GENETICS
Chapter 7
WHAT WE THOUGHT BEFORE MENDEL?
Aristotle (sometime in the 400’s)
 Thought we were composed of “vapors” & “fluids”
“Fluids”- male ejaculate “purified blood” b/c it wasn’t
red
 This first led to the thought that heredity was related
to blood
 “Vapors”- female “invisible particles”


During intercourse these, fluids and vapors
combine, thus creating offspring similar to
parents.
WE CAN SEE EVERYTHING WITH
MICROSCOPES

Theory of Preformation (1600-1700)- Thought
they could see “little people” in sperm when
viewed under a microscope.
EPIGENETICS

Schools in 1800’s taught acquired
characteristics- that you can alter yourself to
affect your offspring.


Ex: playing music, building muscles, smarts
We now call this Epigenetics or changes in gene
expression by mechanisms other than changes in
DNA.
EPIGENETICS CONTINUED…



It was first rejected but now recent evidence has
shown might have been right all along.
This idea was discovered in mice by the work of
Randy Jirtle
He discovered that what a mother mouse ate
during pregnancy can have a effect on gene
expression in the next generation of mice.

This food was high in methyl groups, a substance
that can turn on and off gene expression.
THOSE POOR RATS

August Weismann(1883)- experiment on rats by
cutting off their tails
Rats with cut tails produced rats with tails (~20x)
 Cast doubt on Aristotle’s theory
 Also proposed the idea of Somatic and Germ tissue.

GREGOR MENDEL (1822-1884)

THE Father of Genetics

Worked on Pea Plants (Pisum sativum)

In 1866, his papers were 1st published but
weren’t found until the 1900’s.
WHY PEA PLANTS?
1.
They reproduced quick and with a large amount
of offspring
2.
They contained a wide variety of variation (to
which he studied and composed 7 traits)
3.
Both male and female parts were on one flower
4.
Very convenient to work with.
3 STEPS THAT LED TO HIS DISCOVERY
1.
Had plants self-pollinate until he was sure that
it was a true-bred trait.

2.
He then mated the two opposite traits via crosspollination.

3.
Called this the Parental(P) Generation
Called this the Filial (F1) Generation
He let those offspring then self-pollinate.

F2 Generation. Counted the number of each trait
that was produced.
TERMS AND HOW THEY RELATE TO
GENETICS


Gene- a segment of DNA that transmits
information from parent to offspring.
Allele- Alternative forms of a gene for a trait.


Ex: T, t
Dominant- An allele that has a higher potency.
MORE TERMS


Recessive- a copy of an allele that is not readily
expressed unless it contains another recessive
allele.
Homozygote- 2 alleles are similar. Purebred.


Heterozygote- alleles are different. Hybrid/mutt.


Ex: Rr
Phenotype- Physical expression of a gene.


Ex: RR, rr
Ex: Tall, short
Genotype- Actual genetic construction.

Ex: AA, aa, Aa
MENDEL’S GENETIC LAWS
1.
Law of Dominance
2.
Law of Segregation
3.
Law of Independent Assortment
LAW OF DOMINANCE
Only the dominant allele in a heterozygote is
expressed.
 Dominant is always put first and capitalized
when written out.


Ex:
SS= Smooth
Ss= Smooth
ss= Wrinkled
LAW OF SEGREGATION



2 alleles of a parent separate during Sexual
Reproduction and only one is randomly chosen to
be passed to offspring.
Mendel created this law by saying that his
“factors” split during meiosis.
Walter Sutton- found that chromosomes also
split during meiosis and that Mendel’s “factors”
were on these chromosomes. We call this the
chromosomal theory of inheritance.
T
tt
TT
T
t Tt
Tt
t Tt
Tt
HOW SEGREGATION RELATES BACK TO
MEIOSIS
Genes
Chromosomes
Parental: RR
rr
Parental
Only R
Gametes:
Generation 1:
only r
Rr
LAW OF INDEPENDENT ASSORTMENT


The segregation of 1 pair of alleles occurs
independently of the segregation of any other
pair.
We inherit one and then the other.
USING PROBABILITY AND RATIOS


Mendel was a mathematician so he used math to
predict genetic outcomes.
Probability uses rules that can predict how genes
will be distributed among the offspring of two
parents.
Probability = # of one outcome
# of total outcomes

Ex: Rolling dice or Flipping a coin.
MONOHYBRID CROSS AND PUNNETT
SQUARES

Only using one trait and determining the
outcome.
DIHYBRID CROSS AND LAW OF IA

Using two pairs of contrasting traits.
PUNNETT SQUARE PROBLEMS
HOW MANY DIFFERENT GAMETES?


When given Allele combinations, we can use
math to figure out how many possible gametes
could be produced.
We know that there are only 2 possibilities that
the gamete can have for one trait.


Its either going to have a Dominant or a Recessive
Allele
Ex:
AA
1
Aa
2
aa
1
HOW MANY DIFFERENT GAMETES CAN YOU
HAVE OUT OF THE FOLLOWING COMBINATIONS?


AABb
1 x 2 = 2 possible different gametes (AB or Ab)
CcDdEe
2 x 2 x 2 = 8 possible different gametes
(CDE, CDe, CdE, Cde, cDE, cDe, cdE, cde)
ffGgHhIiJJ
1x2x2x2x1 = 8 possible different gametes
(fGHIJ, fGHiJ, fGhiJ, fGhIJ, fgHIJ, fgHiJ, fghIJ,
fghiJ)

ONE LAST PROBLEM.

AaBbCCDDeeFFGGhhIIjjKkLlMmNNooPP
2x 2 x1 x1 x1 x1 x1 x1x1x1x2x2 x2 x1 x1 x1 = ?
=32 possible different gametes



Now Imagine how our genes work and in each
gamete we have 23,000 different genes.
That would be 2^23,000 = error
That is a whole lot of different gametes.
HOW MANY PHENOTYPES/GENOTYPES?
The secret to this is the mastery of the F.O.I.L.
 First, Outside, Inside, Last
For example: lets look at a monohybrid cross
Aa x Aa

(A a)x(A a)
Four pairs of alleles: AA, Aa, Aa, aa
HOW MANY PHENOTYPES/GENOTYPES
How many Phenotypes are possible in the following
Combination?
Aabb x AaBb
Aa x Aa
bb x Bb
AA, Aa, Aa, aa
Bb, bb, Bb, bb
2
x
2= 4 phenotypes
Genotypes?
Aa x Aa
bb x Bb
AA, Aa, Aa, aa
Bb, bb, Bb, bb
3
x
2
=6
RATIO PROBLEMS AND QUIZ


The handout for today contains the ratio
problems. They will be due on
Also, a quiz over the previous material and the 4
exceptions to Mendel on:
FRIDAY
EXCEPTIONS TO MENDEL
1.
Multiple Alleles
2.
Incomplete Dominance
3.
Co-dominance
4.
Lethality
MULTIPLE ALLELES

More than 2 alleles exist for some gene


Means more phenotypes and genotypes to deal with.
Ex: Coat color in rabbits
C+- agouti
 Cch- chinchilla
 Ch- Himalayan
 C- albino


C+ > Cch > Ch > C
INCOMPLETE DOMINANCE


Heterozygote has a phenotype intermediate to
the two homozygote types.
Ex: Snapdragon color
RR= Red
 Rr= Pink
 rr= White

Phenotype ratio is similar to genotype
 1:2:1

CO-DOMINANCE

Both alleles of a Heterozygote are expressed

Ex: Blood Types

IA – A Antigen

Antigen- Proteins that mark you as being you.
Phenotypes
A
B
AB
O
IB- B Antigen
Genotypes
IAIA, IAi
IBIB, IBi
IAIB
ii
i= no Antigen
LETHALITY


Some offspring have a reduce chance to live
because of their gamete.
Ex: Corn
G- Green color- produces chlorophyll
 g- Yellow Color- no chlorophyll

GG and Gg- Green= live
 gg= Yellow = die b/c no chlorophyll


Genotype ratio- 1:2
ROLE OF THE X AND Y CHROMOSOMES
Females – XX
 Males – XY


Sperm determines sex


Only true of Fruit flies and Humans
Region on Y chromosome that determines sex= SRY
Heterogametic Sex- can make two dif. Gametes
 Homogametic Sex- can only make one Gamete

OTHER SEX ARE DETERMINED
DIFFERENTLY

Fish, bird, reptiles


Homogametic- Males
Heterogametic- Females
Bees and other select insects
They don’t have sex chromosomes but rather male or
female determined by polyploidy.
 Males- n
Females- 2n


Marine worm
Females release pheromones
 No adult females- females

adult females- males
SEX- LINKED GENES
First studied by Thomas Morgan Hunt
 Used Red and white eyed Fruit flies

P: red females and white males
F1: Red females and males
F2: Red females and males and White males
Sex-linked because the gene is located on X
chromosome
SEX- LINKED
Female
WW- Red
Ww- Red
ww – White

Male
WY- Red
wY- White
Hemizygous- only one allele of a gene is present.
WHY DO WE USE FRUIT FLIES FOR
GENETICS?

Long time between generations for humans


Humans produce small # of offspring/ generation


Fruit flies produce ~ 100-200
Humans have a large # of chromosomes


About 2 weeks from generation to next for fruit flies.
Fruit flies have 8
Ethical problems when messing with humans

No one cares that much about flies.
GENETIC DISORDERS
1.
Sex-linked
2.
Sex-Modified
3.
Chromosomal
4.
Sex Chromosome
5.
Recessive disorders
6.
Dominant disorders
SEX-LINKED

Gene expressed usually in one sex
1.
Hemophilia – failure of blood to clot
2.
Deuteronopia (color blindness)
3.
Beards/ Mustache and Breast development
4.
Muscular Dystrophy – wasting away muscles
SEX- MODIFIED

Exhibits a reversal of Dominance between Sexes

Ex: Pattern Baldness
B1- Dominant Male: No hair
 B2- Recessive Male: Hair

B1- Recessive Female: No hair
 B2- Dominant Female: hair

CHROMOSOMAL DISORDERS

Trisomic 13- Patau Syndrome – die w/in 6
months


Trisomic 18- Edwards Syndrome- die w/in 6
months


rare
rare
Trisomic 21- Downs Syndrome- will survive but
with a less than normal lifespan.

1/900

Scan chart here.
SEX CHROMOSOME




Triple X (XXX)– Normal intelligence and are
fertile.
Turner Syndrome (XO) – Short, sterile,
undeveloped 2ndary sex characteristics.
Klinefelter Syndrome (XXY) – Long limbs, sterile,
breast development, underdeveloped genitalia.
Extra Y Syndrome (XYY) - increased risk of
antisocial behavior, fertile.
RECESSIVE DISORDERS




Cystic Fibrosis- mucus clogs lungs, liver, and
pancreas. Don’t survive adulthood.
Sickle cell anemia- Poor blood circulation.
Tay- Sachs- Deterioration of central nervous
system in infancy and don’t last to adulthood.
Phenyl- Ketonuria- Failure of brain to develop in
infancy, if untreated they don’t survive to
adulthood.
DOMINANT DISORDERS

Huntington’s Disease- Gradual deterioration of
brain tissue in middle age, shortened life
expectancy.
TECHNIQUES FOR DETECTING GENETIC
DISORDERS

Pedigrees


Family history of a disorder
Genetic Counseling
PEDIGREES: 3 STEPS ON HOW TO READ
THEM
1.
Sex-Linked?


2.
Dominant or Recessive?


3.
Usually seen only in males because they have 1 X
If Autosomal should be = distribution between sexes
Dom- every offspring that has it should have parent
Rec- parent is a hetero, normal but a carrier
Single gene or several?


If 1 gene, parent: offspring ratio should be 3:1 (25%)
If multiple, ratio and percentage should be lower
GENETIC COUNSELING
Helps families understand the risk of passing the
disorder on by analyzing pedigrees.
 Can also see genetic makeup of embryo by using
prenatal testing through karotyping.

Amniocentesis- withdraw fluid @ week 16 to check
enzyme activities.
 Chorionic villus sampling- part of placenta is tested
early in pregnancy.
 Ultrasound- detect size, position, sex, organ health
 Fetoscopy- direct view of fetus


Gene Replacement- new technique for replacing
bad genes for good genes.