Download Unit 3 - Cerritos College

INTRODUCTION TO BIOLOGY
UNIT THREE
Evolution
Natural Selection
Ecology
Human Impacts on the Environment
Mendelian Genetics
Human Genetics
CERRITOS COLLEGE
SUMMER 2004
LECTURER – L. L. HARRIS
Biology 120
Cerritos College
EVOLUTION
I. INTRODUCTION
A. Evolution : Change over time.
Chemical evolution led to Biological Evolution
B. Charles Darwin envisioned a gradual change over time.
A gentle "unrolling" from simple to more complex.
However, today, given the Earth's age there wasn't enough time
Time involved is probably most difficult concept of all.
Thousands of millions of years – Not enough time?
Today theorists propose ideas based on “hard data” to explain the time dilemma:
Punctuated Equilibrium – Gould & Eldridge – Sudden spurts of evolutionary
progress that are interrupted by periods of relative stability.
C. All Things Evolve: Evolution of the abiotic as well as the biotic. The continents of the planet
today are not as they once were:
1. Wegener's Plate Tectonics: (1928) -- "Continental Drift"
a. Granite plates shifting on molten basalt along “tracks”.
b. "Tracks" are ocean ridges. (ruptures of Earth's crust)
c. Thermal activity (assoc w/ mantle) beneath surface causes spreading
apart of the ridges along their rifts.
2. Continental Drift can explain biogeography of plants/animals present today and it can
also explain animals fossils discovered in some unlikely places.
a) Seychelles Island – granite chunk of India – has frogs on it
b) Marsupials did not originate in Australia, but in South America. Marsupial
fossils are found in Antarctica!
3. Land Bridges – Migrate – Lose Bridge
Siberian Land Bridge (Bering Straight) – Animals and Man just walked from Asia
to North America. Geologists recognize not less than 8 land bridges. Most are
under not much more than 100 meters of water today.
D. Evolution is an Old Idea
Ancient Greeks tried to explain the fossils they found.
By 1800 many of the world's naturalists had accepted that evolution occurred.
Problem was they still had no idea how it came about.
II. LAMARKIAN EVOLUTION:
A. Jean-Baptiste Lamarck:
1. USE - DISUSE THEORY
Example of Use: Giraffe necks
Example of Disuse: Snake legs
2. Inheritance of ACQUIRED CHARACTERISTICS
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B. For the Giraffe Neck: If during your day you need to eat the leaves at higher branches of
a tree, you will stretch your neck (ACQUIRE), eat the leaves (USE),
and pass this long neck to your offspring (INHERIT).
Not entirely without foundation and certainly easy to understand.
Was an extremely popular explanation. However, it is not correct.
II.
DARWINIAN EVOLUTION
A. Charles Darwin:
Captain’s Companion on the H.M.S. Beagle – Trip to South America (1831-1836)
Soon became recognized as the ship’s Naturalist.
1. Influenced by Thomas Malthus: (Population essayist)
Animals increase geometrically (exponentially)
2. Influenced by Lyell (Principles of Geology)
Forces that formed Earth features are still at work.
Earth is millions of years old. Not thousands.
3. Influenced by Adam Smith (Economy essayist)
Strongest survive
4. Observation of animals he saw – Why such DIVERSITY?
Armadillos in Argentina looked like an extinct species in Europe/Asia.
Darwin asked – Related somehow?
Galapagos Island finch species. Different beaks for different foods.
Darwin asked – One common ancestor?
B. Darwin Proposed The Origin Of Species By "Natural Selection":
OBSERVATION 1:
"Organisms tend to increase in a geometric ratio."
OBSERVATION 2:
"However, in spite of this, population numbers tend to remain
more or less constant over a long period of time."
DEDUCTION 1:
"Since not all of the individualists produced can survive, there
must be a struggle for existence."
OBSERVATION 3:
"There is variation in every species."
DEDUCTION 2:
"In the struggle for existence, those individuals that are better
adapted to the environment leave behind more offspring than the
less adapted individuals."
IV. ALFRED WALLACE
Darwin was nearly "scooped" by Alfred Wallace: Alfred Wallace had been a naturalist in
Malaysia. He too noted the diversity of the animals he saw. Text says Wallace sent his
manuscript to Darwin. It outlined the same concept that Darwin had been working out over
the 20 years since voyage of Beagle. In 1859, Wallace and Darwin coauthored the WallaceDarwin Theory of Evolution
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Theory of Natural Selection Made Simple
A. The reproductive potential is great, but
a. Rabbits should cover the Earth, but
B. population tends to remain constant in
size, because
b. they don't, because
C. populations suffer a high mortality.
c. many are caught by predators.
D. Populations exhibit variation which
leads to
d. Some rabbits run faster than others
E. differential survival of individuals.
e. and escape from predators;
F. Individual traits are inherited by their
offspring.
f. so do their young.
G. The composition of the population
g. Populations of rabbits, as a whole,
changes by the selective elimination
tend to run faster than their
of unfit individuals.
predecessors.
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V. DARWIN: THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION (1860)
A. Caused quite a stir: Why a big deal? Man had been using ARTIFICIAL SELECTION for
eons! Man selects "desirable characters" Breeding Livestock, Dogs, Horses, Pigeons, ...
B. Summary of Main Points:
1. Variation – Differences within a population. Weakest point of the theory at the time.
Darwin could not explain mechanism for variation. Mutation is the force at play.
2. Overproduction – More offspring are produced than can possibly survive.
3. Struggle – Competition for survival. Food getting – Animals Light getting – Plants
4. Survival of "fitter" – Nature selects the best variations.
C. In review: Lamarckian Evolution – NATURE CAUSES CHANGE.
Darwinian/Wallace Evolution – NATURE SELECTS CHANGE.
VI. EVIDENCE FOR EVOLUTION
A. Fossil Record
Fossil – Defined as anything indicating existence of former life. Requires immediate
burial. (No chance of decay before burial.)
Petrifications – Every molecule is replaced by a mineral. Good for internal structures.
Molds and Casts – Good for external features. Good for hard parts – inorganic features
Soft bodies don't make good fossils)
Amber –Tree sap oozes over an insect. Trapped! Hardens, glass-like.
Unfortunately fossils are usually of low land animals.
Air, tree, & mountain dwellers usually don't get buried. They just rot in the open air.
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B. Comparative Studies
1. Anatomy
Homologous – Features with a common origin, but not a common function.
Forelimbs: Arms – Wings – Flippers. Different in outward appearance
and function, but similar in bone structure.
Analogous – Features with similar function and similar superficial appearance,
but an entirely different evolutionary background.
Bird wings & Butterfly wings
Whale flippers & Fish fins
2. Physiology
Example: Workings of kidneys. Dealing with Nitrogenous Waste
3. Biochemical
DNA, Cytochromes, Peptide Families, ...
C. Vestigial Organs
Degenerative organ or structure having little or no utility, but which in an earlier stage of
the individual or in preceding organisms performed some useful task.
Humans:
Ear muscles
Hair muscles (erector pili)
Appendix (former stomach?)
Third eye (pineal body)
Wisdom teeth
Little toes (5th digit)
Snakes:
pelvic bones & legs (Boaidae)
Whales:
pelvic bones
VII. POPULATIONS ARE NOT CONSTANT, EVEN WITHIN INTERBREEDING GROUP
A. Forces that lend mechanism to change in population:
1. Mutations – in the individual, not the population.
Random changes in the genetic material.
2. Migration – of organisms between populations. (immigrations & emigrations)
Moves the DNA around from population to population so that it is not constant.
3. Genetic Drift – requires migration of a unique set of genes into a small population.
Small enough that the genes are not "drowned".
"Founder's Principle" – founder that is unique from the initial population sets up
a new population that will be different from the initial population due to the
uniqueness of the founder. Example: Polydactyly in the Amish Community
4. Natural selection – leads to change in population.
B. Hardy-Weinberg Equilibrium
"Under certain conditions of stability both allelic frequencies and genotypic ratios will
remain constant from generation to generation in sexually reproducing populations."
Requirements:
1. Very large population
Large enough so that chance alone could not significantly alter the frequency.
Problem: Populations are not always large enough!
2. Mutations must not occur
Problem: Mutations do occur!
3. No immigration – No emigration
Problem: Almost all populations experience migration
4. Reproduction must be random
Problem: Reproduction is anything but random.
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VIII. KINDS OF SELECTION:
A. Stabilizing selection:
Individuals at both extremes of the population are “selected against”.
Example: Tail Length
Longer tailed rabbits are easier for a fox catch.
Eaten rabbits do not contribute to "gene pool".
Short tailed rabbits are easier for a hawk to grab.
Eaten rabbits do not contribute to "gene pool".
Rabbits with mid-sized tails escape predation.
Contribute their genes to the pool
B. Directional Selection
Individuals at an extreme of the population is “selected for”.
Example: Hummingbird tongues
Those with longer tongues get to nectar. Survive.
Long tongue is a heritable trait.
Offspring get long tongues.
Those with short tongues don't get nectar. Starve.
Dead birds do not contribute to "gene pool".
Environment change can also bring this about.
C. Disruptive Selection
Individuals at the norm are “selected against”. Conversely, those
at extremes are “selected for”. Can lead to speciation.
Example: Rabbit Size
Small ones escape into small holes and crevices.
Very large rabbits "move out" of prey category.
Medium size rabbits get caught – eaten (don't reproduce!)
Evolution is further defined as “a change in the gene pool over time”.
IX. OTHER KINDS OF EVOLUTION
A. Convergent Evolution:
Unrelated organisms come to look alike by similar adaptations to similar environments.
Tuna (osteichthythes)
Pleisiosaur (reptile)
WATER
Fusiform Body Shape.
Gray Whale (mammal)
B. Divergent Evolution: (aka Adaptive Radiation)
Closely related organisms become different in appearance by different adaptations to
different environments.
Bat
Cat
Mammals
Mole
Arthropods (insects) – are
Horse
an excellent example of
Whale
of divergent evolution
C. Co-evolution (HOME ASSIGNMENT)
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X. SPECIATION
A. Processes by which species originate/form. Our focus: Divergent Speciation – one ancestral
species gives rise to two or more descendent species
B. Ernst Mayr – evolutionary biologist whose definition of biological species we will use.
Biological Species – group of interbreeding natural populations that is "reproductively
isolated" with other such groups. They remain same species (despite physical
variations) as long as their matings produce fertile offspring
C. "Reproductive Isolation" – Factors that would prevent interbreeding or gene flow between
similar, but different species. Morphology – Behavior – Physiology
D. Isolation Mechanisms to Prevent Interspecific Hybridizations
1. Prezygotic Isolation – no zygote ever forms
Behavior – receptiveness of one potential mate may rely on a courtship signal
that is lacking in the suitor species
Temporal – ranges of potential mates overlap, but reproduce at different times
Gametic – sex cell non-recognition or are otherwise incompatible. Pollen not
recognized by Stigma or Sperm dies in the female tract
Mechanical – despite attempts to copulate, sperm is not introduced into female
Ecological – potential mates occupy different local microhabitats within same
range. Tolerance to particular environmental factor in adjacent
population can isolate potential mates.
2. Postzygotic Isolation
Zygotic Mortality – egg is fertilized, but zygote or embryo dies
Hybrid Low Fitness – extreme high mortality of hybrid
Hybrid – offspring of genotypically different parents
Hybrid Sterility – more fit hybrids, but is sterile or partially so.
Mare + Male Donkey –> Mule (sterile hybrid)
E. Speciation Models:
1. Allopatric Speciation (allo = different / patric = homeland)
– initial species occupies a given range (gene flow active)
– event geographically isolates species members
– lose land bridge, change in river flow, fire leaves "islands" of life
– daughter species form gradually by divergence
2. Sympatric Speciation (sym = together / patric = homeland)
– initial species occupies a given range (gene flow active)
– behavioral change isolates some members from other members of range.
– tolerances to certain microhabitats or feeding preferences change
3. Parapatric Speciation (para = near / patric = homeland)
– "border effect" gene flow between members of two adjoining ranges. "Hybrid
zone" along border is noted
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Biology 120
Cerritos College
ECOLOGY
I. Introduction
Ecology defined – Study of the interactions of organisms with one another and with the abiotic
factors of their environment. Literally – "study of the home"
II. Life Requires Energy
A. Living Systems are Energy Transforming Systems
Solar  Chemical  Food  ATP
Energy
Energy
Energy Energy
B. Vocabulary
Population: Group of individuals of one species occupying a given area at a given time.
Community: Association of interacting populations Producers
Decomposers
Consumers
Detritivores
Habitat: Site in community where population (or individual) lives
Niche: Role played within the community
Prey
Predator
Top Predator
Ecosystem: Community Members and the many abiotic factors with which they interact
Biosphere: Zones of air, land, & water at surface of Earth that living things occupy
C. Pyramid of Biomass
Trophic Levels – All organisms the same number of energy transfers away from the
original energy source (The Sun)
Transfer Efficiency – Only about 10% of energy obtained by organisms at a trophic level
is "transferred" to organisms of the next higher trophic level. The
other 90% was used to resist Entropy or lost as heat.
TROPHIC LEVELS
The Sun
3o
Consumers
4
(TOP CARNIVORE)
3
2o
Consumers
10%
(CARNIVORE)
10%
2
1o Consumers
(HERBIVORE)
Producers
1
(Autotrophs)
8
10%
D. Food Chains vs. Food Webs
Food Chain:
Seeds  Mouse  Fox  Bobcat
(Key: "" means “eaten by”)
Usually is not this simple, instead a "Food Web": Seeds  Mouse  Fox  Bobcat
A crossing and interlinking of Food Chains
III. Symbiotic Relationships
Most interactions are "NEUTRAL". Some interactions are more involved
A. Symbiotic Relationships: How some populations “live together”
1. Commensalism: One partner benefits & other partner is neither helped nor harmed
Example: Birds or bats roosting and nesting in a tree
2. Mutualism: Both partners benefit
a. Facultative – one can live without the other Example: Birds that pick lice off Rhinos
b. Obligate – one can't live without the other Example: Lichens (algal & fungal partners)
3. Parasitism: One benefits at the other's expense Examples: Mistletoe on a Tree
Nematodes in a host gut
IV. Population Ecology
The Big Question = SURVIVAL Who? What? Where? When? and How?
The General Rule: SURVIVAL WITH AVOIDANCE OF COMPETITION
A. Distributions
How organisms are found in their ecosystem
Bottomline: RESOURCE AVAILABILITY – both the Biotic and the Abiotic
biotic resources – food, shelter (trees, shrubs), mates, …
abiotic resources – water, substrate, elevation, climate, shelter, light, air quality, …
B. Types of Distributions
1. Distributions in Space
Plant Examples
a. Clumped: Willows in Desert Canyons
are found along creekbeds
b. Random: Limestone endemic plants
on limestone outcroppings
c. Uniform: Desert Creosote & Black Bush
(away from asphalt roads)
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2. Distributions Over Time – Changes in the climate, rainfall levels, others
a. Plants – Spring annuals & Winter annuals
b. Animals – Migrations :
Hibernations :
Birds fly South to "Winter"
Deer move to lower elevations (below snowline)
Still there but not "visible"
3. Population Densities
a. Population Density Defined – The numbers of individuals per unit area
b. Populations increase exponentially – a Doubling Effect 2  4
48
8  16
.....
c. Factors Affecting Densities
i. Births and Deaths
ii. Migrations
Births = Natality
Immigrations = enter
Deaths = Mortality
Emigrations = leave
d. POPULATION GROWTH RATE: = (B + I) – (D + E)
As long as there are more adders than subtractors; there is GROWTH
e. ZERO POPULATION GROWTH = (B + I) – (D + E) = 0
For people this just isn't happening – Home Assignment (Breaking the Rules)
People populations and densities are influenced by factors that are very different
from those of most animals and plants. Religion Social Economic Political
Some countries are doing better at population control than others
World's populace – is still experiencing Exponential Growth – “SKYROCKETING”
4. Carrying Capacity of the Environment (= K)
Defined – In an environment, as long as the resource supply is constant, the population
should stabilize at some prescribed size limit. Text – "equilibrium size"
“Logistic Growth”
#
-------------------------- =K
Slow at first
Speeds up
Levels off
Fluctuates, seasonal, year to year
Time
V. Factors Affecting Carrying Capacity – These factors will help to control growth
A. Competition – occurs when a resource is in short supply
1. Intraspecific Competition – Members of same species compete for same resource.
Usually FOOD or TERRITORY (breeding success)
Usually handled in some SOCIAL way
Dominant Members eat first; Subordinate Members eat second
Dominant Males mate with females; Subordinate Males don't get any!
2. Interspecific Competition
Members of different species compete for same resource.
Usually FOOD or TERRITORY (breeding success)
Usually avoided in some SOCIAL way
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a. RESOURCE PARTITIONING: (for somewhat related species)
Birds: Some species eat at higher branches.
Other species stay at lower branches.
Others, still, eat on ground level.
Coral Reef Animals: AM/PM populations
Plant Roots:
Shallow-rooted vs Deep-rooted species
b. Or one species MOVES OUT of the area
The more similar 2 different species are with respect to their
resource requirements – the less likely they are to coexist in
a community. “Competitive Exclusion”
B. Predator-Prey Relations
Prey Species – the hunted; dinner!
Predator Species – the hunter; the diner!
Advantages of a Healthy Relationship : (Evidence to follow shortly!)
1) Holds both species populations at their "K". Therefore, good for both species
2) Encourages Species Diversity. With a healthy predator-prey relationship in
place – more different species can coexist in
the ecosystem.
a. Influences on a Healthy Predator-Prey Relationship:
1. CARRYING CAPACITY for the prey species in the absence of the predator.
What is the number of prey that could be available?
Enough to support the predator species population?
Question of Cost Effectiveness!
2. REPRODUCTIVE RATE of the PREY species.
Will there be bunnies available to feed the foxes?
3. FUNCTIONAL RESPONSE of the PREDATOR.
Catch and Eat the Prey (decrease in prey population)
4. NUMERICAL RESPONSE of the PREDATOR.
Reproduction of Predators (increase in predator population)
Rabbit (––)
Population
Fox (- - -)
Population
Time
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b. Evidence of healthy predator-prey relationships: both populations are maintained at
their “K” value
KIABAB FOREST (associated with Grand Canyon). Removal predators of a
deer population. Reason: Improve Tourism
PREDATORS KILLED
DEER POPULATION DENSITY
200
781 Mtn. Lions
1900: 100 deer/100 acres
556 Bobcats
Deer
1911: 250 deer/100 acres
31 Wolves
1950: 25 deer/100 acres
100
5000 Coyotes
time
1900
Today
BOTTOMLINE: In 11 years the deer population exploded and "overshot" their carrying
capacity. However, they could not be supported by the existing plant supply. The
OVERSHOOT produced a CRASH
CONSEQUENCE: a new, but lower Carrying Capacity due to environmental degradation
c. Evidence that Predation Encourages Species Diversity
1. Darwin's Garden : Lawn composed of several species of low-growing plants. Had
gardener NOT to cut the lawn. (Lawn Mower = Predator and Plants = Prey)
Alternative Hypotheses: 1) Predator removal decreases species diversity due to
increased competition among different prey species.
2) Predator removal increases species diversity due to
increased competition among different prey species.
Results: Of original 20 species in lawn, only 11 remained. Nine species gone!
Interspecific Competition was intensified and resulted in the elimination of
some species. Lost species had been out-competed for space
Conclusion: Predation minimizes competition between Prey species by
minimizing Interspecific Competition.
Thereby, Predation Encourages Species Diversity
2. Paine's Sea Star Experiment: Removal of Sea Stars from a plot in a tidepool.
Of 15 prey species at To – only 8 remained at Tf
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Mussels took advantage of the predator loss and thereby increased their
numbers and crowded out the other species. Mussels had been the preferred
prey of the Stars; but, alas, Mussels were the best competitors for space in their
absence!
C. Parasitism and Disease
1. Parasites differ from Predators
Successful Predators always kill their prey. (Text says predators may or may not
kill prey – hence, "successful" is added)
Successful Parasites live in or on their prey/host. May or may not kill their
prey/host
2. Disease
Viral or bacterial in origin. May kill many or all in population
May affect reproductive success
Disease can have devastating effects on a species population
In the absence of a natural predator – disease will do the job!
D. Density-Dependent vs. Density-Independent Factors Affecting Population Growth Rates
1. Density-Dependent Factors
As density of a population increases there are agents that may affect population
growth
Spread of Disease & Parasites
Extremely Well-fed Predators
Not enough food to go around
2. Density-Independent Factors
Despite reasonable population density (within K) there are agents that may affect
population growth.
Natural Disasters
Hurricanes, Floods, ...
Introduced Species
Feral Pigs, Feral Goats, Feral Cats
Surprise Weather
Changes
Cold-Snaps; Early Thaw – then Freeze
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Biology 120
Cerritos College
GENETICS
I. Introduction
A. Gregor Mendel
Monk at St. Thomas Abbey in Brunn, Austria.
Mathematician and Gardener
Published in 1866 – “Mode of Trait Inheritance in Garden Peas”
His work gives indirect evidence of how some traits are inherited by the offspring
of sexually reproducing organisms
Perspective: Origin of Species by Natural Selection was published in 1859
Mendel was gathering his data even before this date
Mendel's work would support Darwin's premise of variation of inheritable traits
B. "Blending Theory of Inheritance"
Popular inheritance belief in Mendel's Day. Offspring had an equal blending of parental traits.
Never mind the evidence all around them: White Hen & Red Rooster  Pink Chicks (?)
Why not blending? Implies loss of white and red possibilities for future generations. All
chicks of future generations would either be pink or, at best, subject to a dilution factor.
What it all means: (for the BIG picture) Blending would imply uniform populations that
would present no variation whatsoever. Darwin’s idea of Natural Selection would have
no basis! Nothing to “select for”!!
II. MENDEL’S EXPERIMENTS
Attribute to Mendel’s Genius: He had no knowledge of Chromosomes, Genes, DNA, Mitosis,
or Meiosis. They had an idea of the role of egg & sperm  offspring
Attribute to Mendel's LUCK! His experimental subject  Common Garden Pea (Pisum sativum)
True-breeder: Successive generations will have the same traits as the parents. Typically
True of self-pollinators These pea are self-pollinators, but can be cross-pollinated too!
A. Procedure:
0) Use True-Breeding Pea Plants With Contrasting Traits
(Purple Petaled Pea & White Petaled Pea)
1) Collect the Pollen of both Flowers
2) Castrate the Flowers
3) Cross Pollinate the Flowers
4) Wait For Seed Formation / Germination / Flowering
5) Take a tally of the Offspring’s Petal Color
Attribute to luck: Petal color in pea plants has an inheritance pattern called: COMPLETE
DOMINANCE This pattern is the most simple and straightforward mode and follows the laws of
probability
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B. Probability: Number of times that one particular event will occur divided by total number of all
possible outcomes.
P = # of times event happens / # of all possible outcomes
Flipping a coin: Heads?
______________
Rolling a die: Box Cars? ______________
Only after many tosses of a coin will the expected Heads to Tails ratios be true (50:50)
But every time you toss a coin the chances for “heads” is 50:50
C. The Laws of Probability
1. Product Rule – probability of 2 independent events occurring simultaneously is the product of
their individual trial probabilities
Heads for one toss of 2 coins?
___________________
Heads for one toss of 3 coins?
___________________
Snake-Eyes for 2 rolled die?
___________________
2. Sum Rule – probability of either of two mutually exclusive events occurring is the sum
of their individual trial probabilities
Roll a die and get a 2 OR a 6?
___________________
Giving birth to a boy OR a girl?
___________________
Rolling two 4's OR two 6's?
___________________
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Vocabulary:
Genes – Inheritance Units. DNA instructions code for a trait
Locus – The position of a gene on a chromosome
Diploidy – 2n cells inherit 2 forms of a gene for a particular trait. Each gene is located on
separate, but homologous chromosomes (Exception: 23rd pair; behave as
homologues)
Alleles – Different molecular forms of a gene Gene: Hair Color Alleles: Blonde, Brown, Black
Dominant Allele – In a heterozygote, one allele's info "masks" the effect of the other allele.
Recessive Allele – In a heterozygote, one allele's info is "masked" by effect of other allele
Homozygous – When alleles for same trait are identical
Homozygous Dominant – Double dose of the dominant allele for a particular trait.
Written for the diploid cell as BB
Homozygous Recessive – Double dose of the recessive allele for a particular trait.
Written for the diploid cell as bb
Heterozygous – Has both a dominant and arecessive allele for a particular trait.
Written for the diploid cell as Bb
Genotype – Sum total of an individual organism's genes Specifies allelic nature of gene pair
Phenotype – Physical expression of the genotype
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Gametes = Sex Cells, Egg & Sperm: How many possible different gametes could form?
The formula is 2n; where n = the haploid number
2n Organism with 2 chromosomes → Aa [2n = 21 = 2 possible gametes → A & a ]
2n Organism with 4 chromosomes → AaBb [2n = 22 = 4 possible gametes → AB, Ab, aB, & ab ]
2n Organism with 46 chromosomes → H. sapiens [2n = 223 = 8,388,608 possible gametes ]
DISCLAIMER: All figures disregard Cross-Overs & Aberrations
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D. Mendel’s 1st Crossing: True-Breeders of Contrasting Traits
P:
Purple Flowers X White Flowers
AA
X
aa
F1 :
All Purple
The Punnet Square:
Phenotypic Ratio? _____________________ Genotypic Ratio? _______________________
E. Mendel’s 2nd Crossing: Monohybrid Cross
F1 plants were allowed to self-pollinate.
Purple F1 X Purple F1
Aa X Aa
F2 3/4 Purple & 1/4 White
The Punnet Square:
Phenotypic Ratio? ____________________ Genotypic Ratio? _______________________
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F. Mendel’s 3rd Crossing: The Testcross
Testcross -- Cross between an organism whose genotype for a trait is not known with an
organism that is true breeding for the recessive trait
Mendel's Procedure: F1s (Aa) were crossed to a known True-breeder for the same recessive
trait as the recessive parent (aa).
Purple F1 X True White
Mendel's Prediction: The unit of inheritance (recessive allele) in the F1 Hybrid had not lost it's
identity as a result of combining with the dominant allele.
If wrong: Offspring of this cross would all be purple.
If correct: Laws of probability would predict as many purple as white flowered pea plants.
Aa (p = 1/2) and aa (p = 2/2). Therefore, 1/2 x 2/2 = 2/4 = 1/2
Results:
F1 phenotypes: 1/2 Purple & 1/2 White
F1 genotypes: 1/2 Aa & 1/2 aa
What these 3 crosses suggested to Mendel: Unit of Inheritance from Purple Parent is
dominant to that of the unit of inheritance from the White Parent. No blending!
F1 just as purple as purple parent. Hereditary material retains its physical identity.
Units are not altered for having been in the parent's heterozygous condition (state).
MENDEL'S CONCLUSION #1: DOMINANCE
For the gene pair there may be an allele that will display its effect so strongly so as
to mask the expression of the other allele of the gene pair.
MENDEL'S CONCLUSION #2: SEGREGATION
Organisms inherit a pair of genes for each trait. Genes are located on homologue pairs
These genes segregate from each other during meiosis
Each gamete formed will end up with one gene of a trait, but not both.
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Value of a Testcross : Subject displays the dominant phenotype, but what is the
genotype? AA or Aa???
Do a testcross!!: AA x aa (F1 is all Aa)
Aa x aa (F1 is 50% Aa & 50% aa)
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G. Mendel’s 4th Crossing: The Dihybrid Cross
First Step: Repeat Cross #1, but use contrasting forms of 2 traits.
P1 Purple-flowered, Tall X White-flowered, Dwarf
F1 All were Purple-flowered and tall.
Next Step: Cross the F1
Question:
1) Do inheritance units for petal color and stature travel together into gametes?
OR
2) Do units of inheritance for color and stature travel independent of each other?
If stay together -- only 2 gametes possible
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AaBb → AB and ab
Phenotypically: 2 possible combinations possible → Tall, Purple and Short, White
If don't stay together – 4 possible gametes AaBb → AB, Ab, aB, and ab
Phenotypically: 4 possible combinations possible
Results of Cross: 4 combinations of pea plants!! F2 Phenotypic Ratio – 9 : 3 : 3 : 1
Fits Mendel's expectations: Inheritance Obeys the Laws of Probabilities – “Product Rule”
Recall F1 Cross: Aa x Aa -> 3/4 Purple and 1/4 White
Bb x Bb -> 3/4 Tall and 1/4 Dwarf
Probability of a Purple-Petaled Tall?
p = ____________________________
Probability of a Purple-Petaled Dwarf?
p = ____________________________
Probability of a White-Petaled Tall?
p = ____________________________
Probability of a White-Petaled Dwarf?
p = ____________________________
MENDEL'S CONCLUSION #3: INDEPENDENT ASSORTMENT
Each gene pair tends to assort into gametes independent of other gene pairs.
For his pea plants this was true.
Known Today: Hereditary units = Genes Genes for traits may be located on the same
chromosome or may be located on different non-homologous chromosomes.
Amend Conclusion #3 : Each gene pair tends to assort into gametes independent of
other gene pairs located on non-homologous chromosomes.
IV. GENDER (SEX) DETERMINATION
A. The Two Sex Chromosomes
Humans: 22 pairs of autosomal chromosomes
1 pair sex chromosomes (Our 23rd pair!) Typically: Females – XX and Males – XY
The two chromosomes do not look the same.
The two chromosomes do not, to some extent, carry the same information.
Is the Y an X with one of its legs missing? Missing leg = missing genes = missing alleles
Significance? In males, if the expression of a gene on the X does not have a corresponding
allele on the Y that might mask the expression – the X allele is expressed even though it is the
recessive form of the gene.
B. An Example of Linkage – Maleness
Klinefelter's Syndrome – XXY
Caused by a nondisjunction of the X chromosome.
Has 2 X chromosomes, but is male.
Maleness appears to be determined by a gene on the Y chromosome.
TDF – Testis Determining Factor
SRY – Sex-Determinating Region of Y chromosome. Believed to be the Master Gene
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Until 6th or 7th week of embryological development the Wannabe Baby is neither
male nor female. (karyotype could be done to determine gender.)
In Wannabes that carry a Y chromosome the SRY/TDF protein begins to take effect.
If SRY/TDF present – testes form. Testosterone circulates  maleness
If SRY/TDF absent – no testes.
V. VARIATIONS ON MENDEL'S THEMES
A. Incomplete Dominance -- aka Intermediate Inheritance
"Dominant" allele does not completely mask the expression of the "recessive" allele.
Blending of the phenotype, but the physical identity of the two alleles is not altered.
Red petals X White petals
A'A' X AA
F1 genotype : all A'A
F1 phenotype : all pink
Cross F1: Pink petals X Pink petals
A'A X A'A
F2 genotype – 1/4 A'A ': 2/4 A'A : 1/4 AA
(1:2:1)
F2 phenotype – 1/4 red : 2/4 pink : 1/4 white
(1:2:1)
B. Multiple Allele Systems
Although only 2 alleles for a trait exist for any diploid cell, the number of different alleles
for a trait can be large! When there are more than two alleles for a gene locus.
Landsteiner’s Blood Group (ABO) is an example:
A gene codes for an antigen to be inserted into the cell membrane of Red Blood Cells
Or a gene that instructs the absence of an antigen
A antigen
B antigen
No antigen (“O”)
C. Codominance
Both alleles are expressed in the heterozygote. NO MASK / NO BLEND
Both allelic effects are discernable in the phenotype
Example: Landsteiner's blood group (ABO). A and B are codominant and O is recessive.
PHENOTYPES :
A
B
O
AB
GENOTYPES : AA or AO
BB or BO
OO
AB (← Codominance)
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Cross → Individual with blood type A and an AB individual
2 possibilities exist: AO x AB or AA x AB
A
O
A
A
A
B
B
A
D. LINKAGE
Different genes located on same chromosome are "LINKED".
Example: SRY/TDF on the Y is therefore said to be "Y-linked"
LINKAGE – Tendency of genes located on same chromosome to end up in the same gamete.
Genes located on nonhomologous chromosomes tend to assort independently
of each other and segregate into different gametes.
Text makes a good point by stating "sex-linked" when referring to genes along
the 23rd pair of chromosomes is not always true.
COLOR-BLINDNESS & HEMOPHILIA : Not a condition of "sexuality". X-linked Recessive
HAIRY PINNAE: Causes tufts of hair to grow on ears. Y-linked Recessive
Solving X-linked Recessive Inheritance Problems :
Carrier – individual phenotypically “normal”, but has recessive allele in their genotype. Heterozygous.
Colorblind Man x Carrier Female ( XeY x XEXe )
XE
Xe
X
Xe
X’
Y
Y
Normal Male
X’
x Colorblind Female
X’
X
Y
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X’
VI. CHROMOSOMAL ABERRATIONS
All mutagenic agents mentioned before that cause mistakes in DNA and RNA can also bring
about changes in gene arrangements of chromosome. Irradiation, Chemicals, Cancers, Viruses
Occasionally errors occur during the process of meiosis. These errors will affect offspring.
Errors During Prophase I Cross-Over Events:
DELETIONS
INVERSIONS
TRANSLOCATIONS DUPLICATIONS
Errors During Anaphase Segregation Events:
NONDISJUNCTIONS
A. Deletions
A loss of a part of the chromosome. Break and no repair. Or disrepair – parts get left out!
Significance of this event? * Loss of a gene pair in a resulting zygote
* For the unshortened chromosome: Could mean the expression of
genes that ordinarily would not have been expressed
B. Translocations
Part of chromosome to break off and become attached again, but to a nonhomologous
chromosome. It is also possible for whole chromosomes to become attached to a
nonhomologous chromosome.
C. Duplications (Refer to Text Diagram)
During Cross-over events gene pairs of homologous chromosomes may not pair-up correctly.
Result: One gamete will have an excessive amount of gene sequence – other gamete is shorted!
D. Inversions (Refer to Text Diagram)
Enzymes cut out section of chromosome. During "repair" – section gets returned, but “reversed”
E. Nondisjunctions
Anaphase I – Homologous pair fails to separate.
Anaphase II – Failure of a sisters to separate
Trisomy condition
Trisomy (3 bodies) – presence of 3 like chromosomes. [Recall usual is 2 like chromosomes/cell]
Means an increase in chromosome number for zygote. 2n + 1
Also means that there is 2n - 1.
Most cases FATAL
Trisomy 21 → Down's Syndrome = Nondisjunction of Autosome #21 Baby gets 47 chromosomes
Other "common" examples of Nondisjunction are of the 23rd pair of chromosomes.
XXY – Klinefelter's – (2n +1)
XXX – Metafemale – (2n +1)
XYY – Supermale (???) – (2n +1)
XO – Turner's Syndrome – (2n -1)
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VI. INHERITANCE (HOME ASSIGNMENT – fyi)
VARIABLE EXPRESSIVITY – Phenotype is expressed in differing degrees in different individuals
GENE PAIR INTERACTIONS – Many gene pairs will modify, interfere with, or even prevent the
expression of other gene pairs at different loci.
COOPERATION – 2 genes together produce a phenotype that either one alone would not produce.
EPISTASIS – one gene pair masks the other gene pair. Rabbits: B_ – code for melanin distribution
C_ – codes for tyrosinase
cc – no tyrosinase produced
Tyrosinase necessary for melanin production. Melanin is a pigment in the skin of animals
Regardless of BB Bb (black coat) or bb (brown coat) presence – if no tyrosine → albino.
MULTIPLE EFFECTS OF A SINGLE GENE
Pleiotropy – expression of a gene on seemingly unrelated aspects of the individual's phenotype.
Sickle-cell Anemia: Blacks and Mediterraneans
Caused by a recessive gene. Group of symptoms caused by a single gene aberration.
Misshaped RBCs -> lack of oxygen to tissues
Selective Advantage of HbS – resistance to malaria
HbA/HbS - carriers, some portion of RBCs are sickled, show mild symptoms.
HbS/HbS - stricken.
ENVIRONMENTAL EFFECTS ON PHENOTYPE
Siamese Cats – Temperature effects the distribution of melanin.
Tyrosinase has temperature sensitive forms. One is less active at warmer temperatures.
The warmer the temperature – the lower the activity of the enzyme.
Therefore, a lack of pigmentation.
Cooler extremities of cat (ears, tail, paws) are darker due to this effect of
temperature on other parts of the cat's body.
VII. HISTORICAL PERSPECTIVE (fyi)
1866 – Mendel published work.
1882 – Flemming published on: Chromosomes & Mitosis
1887 – Weissman suggested that gametes are formed by another process of cellular division that
halves the chromosome number. The process was later identified and called Meiosis
1920 – Mendel's work was well recognized by scientific community.
What work?: 2n cells have 2 units for each trait and that units segregate during gamete
formation. It will still be a few more years before the "units" are recognized as "genes"
along a chromosome.
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VIII. THE CHROMOSOMAL THEORY OF INHERITANCE (fyi)
A. Genes lie upon chromosomes in a linear array.
B. Diploid cells have 2 chromosomes of each type: HOMOLOGUES
C. The 2 sex chromosomes pair as homologues during Meiosis.
D. After pairing in Prophase I, chromosomes segregates; 2n number is reduced to 1n number.
E. Chromosomes assort independently at meiosis.
F. Genes on same chromosome tend to stay together, but crossing-over can lead to genetic novelty.
G. Chromosomal aberrations occur.
H. Independent assortment, crossing over, and aberrations play roles in evolution.
They offer variations in the make-up of the organism.
Variations can be selected for or selected against
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