CHAPTER 9 Mendel and the Gene Idea… and a little bit about human genetics.

CHAPTER 9

Mendel and the Gene Idea…

and a little bit about human genetics.

What is INHERITANCE?

The study of how physical traits are passed from generation to generation.

How are they passed on???

• GENES within DNA.• DNA is the UNIT OF HEREDITY.• DNA is the molecule that is transmitted

from parent to offspring that determines the physical outcome or plan.

• DNA is instructions to make proteins.• PROTEINS are enzymes, hormones, and

provide structure & color!!!!

GREGOR MENDEL:

•The “father of genetics”•Austrian Monk•First to explain patterns of inheritance.•Analyzed sweet pea plantsover 12 years.•Called the genes- “factors”

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

Mendel studied the inheritance patterns forphysical traits like: 1.) flower petal color or 2) pea pod color or 3) pea shape.

Ex. round or wrinkled peasPurple or White flowers

Mendel’s Experiments: First he created “pure breeding” strains of the plant through self-pollination.

Parent Generation:• Pure for one of two contrasting traits• Result of many generations of self-pollination

Genetic Cross #1

Mendel chose two plants pure for the two contrasting traits, (P1) generation, and crossed them (bred them together).

Mendel’s Experiments:

(P1) Purple x (P1) White =

First Filial Generation (F1):• Offspring of parent generation• All F1 plants looked like only

one of the parents (in this case they all looked purple)

• These were HYBRIDS

Genetic Cross #2:Mendel then crossed two of the

F1 plants (ewww! plant incest)

Mendel’s Experiments:Ppurple x Pwhite = F1purple

F1purple x F1purple = Second Filial Generation (F2):

• The traits from both parents reappear in these offspring

• Some PURPLE• Some white

Mendel found the ratio of expression to be 3:1.

IMPORTANT CONCLUSIONS:

1. If a trait exists in two contrasting forms one is dominant, one is recessive. The recessive one disappears in the F1 generation (ex. White).

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.

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

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.

Alleles are alternate versionsof a gene found on homologousChromosomes.

5) The two chromosomes that contain the same genes are called: homologous.6) If both alleles in a gene pair are the same, we say the pair is: homozygous

(dominant or recessive)7) If both alleles in a gene pair are different, we say the pair is: heterozygous.8) When solving inheritance problems, use capital letters to represent

dominant genes.9) When solving inheritance problems, use lowercase letters to represent

recessive genes.10) A genotype is the combination of genes an individual possesses.11) A phenotype is the appearance of an individual as the genes are being

expressed.

The Punnett Square• Graphic tool used to solve inheritance problems• Predicts the possible gene combinations inherited by

the offspring and their probability of occurring.• For example, a monohybrid cross: involves one

character/trait.

ex. Heterozygous X homozygous recessive (on board)

Segregation of alleles and fertilization as chance eventsIf an individual isHeterozygous (Pp), flipping a coin- landing either headsor tails is similar to the probability of creating a gamete containing either the dominant or recessive gene in it…

Both have a 50% probability or 1 out of 2 chances of occurring.

Fertilization and creating of zygotes is the probability of the 2 gametes happening together. 1/2 x 1/2 or 1/4

A test-cross:

is used to determine the genotype of an individual with a dominant phenotype (cross the individual in question with an individual with a recessive phenotype).

ex. ____ X homozygous recessive---> offspring

DIHYBRID CROSS • a two character/trait cross, with two gene

pairs.• Results in 4 (not 2) phenotypes/looks.• Each parent can create a variety of

gametes… how many?

ex. AaBb X AaBb

There are 4 possible kinds of gametes:

Mendel’s Law of Independent Assortment states when the two gene pairs are located on non-homologous chromosomes they segregate independently of each other. Aa Bb

Results = 4 possible gamete combinations. (use FOIL) first: AB outer: Ab inner: aB last: ab (see board)

Arrogant, Beautiful X Arrogant, Beautiful Man Woman

Both are Heterozygous for both genes…

AaBb X AaBb

What are their chances of having: arrogant, beautiful children? Modest, beautiful? arrogant, ugly? Etc.

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

9:3:3:19/16 Arrogant, Beautiful3/16 Modest, Beautiful3/16 Arrogant, Ugly1/16 Modest, Ugly

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

Now, it can’t be that easy can it?

No it can’t.

BESIDES THIS PATTERN OF INHERITANCE, called DOMINANCE or COMPLETE DOMINANCE, where having just ONE dominant allele results in the dominantphenotype, THERE ARE ALSO THE FOLLOWING PATTERNS:1. Incomplete dominance2. Codominance3. Multiple Alleles4. Polygenic Inheritance5. X linked inheritance

1)Incomplete Dominance: Both alleles are the same strength

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

• For example: Carnation flower color• The alleles are also written differently:

AA’ or CR CW

Figure 14.9 Incomplete dominance in snapdragon color

2)Codominant Genes:• In a heterozygous individual both alleles

affect the phenotype in separate distinguishable ways.

• For example: Roan Horses have red and white hairs

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

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

4) Multiple Alleles: When more than two 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• Antibodies are a class of biological molecules called: Immunoglobulins• The genes for creating these antibodies are determined by the genes for

creating the A or B substance.

Blood Type

(phenotype)

Genotype Antigens on surface

of RBC

Plasma Antibodies

Donate Receive

A

B

AB

O

Blood Type

Genotype Antigens on surface

of RBC

Plasma Antibodies

Donate Receive

A IA IA

IAi

B IB IB

IBi

AB IA IB

O ii

Blood Type

Genotype Antigens on surface

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

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”