Human Biology (BIOL 104)
Talk Twelve:
Genetics
Chapters 19, 20, and 21
Basic Concepts of Heredity
Genes and Inheritance
• Genes provide the instructions for all human
traits, including physical features and how
body parts function
• Each person inherits a particular mix of
maternal and paternal genes
History
• Genetics is the study of genes.
• Inheritance is how traits, or characteristics, are
passed on from generation to generation.
• Chromosomes are made up of genes, which are
made up of DNA.
• Genetic material (genes,chromosomes, DNA) is
found inside the nucleus of a cell.
• Gregor Mendel is considered “The Father of
Genetics"
Gregor Mendel
• Austrian Monk.
• Experimented with “pea plants”.
• Used pea plants because:
– They were available
– They reproduced quickly
– They showed obvious differences in the traits
Understood that there was something that carried
traits from one generation to the next“FACTOR”.
Mendel
In the mid-1800s, the rules underlying
patterns of inheritance were uncovered
in a series of experiments performed
by an Austrian monk named Gregor
Mendel.
Mendel's Plant Breeding
Experiments
Gregor Mendel was one of the first to apply
an experimental approach to the question of
inheritance.
For seven years, Mendel bred pea plants and
recorded inheritance patterns in the
offspring.
Particulate Hypothesis of Inheritance
Parents pass on to their offspring separate
and distinct factors (today called genes)
that are responsible for inherited traits.
Mendelian Genetics
• Dominant traits- traits that are expressed.
• Recessive traits- traits that are covered up.
• Alleles- the different forms of a characteristic.
• Punnett Squares- show how crosses are made.
• Probability- the chances/ percentages that something will
occur.
• Genotype- the types of genes (Alleles) present.
• Phenotype- what it looks like.
• Homozygous- two of the same alleles.
• Heterozygous- two different alleles.
Mendel was fortunate he chose the
Garden Pea
•Mendel probably chose to
work with peas because they
are available in many
varieties.
•The use of peas also gave
Mendel strict control over
which plants mated.
•Fortunately, the pea traits
are distinct and were clearly
contrasting.
To test the particulate hypothesis, Mendel crossed truebreeding plants that had two distinct and contrasting
traits—for example, purple or white flowers.
What is meant by “true breeding?”
Mendel cross-fertilized his plants by hand. Why is it important to control
which plants would serve as the parents?
For each monohybrid cross, Mendel cross-fertilized true-breeding plants
that were different in just one character—in this case, flower color.
He then allowed the hybrids (the F1 generation) to self-fertilize.
Typical breeding experiment
P generation (parental generation)
F1 generation (first filial
generation, the word filial from the
Latin word for "son") are the hybrid
offspring.
Allowing these F1 hybrids to selfpollinate produces:
F2 generation (second filial
generation).
It is the analysis of this that lead
to an understanding of genetic
crosses.
The F1 crossed
produced the F2
generation and the
lost trait appeared
with predictable
ratios.
This led to the
formulation of the
current model of
inheritance.
Mendel studies seven characteristics in the
garden pea
:
Statistics indicated a
pattern.
Law of Dominance
In the monohybrid cross (mating of two organisms that
differ in only one character), one version disappeared.
What happens when the F1’s are crossed?
Alleles: alternative versions of a gene.
The gene for a particular inherited character resides at a
specific locus (position) on homologous chromosome.
For each character, an organism
inherits two alleles, one from each
parent
How do alleles differ?
Dominant allele
Recessive
allele
Recessive allele
Recessive allele
Dominant - a term applied to the trait (allele) that is expressed
irregardless of the second allele.
Recessive - a term applied to a trait that is only expressed when the
second allele is the same (e.g. short plants are homozygous for the
recessive allele).
Probability and Punnett Squares
Punnett square: diagram showing the probabilities of the
possible outcomes of a genetic cross
Genotype versus phenotype.
How does a
genotype ratio
differ from the
phenotype ratio?
Testcross
A testcross is designed to reveal whether an organism that displays the
dominant phenotype is homozygous or heterozygous.
From the monohybrid crosses, Mendel derived 4
hypotheses….combined, we now refer to these as
Mendel’s Principle of
Segregation
– There are alternative forms of
genes, now called alleles
– For each characteristic, each
organism has two genes
– Gametes carry only one allele
for each inherited
characteristic
– Alleles can be dominant or
recessive
A A pair of homologous
chromosomes, each in the
unduplicated state (most
often, one from a male parent
and its partner from a female
parent)
B A gene locus (plural, loci),
the location for a specific
gene on a specific type of
chromosome
C A pair of alleles (each being
one chemical form of a gene) at
corresponding loci on a pair of
homologous chromosomes
D Three pairs of genes (at
three loci on this pair of
homologous chromosomes);
same thing as three pairs of
alleles
Basic Concepts of Heredity
1. Genes
– Humans have ~21,500
– Chemical instructions for building proteins
– Locus: specific location on a chromosome
2. Diploid cells contain two copies of each gene on pairs of
homologous chromosomes.
3. Allele: each version of a gene
4. Homozygous condition: identical alleles
Heterozygous condition: different alleles
5. Dominant allele:
-
Effect masks recessive allele paired with it
Basic Concepts of Heredity
6. Genetic representations
– Homozygous dominant (AA)
– Homozygous recessive (aa)
– Heterozygous (Aa)
7. Genotype: Inherited alleles
– Phenotype: Observable functional or physical traits
Genetic Tools: Testcrosses and Probability
Parent:
homozygous
recessive
alleles segregatecc
male gametes (n)
(sperm)
female gametes (n)
(eggs)
C
c
C
C
c
C
cc
c
C
c
c
Cc
cc
C
c
CC
Cc
Cc
cc
C
C
c
Cc
Cc
cc
c
c
Parent:
homozygous c
dominant
C C Cc
CC
C C Cc
F1
phenotypes
c
c
Cc
Cc
Cc
Cc
Cc
Cc
F1
offspring:
Cc
C c
F1
offspring: C c
C C CC Cc
Cc
c c Cc cc
F2
phenotypes
CC
Cc
Cc
cc
3 dominant (CC, Cc,
Cc) 1 recessive (cc)
Family Pedigrees
Dominant Traits
• Mendel’s
principles apply
to the
inheritance of
many human
traits
Freckles
Widow’s peak
Free earlobe
Recessive Traits
No freckles
Straight hairline
Attached earlobe
Pedigree analysis reveals Mendelian patterns in
human inheritance
In these family trees, squares symbolize males and circles represent
females. A horizontal line connecting a male and female (--) indicates a
mating, with offspring listed below in their order of birth, from left
to right. Shaded symbols stand for individuals with the trait being
traced.
A family pedigree
– Shows the history of a trait in a family
– Allows researchers to analyze human traits
Female
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_
Abigail
Lambert
D_
John
Eddy
Dd
Elizabeth
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_
Hepzibah
Daggett
Dd
Dd
Dd
dd
Male
Deaf
Hearing
Genes can lead to inherited
diseases
• A gene which doesn’t function on an autosomal
chromosome can lead to devastating diseases
• Autosomal chromosomes are 22 pairs of
chromosomes which do not determine gender
• Such diseases can be caused by both a
dominant or a recessive trait
Autosomal Recessive Disorders
• Tay-Sachs Disease:
– Jewish people in USA (E. Euro
descent)
– Not apparent at birth
– 4 to 8 months
• Neurological impairment evident
• Gradually becomes blind and
helpless
• Develops uncontrollable
seizures/paralyzed
•
• Allele is on Chromosome 15
– Lack of enzyme hexosaminidase A
(Hex A)
• Lysosomes don’t work, build up in
brain
From the wikimedia free licensed
media file repository
Autosomal Recessive Disorders
• Cystic Fibrosis
– Most common in USA
(Caucasian)
– 1 in 20 caucasians is a carrier
– Mucus in bronchial and
pancreas thick/viscous
– Breathing and food digestion
problems
• Allele is on chromosome 7
– Cl ions can not pass through
plasma membrane channels
• Cl ions pass –water goes with it.
No water, thick mucus
Location of
CFTR gene
Human chromosome 7
Autosomal Recessive Disorders
• Phenylketonuria (PKU)
– Affects in in 5,000 newborns
– Most common nervous system
disorder
• Allele is on chromosome 12
– Lack the enzyme needed for
the metabolism of the amino
acid phenylalanine
– A build up of abnormal
breakdown pathway
• Phenylketone
• Accumulates in urine. If
diet is not checked, can
lead to severe mental
retardation
From the wikimedia free licensed
media file repository
Autosomal Dominant
Disorders
•
•
•
•
•
Neurofibromatosis
Very common genetic disorder
Tan spots on skin
Later tumors develop
some sufferers have large head
and ear and eye tumors.
• Allele is on chromosome 17
– Gene controls the production
of a protein called
neurofibromin
– This naturally stops cell
growth
From the wikimedia free licensed media file repository
Autosomal Dominant Disorders
• Huntington Disease
• Leads to degeneration of brain
cells
• Severe muscle spasms and
personality disorders
• Attacks in middle age
• Allele is on chromosome 4
– Gene controls the
production of a protein
called huntington
– Too much AA glutamine.
Changes size and shape of
neurons
From the wikimedia free licensed media file repository
DNA: A Double
Helix
DNA is built of four kinds of nucleotides.
• Deoxyribose sugar, a
phosphate group, and one
of four nitrogenous
bases:
•
•
•
•
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
• Forms a double helix
– Structure discovered by
Watson and Crick
DNA Replication
• Adenine (A) always base pairs with thymine (T)
• Guanine (G) always base pairs with Cytosine (C)
• ALL Down to HYDROGEN Bonding
• Requires steps:
– H bonds break as enzymes unwind molecule
– New nucleotides (always in nucleus) fit into
place beside old strand in a process called
Complementary Base Pairing.
DNA: A Double Helix
adenine
A
base with a
double-ring
structure
sugar
(deoxyribose)
guanine
G
base with a
double-ring
structure
thymine
T
base with a
single-ring
structure
cytosine
C
base with a
single-ring
structure
DNA: A Double Helix
Chemical “rules” determine which nucleotide
bases in DNA can pair up.
• Nucleotides can line up in any order
• Specific nucleotides form base pairs,
which are hydrogen molecules bonded
together
– A and T
– G and C
one
base
pair
p408
Passing on Genetic
Instructions
• DNA replication
– Semiconservative replication
• Many enzymes and proteins involved in this
process, e.g., DNA polymerases
• Hydrogen bonds between the strands are broken
• Strands unwind
• Each strand becomes the template for a new
strand of DNA
Passing on Genetic Instructions
A Parent DNA molecule; two
complementary strands of
base-paired nucleotides.
B Replication begins; the two
strands unwind and separate
from each other at specific
sites along the length of the
DNA molecule.
C Each “old” strand serves as
a structural pattern (a
template) for the addition of
bases according to the basepairing rule.
D Bases positioned on each old
strand are joined together into
a “new” strand. Each half-old,
half-new DNA molecule is just
like the parent molecule.
Passing on Genetic Instructions
• Gene mutations
– Changes in the nucleotide sequence of genes
• Two common kinds of gene mutations
– Base-pair substitution
• An incorrect nucleotide is paired with an exposed base during
DNA replication; may be repaired
• If not repaired, one amino acid might replace another during
translation; e.g., sickle-cell anemia
– Deletion
• A base is lost
Passing on Genetic Instructions
Threonine
Threonine
Proline Glutamate Glutamate
Proline
Valine
Glutamate
Lysine
Lysine
part of DNA
mRNA transcribed
from DNA
resulting amino acid
sequence
base substitution in
DNA
altered mRNA
altered amino acid
sequence
deletion in DNA
Threonine
Proline
Glycine
Arginine
altered mRNA
altered amino acid
sequence
Mutations
Figure 3.6a
Mutations
Passing on Genetic Instructions
• Expansion mutation
– Nucleotide sequence is repeated many times
• Huntington’s disease
• Fragile X syndrome
• Transposable elements
– Bits of DNA can move from one location to another
in same DNA molecule or in different one
– Neurofibromatosis
• Genetic disorder
DNA into RNA: The First Step
in Making Proteins
• RNA
– Ribose sugar, a phosphate group, and one of
four nitrogenous bases:
•
•
•
•
Adenine (A)
Guanine (G)
Uracil (U)
Cytosine (C)
– Single-stranded
DNA into RNA: The First Step
in Making Proteins
• Transcription forms
– Ribosomal RNA (rRNA)
• Plus protein and it forms the ribosomes
– Messenger RNA (mRNA)
• Carries the code for proteins
– Transfer RNA (tRNA)
• Picks up a specific amino acid for the growing polypeptide
chain
• mRNA is eventually translated into a protein
DNA into RNA: The First
Step in Making Proteins
In transcription, DNA is decoded into RNA
• Transcription
– Gene serves as a template for RNA synthesis
– Process occurs in the nucleus using RNA
polymerases
• Begins at “promoter” region
• 5' capped end for protection and binding to the
ribosome
• Joins nucleotides together until a termination
sequence is reached
• “Pre-mRNA”: contains introns and exons
• Exons: sequences for the proteins
DNA into RNA: The First Step
in Making Proteins
gene region
RNA polymerase, the enzyme that
catalyzes transcription
A
forming RNA
transcript
B
DNA template
winding up
DNA template
unwinding
DNA into RNA: The First
Step in Making Proteins
transcription site
C
D
growing RNA transcript
DNA into RNA: The First
Step in Making Proteins
Gene transcription can be turned on or off.
• Each cell determines which genes are active or
inactive
– Some genes switched on and off throughout a person’s
lifetime
– Some turned on only at certain times
– Some turned off permanently before birth
• Regulatory proteins
– Speed up or halt transcription
– May bind with noncoding DNA sequences and affect the
transcription of a neighboring gene
The Genetic Code
• Codons
– Three nucleotide bases
on mRNA
– Sixty-four codons make
up the genetic code
– Most amino acids have
more than one codon
• Start codon
– AUG; establishes the
“reading frame” for
translation
• Stop codons: UAA,
UAG, and UGA
The Genetic Code
DNA
mRNA
mRNA
codons
amino
acids
B
Threonine
Proline
Glutamate
Glutamate
Lysine
tRNA and rRNA
anticodon
• Has a “hook” for the
attachment of an amino
acid
• Anticodon base pairs
with the codon
• Sixty-four codons; fewer
tRNAs, due to
redundancy of the
genetic code
– Example, amino acid
proline codons: CCU, CCC, amino acid attachment site
CCA, CCG
trp
tRNA and rRNA
rRNAs are ribosome building blocks.
• Two subunits are built in the nucleus from rRNA and
proteins
• Shipped to the cytoplasm
• Combine into ribosomes during translation
tunnel
small ribosome
subunit
+
large ribosome
subunit
assembled
ribosome
The Three Stages of
Translation
• Initiation
– Initiator tRNA with AUG attached binds to mRNA
codon
– AUG binds with the small ribosome subunit
– Large ribosome subunit binds to form the initiation
complex
• Elongation
– Peptide bonds form between incoming amino acids
• Termination
– Stop codon; detachment of the new polypeptide
The Three Stages of Translation
start codon
(AUG)
initiator tRNA
first amino acid
of polypeptide
Ribosome subunits
1
and an initiator
tRNA converge
on an mRNA.
A second tRNA
binds to the second
codon.
A peptide bond
forms between
the first two amino
acids.
2
peptide bond
The Three Stages of Translation
3
The first tRNA
is released and the
ribosome moves to
the next codon. A
third tRNA binds to
the third codon.
A peptide bond
4
forms between the
second and third
amino acids.
5
The second tRNA is
released and the ribosome
moves to the next codon. A
fourth tRNA binds the fourth
codon.
6
A peptide bond forms
between the third and fourth
amino acids. The process repeats
until the ribosome encounters a
stop codon in the mRNA.
The Three Stages of
Translation
• Polysomes
– Cluster of ribosomes all
translating the same
mRNA at the same time
• Where do the new
polypeptide chains go?
– Some stay in the
cytoplasm
– Others modified in the
ER to be sent out of the
cell
polysomes
The Three Stages of Translation
polysomes
ribosome
subunits
tRNA
Convergence of RNAs
mRNA
polypeptide
A
Translation
Human Variation
• With origins in Africa, modern man has spread around the
globe. In doing so, modern man adapted to the
surroundings.
Human Variation
• Arms and legs are longer
and thinner in warm
areas of the planet –
shorter and thicker in
cold regions.
• Conserves heat in cold
regions by reducing
surface area
• Skin pigmentation is
darker the nearer the
equator to protect the
skin from UV.
Polygenic Inheritance
• The additive effects of two or more genes on a
single phenotype
Polygenic
inheritance
Single trait
(e.g., skin color)
Multiple genes
Visual Summary 9.5
Human Variation
• There is a gradient of
melanosome size and number in
dark, intermediate, and light
skin; in addition, melanosomes
of dark skin are more widely
dispersed.
• This diagram is based on one
published by Sturm et al.
(1998) and summarizes data
from Szabo et al. (1969), Toda
et al. (1972), and Konrad and
Wolff (1973) based on
individuals whose recent
ancestors were from Africa,
Asia, or Europe.
PLoS Biology | http://biology.plosjournals.org
Human Variation
•
A traditional skin color map by based on geographical location on a
global scale. Taken from: PLoS Biology | http://biology.plosjournals.org
Polygenic Inheritance
• Three genes inherited
separately
• The dark-skin allele for each
gene (A, B, and C) contributes
one “unit” of darkness to the
phenotype and is incompletely
dominant to the other alleles
(a, b, and c).
aabbcc
(very light)
AaBbCc
Eggs
AABBCC
(very dark)
AaBbCc
Sperm
• An AABBCC person would have
very dark skin
• An aabbcc person would have
very light skin
Figure 9.22
Ice age Europe (18,000 years ago)
• Glacial ice 2km thick covers
much of Northern Europe
and the Alps.
• Sea levels are approx. 125m
lower than today and the
coastline differs slightly
from the present day.
• Human populations that
began their migration from
Africa 60,000 years earlier
were stopped by the ice.
Ice age Europe (18,000 years ago)
•
Due to the cold and the
need for food, the
populations of the day
waited the ice age out in
the three locations
shown on the map.
• These were the Iberian
Peninsula, the Balkans
and the Ukraine.
After the Ice age – 12,000 years
ago
• 12,000 years ago, the ice
retreated and the land has
become much more
supportive to life.
• The three groups of humans
had taken refuge for so long
their DNA had naturally
picked up mutations
• These three major population
groups account for approx
80% of Europe's present-day
population
Finally, from 8,000 years
ago
• Peoples from Africa that had
moved to the Middle East
developed the new technology
of agriculture and began
moving back into Europe.
• This was the last migration of
human population into Europe.
• Body shape and skin
pigmentation all changed due
to environmental pressure on
the genomes of these
separate populations
The concept of racism
• Racism has many meanings:
– All of them come down to the belief that some
group of people are better than others.
• In most cases, the motivation to conquer a
region comes first, the racist ideology comes
later
• Came about because people thought that a
different genetic trait was inferior to one(s)
they processed.
The concept of racism
• They also believed that their “group
identity” was inherited and could not be
changed
– A view which has no basis in genetics
• In the 1940’s the Nazis exterminated 11
million Jews, gypsies and other groups
– But not before theses groups were declared
“inferior”.
• Most people now regard racism as unethical
– Denies basic human rights, results in crime
and human conflicts.
Central High School in Little
Rock, Ark. 1957.
Elizabeth Eckford : Used
with permission from the media file repository of USHistory.com
Systemic Power and Race
• P3 = power X Power X Power
• Power1 = Power over people of color
• Power2 = Power which gives and preserves privilege
and advantages for white people
• Power3 = Power which socializes all of us into the
racial rules
• Racism’s ultimate power – to control and racialize all
of us
The concept of racism
• Human populations have always been
variable.
– adapt and change under selective pressure
• Skin pigmentation is determined by a
selective environmental pressure due to the
total amount of sunlight a population exists
with.
• Taught hatred for different populations of
people
Final thought……
The end!
Any Questions?