CHAPTER 14Mendel and the Gene Idea

Big Idea 3: Living systems store, retrieve, transmit and respond to

information essential to life processes.

• Mendel was able to describe a model of inheritance of traits, and his work represents an application of mathematical reasoning to a biological problem.

• However, most traits result from interactions of many genes and do not follow Mendelian patterns of inheritance (ex. hair color, eye color.)

• Understanding the genetic basis of specific phenotypes (physical outcomes due to proteins assembled) and their transmission in humans can raise social and ethical issues.

What is INHERITANCE?

The study of how genes

and traits are passed from generation to generation.

Genetic information provides for continuity of life and, in most cases,

this information is passed from parent to offspring via DNA.

Review meiosis w/ your partner.

1. What are the 3 sources of genetic variation for sexually reproducing organisms?

2. Why is it beneficial to be a “diploid” organism?

1) Crossing over makes recombinant chromosomes that are unique.

• Independent assortment makes new combinations of chromosomes (one of each kind) in the gametes by “shuffling” the pairs of chromosomes.

• Fertilization joins gametes from two separate gene lineages (hopefully) to make a diploid zygote with new gene pairs.

2) Gene pairs = “back up copy” in case one gene is mutated/nonfunctional.

GREGOR MENDEL:

The “father of genetics”

Austrian Monk

First to explain patterns of inheritance.

Analyzed sweet pea plantsover 12 years.

Figure 14.x2 Round and wrinkled peas

Characteristic comesin 2 contrasting traits:Ex. PEA SHAPERound Wrinkled

Ex. FLOWER COLORPurpleWhite

Mendel’s Experiments:Parent Generation (P generation):• Pure for one of two contrasting traits• Result of many generations of self-pollinationChose two plants pure for the two contrasting traits for

the characteristic flower color & crossed them.purple X white

Mendel’s Experimental RESULTS:

(F1) First Generation :

• Offspring of parent generation

• All F1 plants looked like only one of the parents (that was the dominant trait)

• The recessive trait “disappeared”

• Hybrid offspring

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Mendel’s 2ndExperiment:

Mendel then crossed two F1 plants (“Ewww incest!” jk)

(F2) Second Filial Generation :

• The traits from both parents reappear in these offspring

Mendel found the ratio of expression to be 3:1

(3 dominant:1 recessive)

* note, not EXACTLY 3:1

IMPORTANT CONCLUSIONS:

1. If a characteristic exists in two contrasting forms one is dominant, one is recessive.

2. Factors controlling traits occur in pairs.3. The dominant factor prevents the recessive factor

from being expressed.4. Recessive factors are only expressed when both

factors in the pair are recessive.5. When pea plants reproduce, a factor pair is

segregated (split) and each factor ends up in a separate gamete. Mendel’s law of segregation.

In modern terms:1. Mendel’s factors are called

genes. Some genes are dominant, others recessive.

2. Different forms of a single gene are called alleles.

3. Genes occur in pairs, at the same position on two chromosomes.

4. The gene position is called the locus.

5) The two chromosomes that contain the same genes are called: homologous.6) When solving inheritance problems, use capital letters to represent dominant

genes. T7) When solving inheritance problems, use lowercase letters to represent

recessive genes. t8) If both alleles in a gene pair are the same, we say the pair is: homozygous

(dominant or recessive) TT, tt9) If both alleles in a gene pair are different, we say the pair is: heterozygous. Tt10) A genotype is the combination of genes an individual possesses. (for one

trait shown w/ pair of letters) TT11) A phenotype is the appearance of an individual as the genes are being

expressed & proteins are being made or not made.

The Punnett Square• Used to solve inheritance problems• Predicts the possible gene combinations inherited by

the offspring• For example, a monohybrid cross: involves one

character/trait.ex. Heterozygous X Heterozygous (on board)

Figure 14.8 Segregation of alleles and fertilization as chance events

POSSIBLE GENE COMBINATIONS FOR OFFSPRING ARE A RESULT OF 3 CHANCE EVENTS:1.Segregation of alleles into eggs2.Segregation of alleles into sperm3.Fertilization of WHICH egg by WHICH sperm

Figure 14.4 Mendel’s law of segregation (Layer 2)

Figure 14.6 A testcross

A test-cross: used to determine the genotype of an individual with a dominant phenotype…Not sure if they are Homo dom: PPOr Hetero: Pp

cross the individual in question with an individual with a recessive phenotype____ X homo rec

-> offspring phenotypeIf all dominant -> PPIf ½ recessive -> Pp

For another example, a DIHYBRID cross: a two character/trait cross, with two gene pairs.

ex. AaBb X AaBb

(see board)

Attractive, Boring X Attractive, Boring Man Woman

Both are Heterozygous for both genes…AaBb X AaBb

1. What gametes can each make?2. What are their chances of having: Attractive Boring kids? List the genotypes possible. #/16Attractive Exciting kids? List the genotypes possible. #/16Unattractive Boring kids? List the genotypes possible. #/16Unattractive Exciting kids? List the genotypes… #/16

Mendel’s Law of Independent Assortment states when the two gene pairs are located on non-homologous chromosomes they segregate independently of each other. Results = 4 possible gamete combinations. (use FOIL) first: AB

outer: Ab inner: aB last: ab

Attractive, Boring X Attractive, Boring Man Woman

Both are Heterozygous for both genes…AaBb X AaBb

• What gametes can each make? AB, Ab, aB, ab• What are their chances of having: Attractive Boring kids? List the genotypes possible. #/16AABB, AaBB, AaBb, AABbAttractive Exciting kids? List the genotypes possible. #/16Aabb, AAbbUnattractive Boring kids? List the genotypes possible. #/16aaBB, aaBbUnattractive Exciting kids? List the genotypes… #/16aabb

AaBb X AaBb

AaBb X AaBb

Ab abaBAB

Ab

aB

ab

AB

AaBb X AaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

Ab abaBAB

Ab

aB

ab

AB

AaBb X AaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

Ab abaBAB

Ab

aB

ab

AB

Phenotype Ratio of dihybrid cross

9:3:3:19/16 Attractive, Boring3/16 Attractive, Exciting3/16 Unattractive, Boring1/16 Unattractive, Exciting

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

Now, it can’t be that easy & boring can it?

No it can’t.

1)Incomplete Dominance: Both alleles are the same strength

• Heterozygous individuals…express BOTH alleles, The resulting phenotype is a mix (blend) of the two.

• 3 phenotypes possible.• For example: Carnation

flower color• The alleles are also written

differently: AA’ or CR CW

CRCR CR CW CWCW

2) Codominant Genes:• In a heterozygous individual

both alleles affect the phenotype in separate distinguishable ways.

• For example: Roan Horses have red (or black) and white hairs

• Another example: A, B, and AB blood groups in humans.

3) Multiple Alleles: When more than

2 forms of the gene exist

• For example: Human Blood Types (ABO blood groups)• Blood cells have a carbohydrate marker that is found on the

surface of red blood cells.• These markers are called: A substance or B substance.• They are recognized by antibodies present in the blood serum

of individuals for foreign carbohydrate markers.• There are two types of RBC antibody: Anti-A & Anti-B• The genes for creating these antibodies are determined by the

genes for creating the A or B substance.

Blood Type

Genotype Antigens on surface

of RBC

Plasma Antibodies

Donate Receive

A

B

AB

O

Write the possible genotypes for the blood types A, B, AB and O

Blood Type

Genotype Antigens Plasma Antibodies

Donate Receive

A IA IA

IAi

B IB IB

IBi

AB IA IB

O ii

What are the actual antigens (surface molecules) found on the RBCs?

Blood Type

Genotype Antigens on surface

Plasma Antibodies

Donate Receive

A IA IA

IAi

Substance A

B IB IB

IBi

Substance B

AB IA IB Substance

A and B

O ii none

What kinds of antibodies does Blood Type A make? Who can he/she Donate blood to? Receive it from?

Blood Type

Genotype Substance on surface of

RBC

Plasma Antibodies

Donate Receive

A IA IA

IAi

Substance A

Anti-B Type A

& AB

Type A

& O

B IB IB

IBi

Substance B

AB IA IB AB

O ii none

Fill in the rest… check your work. Which kind are you? Which kindWhich is called the “universal donor”? The “universal recipient?”

Blood Type

Genotype antigens Plasma Antibodies

Donate Receive

A IA IA

IAi

Substance A

Anti-B Type A

& AB

Type A

& O

B IB IB

IBi

Substance B

Anti-A Type B

& AB

Type B

& O

AB IA IB AB none TYPE

AB

A, B, AB & O

O ii none Anti-A

Anti-B

All blood types

Type O

Type AB is the universal recipient, Type O is the universal donor.

Figure 14.10 Multiple alleles for the ABO blood groups

Figure 14.10x ABO blood types

• Remember, the IA & IB alleles are CODOMINANT.

• With human blood type, another marker called the Rh factor is denoted as + or -.

• So, type O -, is the worst to be in terms of getting a blood transfusion… but is the best donor.

• “universal donor”

1) Pleiotropy: One gene influences many traits.

• (i.e. disease genes)• PKU (phenylketonuria). Mutation in a single gene

that codes for an enzyme results in: mental retardation, reduced hair, and skin pigmentation.

• Enzyme phenylalanine hydroxylase converts the amino acid phenylalanine to tyrosine.

• Absence of the enzyme causes phenylalanine to accumulate in toxic levels.

Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

2) Epistasis: When two genes control the expression of a single trait. One gene pair

interferes with the expression of the other.

• Common for expression of pigment • For example: Fur color in mice. • Gene 1 = Fur color (B = Black fur, b = brown fur)• Gene 2 = Depositing pigment in the hair

(C = deposits color, c = deposits no color (albino))• What is the phenotypic ratio that results from the

cross a Black haired mouse, heterozygous both gene pairs and a Brown Haired mouse- heterozygous for gene 2?

Figure 14.11 An example of epistasis

9:3:4

3) Polygenic inheritance: When one phenotype is under the control of multiple gene pairs

• For example: Human Skin Color• Alleles are… Dark and light• At least… three genes control the color of your

skin• The phenotype is a result of… the cumulative

effects of the dominant genes• Human skin color exists… as a gradient

Figure 14.12 A simplified model for polygenic inheritance of skin color

Figure 14.13 The effect of environment of phenotype

Multifactorial characters are influenced by genetics and environment.

Ph differences control the color of the hydrangeas.

TAN=genes + Sun exposure / melanin production.

"temperature sensitive" or ts mutants

Conditional mutants that grow at a low temperature but not at a high temperature are are called "temperature sensitive" or ts mutants.

Conditional mutants are not necessarily

associated with lethality.

The dark ear tips, nose and feet of a Siamese cat are the phenotype of a temperature sensitive mutation in the c locus (determining fur color).

The enzyme encoded is not functional at higher temperatures, but is functional at lower temperatures, such as the extremities of the cat.

Hence the fur on these parts of the Siamese cat’s body is pigmented.

PEDIGREE PROJECT (Complete over Thanksgiving)3 generationsExamples of TRUE mendelian traits (show dominance)WET EARWAX vs. dry earwax (recessive)PIGMENT vs. AlbinismRecessive: magenta urine after eating beats

Pedigree• An analysis of the results of mating that

have already occurred- especially when studying a species with a long generation time.

• As much information as possible is collected about a family’s history for a particular trait.

• Information is assembled into a family tree describing the interrelationships across the generations.

• Helps us understand the past and predict the future.

For those traits exhibiting dominant gene action:•affected individuals have at least one affected parent•the phenotype generally appears every generation•two unaffected parents only have unaffected offspring

And for those traits exhibiting recessive gene action:•unaffected parents can have affected offspring•affected progeny are both male and female

Figure 14.16 Large families provide excellent case studies of human genetics

Figure 14.17 Testing a fetus for genetic disorders