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