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Brief History of Genetics
• prehistory,
– “artificial selection” non-random breeding with no
guarantee of results,
• human mediated natural selection
• Canis lupis (wolf) to Canis domesticus (dog),
– helpful, friendly companions lived to breed,
– savage, misbehaving wolves = stewpot,
– oldest undisputed dog bones, 20,000 years at an
Alaskan settlement.
Gourds!
Lagenaria vulgaris
Why I Am a Biologist?
‘Prehistory’ of Genetics
• By around 10,000 years ago, the same
approach yielded,
– reindeer, sheep, goats, pigs, cattle, fowl, etc.
– rice, barley, wheat, lentils, corn, squash, tomatoes,
potatoes, peppers, yams, peanuts, gourds, etc.
– yeast and bacteria for fermentation, etc.
Northwest Palace of Ashurnasirpal II
(833-859 B.C.)
• Selective breeding:
purposeful control of
mating by choosing
parents for the next
generation.
• 1929 survey of 3 oases in
Egypt identified 400 varieties
of dates.
• DNA evidence now allows us
to unravel prehistorical genetic
manipulations.
History of Modern Genetics
• By the 19th century, precise techniques for selective
breeding allowed the systematic creation of strains in
which offspring often had prized traits.
– However, the traits would unpredictably disappear in some
generations and return in others.
Moravian Sheep Breeders Association (1837)
One breeder’s dilemma…
I have an outstanding ram that would be priceless “if its advantages are
inherited by its offspring…if they are not inherited, then it would be worth no
more than its wool, meat and skin.”
Abbot Cyril Napp
• In concluding remarks to the Moravian Sheep
Breeders Society, Abbot Cyril Napp proposed
that breeders could improve predictions of traits in
offspring if they determined the answers to three
basic questions;
– What is inherited?
– How is it inherited?
– What is the role of chance in heredity?
Monastery of St. Thomas
Brno, 1843
• Abbot Cyril Napp, master of the monastary
admitted Johann Mendel, a gifted student from a
poor peasant family,
– Johann changed his name to Gregor,
– was sent to the University of Vienna,
• studied physics, chemistry, botany, paleontology
and plant physiology,
• resolved to answer Abbot Napps three questions.
Prevailing Genetic ‘Philosophies’
• Philosophy 1: one parent
contributes most to the offspring,
– the homunculus did it,
• Aristotle contended that it was the
male, via a fully formed being in the
sperm,
• Respected 19th microscopists staked
their reputation that they could see the
homunculus in sperm.
Prevailing Genetic ‘Philosophies’
Philosophy 2: blended inheritance,
– parental traits are mixed and become forever
changed in the offspring.
To begin a Science of Genetics
• careful observation, over time, of large numbers of
organisms,
– identify significant variables,
– measure the variables meticulously,
• rigorous (i.e. mathmetical) analysis of these observations,
• development of a theoretical framework to explain the
observations.
Napp’s Questions
• Napp…
– “What is inherited?”
– “How is it inherited?”
– “What is the role of chance in heredity?”
Mendel Insight 1
• Use the pea,
Insight 2
• alternate forms,
Insight 3
• True breeding lines,
– “Permit me to state that, as an empirical worker, I must
define constancy of type as the retention of character
during the period of observation”. -Mendel
– Mendel observed his ‘true-breeding’ lines for up to 8
generations.
• Used the pure-breeding line to form hybrid lines,
– offspring of genetically dissimilar parents.
Insight 4
• Expert plant breeder,
– carefully controlled the matings,
• prevented the intrusion of any pollen foreign to the desired
mating,
– made reciprocal crosses:
• reversing the traits of the male and female parents,
– male wrinked x female smooth,
– female wrinkled x male smooth.
Insight 5
• Used large numbers of subjects,
– applied statistical analysis to his data!
• uncovered the patterns of transmission that we
“eventually” will take for granted.
Insight 6
• Controlled for environmental factors,
– for example, when looking at the short and tall
plants, he made sure that all subjects received
equal light,
• from his studies of plant physiology, he knew that
light mediates stem elongation.
Insight Summation
•
•
•
•
•
•
Used the pea,
Identified alternate forms,
Identified and used true breeding lines,
Expert plant breeder,
Used statistical analysis,
Controlled for environmental factors.
Set up a simple ‘black and white’ system,
and then figured out how it worked.
Monohybrid Cross
Mating between individuals that
differ in only one trait.
Monohybrid Cross
Generation
Parental (P)
yellow pea
(pollen)
green pea
(eggs)
x
First Filial (F1)
all yellow
Second Filial (F2)
grow plants, cross
pollinate
grow, allow to selffertilize
6022 yellow : 2001 green
3:1
Reappearance of Trait in F2
Generation Disproves Blending
• Blending did not occur, in fact over 2000 peas
retained the information necessary to make green
peas,
• Mendel concluded that there must be two types of
yellow peas,
– those that breed true like the parent plant,
– those that can yield some green peas, like some of the
F1 hybrids.
Reciprocal Crosses Disproved
Influential Parent Myths
•
In all monohybrid crosses, the ratio of contrasting traits was approximately 3:1,
– in the yellow(male) x green (female) pea cross, three yellow peas were produced for
every green pea in the F2 generation.
•
Same ratio independent of which parent carried the dominant trait...
Dominant vs. Recessive Traits
x
P
F1
The trait that appears in the F1 generation is the DOMINANT trait.
The trait that disappears in the F1 generation is termed RECESSIVE.
Nomenclature
• Dominant unit factors are designated with a
capital letter, often (but not always) with the first
letter of the description,
– Y = yellow,
– V = violet,
– T = tall,
• Recessive unit factors are represented by small
letters,
– y = green,
– v = white,
– t = dwarf,
Nomenclature II
…it’s not my fault.
• Dominant unit factors are designated with a
capital letter,
– G = yellow,
– W = violet,
– D = tall,
• Recessive unit factors are represented by small
letters, often (but not always) with the first letter
of the description,
– g = green,
– w = white,
– d = dwarf,
Mendel’s First Postulate
Unit Factors in Pairs
• Genetic characteristics are controlled by
unit factors that exist in pairs in individual
organisms,
– each individual receives one unit factor from each
parent,
– in a monohybrid cross, three combinations of unit
factors are possible,
Definitions to Know
• Homozygous: the unit factors that
determine a particular trait are the same,
– YY = homozygous dominant,
– yy = homozygous recessive,
• Heterozygous: the unit factors that
determine a particular trait are different,
– Yy = heterozygous.
Mendel’s Second Postulate
Dominance/Recessiveness
• When two unlike unit factors are present in
a single individual, one unit factor is
dominant to the other, which is said to be
recessive.
Unlike Unit Factors
=
Alternate Forms of the Same Gene
=
Alleles
Unit Factors = Genes
• three combinations of alleles are possible,
YY
Yy
yy
Molecular Alleles
Fig. 2.4
Mendel’s Third Postulate
Segregation
• During the processes of heredity, the paired unit
factors separate so that the offspring receives one unit
factor from each parent,
• The unit factors segregate to offspring randomly.
When Unit Factors Separate
Two Unit Factors = Diploid
One Unit Factor = Haploid
During Gamete formation, Unit Factors Separate
More Definitions to Know
• Phenotype: an observable trait,
• Genotype: the actual alleles present in an
individual.
Mendel’s First Three Postulates
Unit Factors in Pairs
Dominance/Recessiveness
Random Segregation
Postulates 1-3 Applied
P1 - F1 Generation
Postulates 1-3 Applied
F1 - F2 Generation
Yellow
F1:
Yy
F1 Self-Cross: Yy
Gametes:
F2:
Yy
Y or y
YY
Yy
Y or y
Yy
yy
3 : 1 Phenotypic Ratio
(1:2:1 Genotypes)
1
Yellow
F2:
YY
homozygous
dominant
:
1
:
1
:
Yellow
Yy
Yellow
Yy
heterozygous
heterozygous
1
Green
yy
homozygous
recessive
Punnett Squares
Y
y
Y
YY
Yy
Y
YY
Yy
gametes
Parent 2
gametes
Parent 1
Predicted
Offspring In
Squares
Back to the Moravians
• So, you’ve got a prize ram, how do you tell
it’s not a dud dad?
S = stud
s = dud
SS or Ss?
Test Cross
• Your ram has a stud phenotype, but unknown genotype,
– cross it to a homozygous recessive individual,
s
s
S
Ss
Ss
S
Ss
Ss
all studs
s
s
S
Ss
Ss
s
ss
ss
half studs, half duds
Test Cross
• Your ram has a stud phenotype, but unknown genotype,
– cross it to a homozygous recessive individual,
SS x ss
Ss x ss
all stud
1/2 stud, 1/2 dud
The phenotypic ratio is the same as
the allele ratio in the tested parent!
Extra Credit
(formatted for lecture)
No Genetic Reason to Discourage
Cousin Marriage, Study Finds
April 3, 2002
Byline: Denise Grady, NYT
Who: Dr. Arno Motulsky
Where: U. Washington
• Unrelated couples have a 3-4% chance of having an offspring with a
genetic based birth defect. Cousin-mating only raises the risk by 1.7 2.8% more. The report compiled the results of six studies on thousands
of cousin-couplings.
• Although there is an increased risk, it is very small. Significantly, the
additional risk is mitigated by standard genetic counseling.
• It is illegal in 30 states to marry your first cousin,
– for many cultures, familial marriages are the cultural norm,
– legal issues, social stigma, family strife, and decisions such as abortions
resulting from overestimates of risks may need to be reconsidered.
Chance and Probability
Chance
Probability
100%
1
50%
0.5
%0
0
(1/2), fractions are
often used in
genetics.
* Know how to multiply, divide, subtract and add fractions! *
Laws of Probability
• Product Law: the probability of two or more
independent outcomes occurring is equal to the product
of their individual probabilities.
…text uses the term (Multiplication Law).
Laws of Probability
Product Law Example
The probability IF the parent was Ss…
•
•
•
•
1 Stud (S_) sheep offspring = .5
2 Stud sheep offspring = .5 x .5 = .25
3 Stud sheep offspring = .5 x .5 x .5 = .125
8 Stud sheep offspring = .5 x .5 x .5 x .5 x .5 x .5 x .5 x .5 = .004
• 10 stud sheep offspring = .001 = 0.1%
– or, you have a 99.9% chance that your Ram is a Stud.
Mendel’s Forth Postulate
Independent Assortment
• How do two traits segregate in the offspring
of an individual that is heterozygous for
both traits?
Dihybrid Crosses
• Monohybrid Cross,
– one set of contrasting traits,
• Y (yellow) versus y (green).
• S (smooth) versus s (wrinkled).
• Dihybrid Cross,
– SSYY x ssyy
Dihybrid Cross,
phenotype
genotype
gametes
genotype
Dihybrid Cross,
F1
SY
F2?
phenotype
genotype
smooth/yellow
SsYy
sY
Sy
sy
gametes
genotype
Gamete Formation in F1
Dihybrids
S
s
Y
y
genotype
SY
Sy
sY
sy
gametes
.25
.25
.25
.25
probability
Mendel’s Results
(phenotypes)
• Y_ S_
= 315
=9
• yyS_
= 108
=3
• Y_ss
= 101
=3
• yyss
= 32
=1
• S_ Y_
=9
• S_yy
=3
• ss Y_
=3
• ssyy
=1
Independent Assortment
• Y_
• yy
= 12
= 4
• S_
• ss
= 12
= 4
Still 3:1 ratios!
Monohybrid Cross
…and probability
P:
GG
F1:
gg
Gg
Gametes:
F2:
x
G
g
1/2
G
g
1/2
1/2
1/2
GG
Gg
gG
gg
1/2 x 1/2
1/2 x 1/2
1/2 x 1/2
1/2 x 1/2
1/4
1/4
1/4
1/4
Laws of Probability
Sum Law: The probability of an outcome that
can occur in more than one way is the sum
of the probabilities of the individual events.
Random Segregation
F2:
GG
1/4 GG
1/4 GG
Gg
gG
gg
1/4 Gg
1/4 gG
1/4 gg
1/2 Gg
1/4 gg
Problem
 You cross YYSs x YySs individuals.
 What are the expected phenotypic ratios?
 use a Punnett Square, and/or probability
calculations to get your answer.
YYSs x YySs
YS
YS
Ys
Ys
YS
YS
YS
YS
YS
YS
Ys
YS
Ys
• S_ Y_
Ys
YS
Ys
YS
Ys
Ys
Ys
Ys
Ys
• S_yy
yS
YS
yS
YS
yS
Ys
yS
Ys
yS
• ss Y_
ys
YS
ys
YS
ys
Ys
ys
Ys
ys
• ssyy
= 12
=4
Using Probability
WWMD
YYSs x YySs
YY x Yy
Independent
Assortment
Ss x Ss
gametes
YY or Yy
.5
.5
(p) Y_ = 1
SS
Ss
ss
.25 .5
.25
Random Segregation
probability
(p) S_ = .75
Product Rule: (p) Y_S_ = .75
(p) ss = .25
Product Rule: (p) Y_ss = .25
Assignment
• Probability, know it well,
• Start mastering problems 2.1 - 2.19 (all),
• Central Dogma Quiz Wednesday