Download - Chapter 14: Mendel and the Gene Idea
Chapter 14: Mendel and the Gene Idea
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).
Essential Knowledge
1. Blending Theory - traits were like paints and mixed evenly from both parents
2. Incubation Theory - only one parent controlled the traits of the childrenEx: Spermists and Ovists
3. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring
Past/Present Genetic Hypotheses
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”
Gregor Mendel
Used an experimental approach (scientific method)
Applied mathematics to the study of natural phenomena Ratios and probability
Kept good records and observations Large test sample/size
Reasons for Mendel's Success
1. Short life span 2. 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
Why Use Peas?
Cross between two different parents Results in hybrid offspring
◦The offspring may be different than the parents.
Cross-pollination
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??? NO…you still have gametes
Self-pollination
Used seven characters, each with two expressions or traits
Example: ◦Character - height◦Traits - tall or short
Mendel's Work
Mono = one Crosses that work with a single character at a time◦Example - Tall X short
Monohybrid or Mendelian Crosses
The Parental generation or the first two individuals used in a cross◦Example - Tall X short
Mendel used reciprocal crosses, where the parents alternated for the trait
P Generation
F1 - first filial generation◦Filial – Latin for “son”
F2 - second filial generation,◦Bred by crossing two F1 plants together
or allowing a F1 to self-pollinate
Offspring
Notice: only ONE plant shown
(self-fertilz.)
Notice: TWO P1 plants shown (cross fertilz.)
P1 Tall X short (TT x tt)F1 all Tall (Tt)F2 3 tall to 1 short (1 TT: 2 Tt: 1 tt)
Another Sample Cross
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 %)
Results - Summary
1. Genes can have alternate versions called alleles
2. Each offspring inherits two alleles, one from each parent He made this conclusion without
having knowledge of chromosomes/DNA makeup
Mendel's Hypothesis
** Remember: Each diploid cell has a pair of homologous chromosomes
-Therefore, any gene has 2 loci *one on maternal chromo
*one on paternal chromo
Homologous chromosomes
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
Mendel's Hypothesis
4. The two alleles for each trait separate during gamete formation (meiosis) This now called Mendel's Law of
Segregation
Mendel's Hypothesis
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
Genetics Vocabulary
VocabularyHomozygous - 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 Genotype PhenotypeTT X tt all Tt all DomTt X Tt 1TT:2Tt:1tt 3 Dom: 1 ResTT X TT all TT all Domtt X tt all tt all ResTT X Tt 1TT:1Tt all DomTt X tt 1Tt:1tt 1 Dom: 1 Res
6 Mendelian Crosses are Possible
Notice the 3:1 ratio!!!
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
Test Cross
Cross with two genetic traits◦Di = two
Need 4 letters (two for each trait) to code for the cross◦Ex: TtRr (Mono = Tt OR Rr)
Each Gamete - Must get 1 letter for each trait◦Ex. TR, Tr, etc. (when combine = 4
letters)
Dihybrid Cross
Critical to calculating the results of higher level crosses
Look for the number of heterozygous traits
Number of Kinds of Gametes
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)
Ex: TtRR, n = ?◦21 or 2 different gametes are possible
Equation
TtRr X TtRr Each parent can produce 4 types of gametes. (n=2; 22=4)◦TR, Tr, tR, tr
Cross is a 4 X 4 = 16 possible offspring
Dihybrid Cross
9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered
Or: 9:3:3:1 ratio
Results
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 This relates to dihybrid crosses – one character’s inheritance is NOT connected to the inheritance of another!
Law of Independent Assortment
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
Comment
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
Probability
The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization
Ex: 3:175% chance of the dominant25% chance of the recessive
Genetic Ratios
The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities
Steps to determining probability:◦ 1) Determine ratios for each character/trait
How? Do “little” Punnett squares for EACH trait
◦ 2) Multiply ratios together
Rule of Multiplication
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
Example: TtRr X TtRr
Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares
Ex: TtrrGG X TtRrgg
Product Rule
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
Solution
1. Incomplete Dominance2. Codominance3. Multiple Alleles4. Epistasis5. Polygenic Inheritance
Variations on Mendel
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!!!!!
Incomplete Dominance
Not enough red pigment made
No hidden recessive 3 phenotypes and 3 genotypes
(Hint! – often a “dose” effect)◦ Red = CR CR
◦ Pink = CRCW
◦ White = CWCW
Result from Inc Dominance
Another example
Both alleles are expressed equally in the phenotype
NOT an intermediate (like incomplete dominance
Ex. MN blood group◦ MM, MN, NN
Ex: Rooster/chicken feathers Ex: flower petal color
Codominance
No hidden recessive 3 phenotypes and 3 genotypes (but not a
“dose” effect)
Result from Codominance
When there are more than 2 alleles for a trait◦ *Remember: only 2 alleles exist for Mendel’s
pea plants
Ex. ABO blood group◦ IA - A type antigen◦ IB - B type antigen◦ i - no antigen
Multiple Alleles
Multiple genotypes and phenotypes Very common event in many traits
Result from Multiple Alleles
Phenotypes Genotypes A IA IA or IAi B IB IB or IBi AB IAIB O ii
Alleles and Blood Types
IA 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
Blood types
Rh blood factor is a separate factor from the ABO blood group
Rh+ = dominant Rh- = recessive
Rh factor
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!
Blood Type Problem
Factors that are expressed as continuous variation
Lack clear boundaries between the phenotype classes
Ex: skin color, height
Polygenic Inheritance
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
Genetic Basis
Mendelian ratios fail Traits tend to "run" in families Offspring often intermediate between the
parental types Trait shows a “bell-curve” or continuous
variation
Result from Polygenic Inheritance
Often done by Pedigree charts Why?
◦ Can’t do controlled breeding studies in humans◦ Small number of offspring◦ Long life span
Genetic Studies in Humans
Male
Female
Person with trait
Pedigree Chart Symbols
Sample Pedigree
Dominant Trait Recessive Trait
Several thousand known! Some examples:
◦Albinism◦Sickle Cell Anemia◦Tay-Sachs Disease◦Cystic Fibrosis◦PKU◦Galactosemia
Human Recessive Disorders
Most common inherited disease among African-Americans
Single amino acid substitution results in malformed hemoglobin
Reduced O2 carrying capacity Codominant inheritance
Sickle-cell Disease
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
Tay-Sachs
Most common lethal genetic disease in the U.S.
Most frequent in Caucasian populations (1/20 a carrier)
Produces defective chloride channels in membranes
Cystic Fibrosis
Usually rare Skips generations Occurrence increases with consaguineous
matings (people descended from the same ancestor)
Often an enzyme defect Affects males and females equally
Recessive Pattern
Less common then recessives Affects males and females equally Ex:
◦Huntington’s disease◦Achondroplasia◦Familial Hypercholesterolemia
Human Dominant Disorders
Each affected individual had one affected parent.
Doesn’t skip generations. Homozygous cases show worse phenotype
symptoms. May have post-maturity onset of
symptoms.
Inheritance Pattern
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
Newborn Screening
Where Genetic and Environment Factors interact to cause the disease
Ex: Heart Disease factors◦ Genetics◦ Diet◦ Exercise◦ Bacterial infections
Multifactorial Diseases
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.
Summary