Chapter 14: Mendel and the Gene Idea

Essential Knowledge 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).

Past/Present Genetic Hypotheses 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

Gregor Mendel 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”

Reasons for Mendel's Success 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

Why Use Peas? Short life span Bisexual Many traits known *Both sexes in one flower/plant *Stamens and carpels Many traits known *Easy to see/observe traits Cross- and self-pollinating *Easy to control reproduction You can eat the failures 

Cross-pollination Cross between two different parents Results in hybrid offspring The offspring may be different than the parents.

Self-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

Mendel's Work Used seven characters, each with two expressions or traits Example: Character - height Traits - tall or short

Monohybrid or Mendelian Crosses Mono = one Crosses that work with a single character at a time Example - Tall X short

P Generation 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

Offspring F1 - first filial generation F2 - second filial generation, Filial – Latin for “son” 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.)

Another Sample Cross P1 Tall X short (TT x tt) F1 all Tall (Tt) F2 3 tall to 1 short (1 TT: 2 Tt: 1 tt) Tall Short

Results - Summary 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 %)

Mendel's Hypothesis Genes can have alternate versions called alleles Each offspring inherits two alleles, one from each parent He made this conclusion without having knowledge of chromosomes/DNA makeup

Homologous chromosomes ** Remember: Each diploid cell has a pair of homologous chromosomes -Therefore, any gene has 2 loci *one on maternal chromo *one on paternal chromo

Mendel's Hypothesis 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 The two alleles for each trait separate during gamete formation (meiosis) This now called Mendel's Law of Segregation

Law of Segregation

Genetics Vocabulary Phenotype - the physical appearance of the organism Genotype - the genetic makeup of the organism, usually shown in a code T = tall t = short

Vocabulary 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)

6 Mendelian Crosses are Possible Notice the 3:1 ratio!!! Cross Genotype Phenotype TT X tt all Tt all Dom Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res TT X TT all TT all Dom tt X tt all tt all Res TT X Tt 1TT:1Tt all Dom Tt X tt 1Tt:1tt 1 Dom: 1 Res

Test Cross 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

Dihybrid 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)

Number of Kinds of Gametes Critical to calculating the results of higher level crosses Look for the number of heterozygous traits

Equation 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

TtRr X TtRr Dihybrid Cross 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

Results 9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered Or: 9:3:3:1 ratio

Law of Independent Assortment 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!

Comment 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

Probability 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

Genetic Ratios 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

Rule of Multiplication 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

Example: TtRr X TtRr 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

Product Rule 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 Solution “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

Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance

Incomplete Dominance 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!!!!!

Not enough red pigment made

Result from Inc Dominance No hidden recessive 3 phenotypes and 3 genotypes (Hint! – often a “dose” effect) Red = CR CR Pink = CRCW White = CWCW

Another example

Codominance 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

Result from Codominance No hidden recessive 3 phenotypes and 3 genotypes (but not a “dose” effect)

Multiple Alleles 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

Result from Multiple Alleles Multiple genotypes and phenotypes Very common event in many traits

Alleles and Blood Types Phenotypes Genotypes A IA IA or IAi B IB IB or IBi AB IAIB O ii

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

Rh factor Rh blood factor is a separate factor from the ABO blood group Rh+ = dominant Rh- = recessive

Blood Type Problem 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!

Polygenic Inheritance Factors that are expressed as continuous variation Lack clear boundaries between the phenotype classes Ex: skin color, height

Genetic Basis 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

Result from Polygenic Inheritance Mendelian ratios fail Traits tend to "run" in families Offspring often intermediate between the parental types Trait shows a “bell-curve” or continuous variation

Genetic Studies in Humans Often done by Pedigree charts Why? Can’t do controlled breeding studies in humans Small number of offspring Long life span

Pedigree Chart Symbols Male Female Person with trait

Sample Pedigree

Recessive Trait Dominant Trait

Human Recessive Disorders Several thousand known! Some examples: Albinism Sickle Cell Anemia Tay-Sachs Disease Cystic Fibrosis PKU Galactosemia

Sickle-cell Disease Most common inherited disease among African-Americans Single amino acid substitution results in malformed hemoglobin Reduced O2 carrying capacity Codominant inheritance

Tay-Sachs 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

Cystic Fibrosis Most common lethal genetic disease in the U.S. Most frequent in Caucasian populations (1/20 a carrier) Produces defective chloride channels in membranes

Recessive Pattern Usually rare Skips generations Occurrence increases with consaguineous matings (people descended from the same ancestor) Often an enzyme defect Affects males and females equally

Human Dominant Disorders Less common then recessives Affects males and females equally Ex: Huntington’s disease Achondroplasia Familial Hypercholesterolemia

Inheritance Pattern Each affected individual had one affected parent. Doesn’t skip generations. Homozygous cases show worse phenotype symptoms. May have post-maturity onset of symptoms.

Newborn Screening 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

Multifactorial Diseases Where Genetic and Environment Factors interact to cause the disease Ex: Heart Disease factors Genetics Diet Exercise Bacterial infections

Summary 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.