending Hypothesis
Vital spark Hypothesis
Homunculus Hypothesis
Actual results:
Generation II:
Hybrid pea
plants
The yellow trait returns in generation III
True-breeding
green pea pod
strain
Generation
I:
Generation II:
x
True-breeding
yellow pea pod
strain
Hybrid pea
plants
Cross hybrid plants to one another (or self-cross)
Generation III:
Identical findings seen with other traits…
Parental Phenotypes
Gen II
Gen III
Ratio (gen III)
1. Green X yellow pod
Green
428 Green
152 yellow
2.82 : 1
2. Yellow X green seed
Yellow
6022 yellow
2001 green
3.01 : 1
3. Purple X white petal
Purple
705 purple
224 white
3.15 : 1
4. Inflated X pinched pod Inflated 882 inflated
299 pinched
5. Round X wrinkled seed
Round
6. Axial X terminal flowers Axial
7. Long X short stem
Long
2.95 : 1
5474 Round
2.96 : 1
1850 wrinkled
651 axial
207 terminal
3.14 : 1
787 long
277 short
2.84 : 1
Mendel’s interpretations
Both parents contribute a “determinant” (gene) that
influences the seed pod color trait
Truebreeding
green pea
pod strain
x
Truebreeding
yellow pea
pod strain
Mendel’s interpretations
There are two forms of a gene (alleles) for the seed pod color
trait; the trait conferred by one allele (recessive) can be
masked by the trait conferred by the other allele (dominant)
Truebreeding
green pea
pod strain
x
Truebreeding
yellow pea
pod strain
The g allele (which confers yellow seed pods) is recessive
to the dominant G allele (which confers green seed pods).
Mendel’s interpretations
Genes are particulate (i.e., do not mix); recessive traits that
are not evident in heterozygotes can be unmasked in progeny
True-breeding
(homozygous)
green pea pod
strain
Generation
I:
Generation II:
x
True-breeding
(homozygous)
yellow pea pod
strain
Hybrid
(heterozygous)
pea plants
Cross hybrid plants to one another (or self-cross)
Generation III:
The recessive trait
reappears intact in
generation III
How did Mendel explain the 3:1 ratio?
-The Punnett Square
x
male gametes
female gametes
G
g
G
g
General conclusions of Mendel’s work
1. Many traits (phenotypes) are determined by genes
2. Gene variants (alleles) can confer dominant or
recessive traits (phenotypes)
3. There are two copies of each gene
4. Each parent randomly transmits only one of their two
alleles of a given gene to their offspring
Some vocabulary
Gene: unit of information passed from one generation to
the next.
Alleles : variants of a gene (e.g., yellow vs. green)
Homozygote: both copies of the gene are the same
Heterozygote: The two copies of the gene are different
Genotype: the information specifying a trait
Phenotype: the manifestation of the trait itself
Genotypes?
Phenotypes?
Information passes from one
generation to the next!
Applying Mendel’s principles to CF
Two unaffected individuals have eight
children, the two of whom have cystic fibrosis
C = common allele
c = cystic
fibrosis allele
The Punnett Square
Cc
Heterozygous parents
C
C
Cc
c
c
The cystic fibrosis gene specifies
a membrane protein
Proteins are the workhorses of the cell
• Many sizes and shapes
– Rod-like, globular
– Single subunit, multimeric
• Many distinct properties
– Water soluble, lipid loving
• Many functions
– Structure, catalysts, motors, signals, pumps
• Mutations often alter proteins
Cystic fibrosis is recessive
Cystic Fibrosis
Homozygous (wild-type)
Heterozygous
CFTR+
CFTR+
CFTR+
CFTR-
Homozygous (mutant)
NO
NO
CFTRCFTR-
YES
But what are proteins (chemically)?
Polymers of 20 different amino acids
(only 11 can be made by humans, others
must be obtained from the diet)
The 20 amino acids
Proteins adopt a variety of structures
•Average protein = 300 to 400 aa’s
•Variety of linear amino acid sequences is almost infinite...
e.g., a protein of 100 amino acids made with the 20
different known amino acids can have 20100 different
linear sequences
•most often has a globular (spherical) 3-D shape & is
negatively charged
•E. coli (human intestinal bacteria) makes about 3,000
proteins
•humans make about 100,000 different proteins with
25,000 genes (WOW!)
Distinct proteins are different length
chains of different amino acids
Insulin -- Met-ala-leu-trp-met … glu-gln-tyr-cys-gln (110 aa)
Collagen -- Met-his-pro-gly-leu … cys-met-lys-ser-leu (1678 aa)
ß-Hemoglobin -- Met-val-his-leu … ala-his-lys-tyr-his (147 aa)
Protein
Function
Actin, myosin
Muscle contraction
Antibodies
Immunity
Hemoglobin, myoglobin Oxygen transport
Insulin, glucagon
Collagen
Kinases
Blood glucose control
Tendons,dermis
Modulate protein activity
Dehydrogenases
Thrombin, fibrinogen
Keratin
Trypsin, proteases
Metabolism
Blood clotting
Hair and skin
Digestion
Polymerases
NaATPases
DNA, RNA synthesis
Ion pumps
Collagen
Albumin
Cells -- the basic unit of life
• Organisms can be single cells (e.g., bacteria, yeast)
or collections of many cells
• Prokaryotes (bacteria) lack a nucleus
The Basic Unit of Life
• Eukaryotes have a nucleus and other compartments
33
An animal cell
• Surrounded by the
plasma membrane
• Contains a nucleus
(where >99% of the
genes are located) and
cytoplasm with
specialized organelles
•Come in many
different shapes
The plasma membrane
The cystic fibrosis gene specifies
a membrane protein
Mitochondria
• Site of ATP
(energy) production
• Has its own
circular DNA (<1%
of the cellular
genes located here)
• Mitochondrial
genes are inherited
from the mother
Human Cells
• Hundreds of cell types
• Several categories
– Epithelial (skin, intestinal, lung,
but also pancreas, liver, kidney)
– Muscle
– Nerve
– Connective
– Blood
Levels of Organization
•
•
•
•
•
Organism
Organ systems
Organs
Tissues
Cells
Next time…
DNA is the genetic material
Structure of DNA reveals a digital code
Replication of DNA
CFTR regulates Cl- transport across membranes
Gene responsible for Cystic Fibrosis
Cystic Fibrosis
Affected persons can have unaffected parents
Disease can skip generations
Both sexes equally affected
Genetics of Cystic Fibrosis (CF)
* Autosomal recessive trait
* ~1/25 Caucasians is a carrier
* ~1/65 Africans is a carrier
* ~1/90 Asians is a carrier
* Gene lies chromosome 7q31.2
* Gene encodes a chloride channel expressed in
lung, skin and pancreas
* DNA diagnosis in utero