Chapter 14: Mendel and the Gene Idea

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3.a.3 – The chromosomal basis of inheritance
provides an understanding of the pattern of
passage (transmission) of genes from parent
to offspring (14.1-14.4).
4.c.2 – Environmental factors influence the
expression of the genotype in an organism –
(14.3).
4.c.4 – The diversity of species within an
ecosystem may influence the stability of the
ecosystem (14.3).
1. Blending Theory - traits were like
paints and mixed evenly from both
parents
2. Incubation Theory - only one
parent controlled the traits of the
children
Ex: Spermists and Ovists
3. Particulate Model - parents pass
on traits as discrete units that retain
their identities in the offspring
Father of Modern
Genetics
 Mendel’s paper
published in 1866,
but was not
recognized by science
until the early 1900’s
 Died prior to his
“fame”
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Used an experimental approach (scientific
method)
Applied mathematics to the study of natural
phenomena
 Ratios and probability
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Kept good records and observations
Large test sample/size
1.
2.
Short life span
Bisexual
*Both sexes in one flower/plant
*Stamens and carpels
3.
Many traits known
*Easy to see/observe traits
4.
Cross- and self-pollinating
*Easy to control reproduction
5.
You can eat the failures 
Cross between two different parents
 Results in hybrid offspring
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◦ The offspring may be different than the
parents.
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Cross with only one flower
◦ Stamens/carpels fertilize each other!
Naturally occurring event in pea
plants
 Results in pure-bred offspring where
the offspring are identical to the
parents
 Is this asexual reproduction???
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 NO…you still have gametes
Used seven characters, each with two
expressions or traits
 Example:
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◦ Character - height
◦ Traits - tall or short
Mono = one
 Crosses that work with a single
character at a time
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◦ Example - Tall X short
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The Parental generation or the first
two individuals used in a cross
◦ Example - Tall X short
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Mendel used reciprocal crosses, where
the parents alternated for the trait
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F1 - first filial generation
◦ Filial – Latin for “son”
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F2 - second filial generation,
◦ Bred by crossing two F1 plants together or
allowing a F1 to self-pollinate
Notice: TWO P1
plants shown
(cross fertilz.)
Notice: only
ONE plant
shown
(self-fertilz.)
P1
F1
F2
Tall X short (TT x tt)
all Tall (Tt)
3 tall to 1 short
(1 TT: 2 Tt: 1 tt)
Tall
Short
 Mendel
observed SAME pattern
in ALL 7 characters
◦ F1 generation showed only one of
the traits (regardless of sex)
◦ The other trait reappeared in the
F2 at ~25%
 3:1 ratio; 3 dominant – 1 recessive
 Remember: the % are estimates (still have
mutations that could change %)
1.
2.
Genes can have alternate versions
called alleles
Each offspring inherits two alleles,
one from each parent
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He made this conclusion without
having knowledge of
chromosomes/DNA makeup
** Remember: Each diploid cell
has a pair of homologous
chromosomes
-Therefore, any gene has 2 loci
*one on maternal chromo
*one on paternal chromo
3.
If the two alleles differ, the
dominant allele is expressed
 The recessive allele remains
“hidden” (unseen) unless the
dominant allele is absent
 Now called Mendel’s Law of Dominance
4.
The two alleles for each trait
separate during gamete formation
(meiosis)
 This now called Mendel's Law of
Segregation
 Phenotype
- the physical
appearance of the organism
 Genotype - the genetic makeup of
the organism, usually shown in a
code
◦ T = tall
◦ t = short
 Homozygous
- When the two
alleles are the same (TT/tt)
 Heterozygous- When the two
alleles are different (Tt)
 Notice (for single-gene traits:
◦ Three choices for genotypes
◦ Homo Dom (TT), Homo Rec (tt),
Hetero (Tt)
Cross
TT X tt
Tt X Tt
TT X TT
tt X tt
TT X Tt
Tt X tt
Genotype
all Tt
1TT:2Tt:1tt
all TT
all tt
1TT:1Tt
1Tt:1tt
Phenotype
all Dom
3 Dom: 1 Res
all Dom
all Res
all Dom
1 Dom: 1 Res
Notice the
3:1 ratio!!!
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Cross of a suspected heterozygote
with a homozygous recessive
◦ Goal: to determine genotype of unknown
Ex: T? X tt
*If TT - all Dominant
*If Tt - 1 Dominant: 1 Recessive
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Cross with two genetic traits
◦ Di = two
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Need 4 letters (two for each trait) to
code for the cross
◦ Ex: TtRr (Mono = Tt OR Rr)
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Each Gamete - Must get 1 letter for
each trait
◦ Ex. TR, Tr, etc. (when combine = 4
letters)
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Critical to calculating the results of higher
level crosses
Look for the number of heterozygous traits
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The formula 2n can be used, where “n”
= the number of heterozygous traits.
Ex: TtRr, n=2 (2 heterozygous traits)
◦ 22 or 4 different kinds of gametes are
possible (TR, tR, Tr, tr)
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Ex: TtRR, n = ?
◦ 21 or 2 different gametes are possible
TtRr X TtRr
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Each parent can produce 4 types of
gametes. (n=2; 22=4)
◦ TR, Tr, tR, tr
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Cross is a 4 X 4 = 16 possible
offspring
9
3
3
1
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Tall, Red flowered
Tall, white flowered
short, Red flowered
short, white flowered
Or: 9:3:3:1 ratio
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The inheritance of 1st genetic trait is
NOT dependent on the inheritance of
the 2nd trait
◦ Ex: Inheritance of height is independent of
the inheritance of flower color
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This relates to dihybrid crosses – one
character’s inheritance is NOT
connected to the inheritance of
another!
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Ratio of Tall to short is 3:1
Ratio of Red to white is 3:1
The cross is really a product of the ratio of
each trait multiplied together.
(3:1) X (3:1) = 9:3:3:1
◦ *Use FOIL method to attain ratio
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Genetics is a specific application of the
rules of probability
Probability - the chance that an event will
occur out of the total number of possible
events
The monohybrid “ratios” are actually
the “probabilities” of the results of
random fertilization
Ex: 3:1
75% chance of the dominant
25% chance of the recessive
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The probability that two alleles will
come together at fertilization, is equal
to the product of their separate
probabilities
 Steps to determining probability:
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◦ 1) Determine ratios for each character/trait
 How? Do “little” Punnett squares for EACH trait
◦ 2) Multiply ratios together
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The probability of getting a tall offspring
is ¾.
The probability of getting a red offspring
is ¾. (use same Punnett square as above – only with R/r)
The probability of getting a tall red
offspring is ¾ x ¾ = 9/16
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Use the Product Rule to calculate the results
of complex crosses rather than work out the
Punnett Squares
Ex: TtrrGG X TtRrgg
TtrrGG X TtRrgg
“T’s” = Tt X Tt = 3:1
“R’s” = rr X Rr = 1:1
“G’s” = GG x gg = 1:0
Product is:
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
Epistasis
Polygenic Inheritance
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When the F1 hybrids show a
phenotype somewhere between the
phenotypes of the two parents
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white
 NOT BLENDING!!!!!
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Not enough
red pigment
made
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No hidden recessive
3 phenotypes and 3 genotypes
(Hint! – often a “dose” effect)
◦ Red = CR CR
◦ Pink = CRCW
◦ White = CWCW
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Both alleles are expressed equally in the
phenotype
NOT an intermediate (like incomplete
dominance
Ex. MN blood group
◦ MM, MN, NN
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Ex: Rooster/chicken feathers
Ex: flower petal color
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No hidden recessive
3 phenotypes and 3 genotypes (but not a
“dose” effect)
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When there are more than 2 alleles for a
trait
◦ *Remember: only 2 alleles exist for Mendel’s
pea plants
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Ex. ABO blood group
◦ IA - A type antigen
◦ IB - B type antigen
◦ i - no antigen
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Multiple genotypes and phenotypes
Very common event in many traits
Phenotypes
A
B
AB
O
Genotypes
IA IA or IAi
IB IB or IBi
IAIB
ii
 IA
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and IB are dominant
A and B are CODOMINANT
A and B are the names for two different
carbohydrates found on the surface of RBCs
◦ Blood types are actually ways of differentiating the
type of antigens on a person's red blood cells
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Rh blood factor is a separate factor from
the ABO blood group
Rh+ = dominant
Rh- = recessive
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Wife is type A
Husband is type AB
Child is type O
Question - Is this possible?
Comment - Wife’s boss is type O…There’s
some explaining to be done!
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Factors that are expressed as continuous
variation
Lack clear boundaries between the phenotype
classes
Ex: skin color, height
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Several genes govern the inheritance of the
trait
Ex: Skin color is likely controlled by at least 4
genes
◦ Each dominant gives a darker skin
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Mendelian ratios fail
Traits tend to "run" in families
Offspring often intermediate between the
parental types
Trait shows a “bell-curve” or continuous
variation
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Often done by Pedigree charts
Why?
◦ Can’t do controlled breeding studies in humans
◦ Small number of offspring
◦ Long life span
Male
Female
Person with trait
Dominant Trait
Recessive Trait
Several thousand known!
 Some examples:
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◦ Albinism
◦ Sickle Cell Anemia
◦ Tay-Sachs Disease
◦ Cystic Fibrosis
◦ PKU
◦ Galactosemia
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Most common inherited disease among
African-Americans
Single amino acid substitution results in
malformed hemoglobin
Reduced O2 carrying capacity
Codominant inheritance
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Only affects Eastern European Jews
Brain cells unable to metabolize type of lipid;
accumulation of the lipid causes brain damage
Death in infancy or early childhood
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Most common lethal genetic disease in the
U.S.
Most frequent in Caucasian populations (1/20
a carrier)
Produces defective chloride channels in
membranes
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Usually rare
Skips generations
Occurrence increases with consaguineous
matings (people descended from the same
ancestor)
Often an enzyme defect
Affects males and females equally
Less common then recessives
 Affects males and females equally
 Ex:
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◦ Huntington’s disease
◦ Achondroplasia
◦ Familial Hypercholesterolemia
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Each affected individual had one affected
parent.
Doesn’t skip generations.
Homozygous cases show worse phenotype
symptoms.
May have post-maturity onset of symptoms.
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Blood tests for recessive conditions that can
have the phenotypes treated to avoid
damage
Genotypes are NOT changed
Ex: PKU
◦ Required by law in all states
◦ Tests 1- 6 conditions
◦ Required of “home” births too
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Where Genetic and Environment Factors
interact to cause the disease
Ex: Heart Disease factors
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Genetics
Diet
Exercise
Bacterial infections
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Recognize Mendel's experiments and their role in the
scientific discovery of genetic principles.
Identify Mendel's Laws of Genetics.
Recognize the use and application of probability in
genetics.
Recognize the basic Mendelian crosses and genetic
terminology.
Recognize various extensions of Mendelian genetics and
their effect on inheritance patterns.
Identify human traits that exhibit Mendelian inheritance
patterns.
Recognize methods used in genetic screening and
counseling.