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Chapter 8- Mendel And Heredity

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I. The origins of Genetics

A. The passing of traits from parents to offspring is called heredity.

 

1. Mendel was a mathematician and looked at genetics differently than anyone in the past ever did.

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2. Genetics is the branch of biology that focuses on heredity.

 

3. Mendel counted the number of each kind of offspring and analyzed the data.

4. Quantitative approaches to science-those that include measuring and counting.

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B. Useful features in peas

  1. The garden pea is a good subject for studying heredity for several reasons.

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a. The garden pea has many traits that have two clearly different forms that are easy to tell apart.

 b. The mating of the garden pea

flowers can be easily controlled. 

c. The garden pea is small, grows easily, matures quickly and produces many offspring.

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C. Mendel observed that traits are expressed as simple ratios

 1. Mendel’s initial experiments

were monohybrid crosses. A cross with ONE pair of contrasting traits.

 2. Mendel allowed true breeding

or self-pollination to occur.

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3. These true-breeding plants served as the parental generation in Mendel’s experiments.

 

4. The parental generation, or P generation are the first two individuals that are crossed.

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5. Mendel crossed the P generation to get the F1 generation. First generation. He then examined each F1 plant and recorded the number of F1 plants and their traits.

 

6. Mendel then mated the F1 generation to get a F2 generation. The second generation and then again examined each of their traits.

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D. M

ende

l’s

resu

ltsP generation

(true-breedingparents)

Purple flowers White flowers

F1 generationAll plants havepurple flowers

F2 generation

Fertilization among F1 plants (F1 F1)

of plantshave purple flowers

3–4 of plants

have white flowers

1–4

Mendel’s Results

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II. Mendel’s theory

A. Mendel’s work became a theory of heredity.

 

1. The four hypotheses Mendel developed were based directly on the result of his experiments.

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a. For each inherited trait, an individual has two copies of genes- one from each parent.

 

b. There are alternative versions of genes. These are called alleles.

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c. When two different alleles occur together, one of them bay be completely expressed, while the other may have no affect on the appearance. Mendel explained these to be dominant and recessive.

 

d. When gametes are formed, the alleles for each gene in an individual separate independently of one another.

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B. Mendel’s findings in modern terms

 

1. If two traits are the same they are called homozygous.

 

2. If two traits are different they are called heterozygous.

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3. Dominant genes are expressed with capital letters.

 4. Recessive genes are expressed with lower caps.

 5. Example Brown is dominant= B, blue is recessive =b.

 6. A set of alleles that an individual has is called the genotype.

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7. What the individual expresses is called the phenotype.

 

8. Example: BB, Bb and bb are genotypes.

 

9. BB, and Bb would have brown eyes, and bb would have blue eyes. The colors are the phenotypes.

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Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

Phenotype

Dihybrid Cross

Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green).

Mendel’s dihybrid crosses:

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Pea Traits:

Dihybrid Cross Example:

Dihybrid crosses:

SsYy SsYy

Dihybrid Crosses:

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B. Mendel’s Ideas gave rise to the laws of heredity.

 

1. The first law, the law of segregation, states that the two alleles for a trait segregate or separate when gametes are formed.

P plants

1–2

1–2

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)

All

All Pp

Sperm

Eggs

PP

p

ppPp

Pp

P

pP

pP

P

p

PPpp

All

Gametes

F2 plants

The Law of Segregation

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2. The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation.

P generation

1–2

Hypothesis: Dependent assortment Hypothesis: Independent assortment

1–2

1–2

1–2

1–4

1–4

1–4

1–4

1–4

1–4

1–4

1–4

9––16

3––16

3––16

1––16

RRYY

Gametes

Eggs

F1

generation

SpermSperm

F2

generation

Eggs

Gametes

rryy

RrYy

ryRY

ryRY

ry

RY

Hypothesized(not actually seen)

Actual results(support hypothesis)

RRYY rryy

RrYy

ryRY

RRYY

rryy

RrYy

ry

RY

RrYy

RrYy

RrYy

rrYYRrYY

RRYyRrYY

RRYy

rrYy

rrYy

Rryy

Rryy

RRyy

rY

Ry

ry

Yellowround

Greenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

The law of independent assortment

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II. Studying heredity

A. Punnet squares

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F1 genotypes

1–2

1–2

1–2

1–2

1–4

1–4

1–4

1–4

Formation of eggs

Bb female

F2 genotypes

Formation of sperm

Bb male

B

BB B B

B

b

b

bbbb

B. Probability

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C. Pedigrees

SymbolsPedigree Sample

• Rather than manipulate mating patterns of people, geneticists analyze the results of matings that have already occurred.

• In a pedigree analysis, information about the presence/absence of a particular phenotypic trait is collected from as many individuals in a family as possible and across generations.

• The distribution of these characters is then mapped on the family tree.

Pedigree analysis

For example, the occurrence of widows peak (W) is dominant to a straight hairline (w).

The relationship among alleles can be integrated with the phenotypic appearance of these traits to predict the genotypes of members of this family.

Pedigree analysis

Examples: Widow’s Peak & Earlobes

Is widow’s peak a dominant or recessive trait?

Widow’s Peak No Widow’s Peak

Is an attached earlobe a dominant or recessive trait?

Attached Earlobe Free Earlobe

D. Sex-LinkedTraits

Sex-LinkedTraits

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Color-Blindness

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What type of inheritance pattern?

Color blindness Red-Green

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

Hemophilia in the royal family of Russia

XN Y

XN

Xn

XNXN XNY

XNXn XnY

Hemophilia Example

N = Normal Blood n = Hemophilia

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III. Patterns of Heredity can be complex

A. Traits influenced by several genes

 

1. When several genes influence a trait, the trait is said to be polygenetic.

Polygenic Inheritance Example

Polygenic (multiple Genes) for a given phenotypic character.

Example: Skin Color, Eye Color, Hair Color, & Rh Factor.

Polygenic Inheritance Example: Skin Color

The melanin factor is located on three loci of each chromosome. (total of six locations)

Polygenic Inheritance: Human skin color is a good example of polygenic (multiple gene) inheritance.

AABBCCMost Melanin

aabbccLeast Melanin

Assume that three "dominant" capital letter genes (A, B and C) control dark pigmentation because more melanin is produced. The "recessive"alleles of these three genes (a, b & c) control light pigmentation because lower amounts of melanin are produced.

Polygenic Inheritance ExampleLet’s do a cross.

A B C

A B C

a b c

a b cX

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Polygenic Inheritance Example

Result would be the F1 generation would be AaBbCc.

abcabc

ABCABC

abcABC

abcABC

F1

Polygenic Inheritance Example

Let’s cross the F1 with another F1.

A B C

a b cX

A B C

a b c

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Pol

ygen

ic I

nher

itan

ce E

xam

ple

F2

F1

F2

Gametes  ABC   ABc   AbC   Abc   aBC   aBc   abC   abc 

ABC 6 5 5 4 5 4 4 3

ABc 5 4 4 3 4 3 3 2

AbC 5 4 4 3 4 3 3 2

Abc 4 3 3 2 3 2 2 1

aBC 5 4 4 3 4 3 3 2

aBc 4 3 3 2 3 2 2 1

abC 4 3 3 2 3 2 2 1

abc 3 2 2 1 2 1 1 0

Polygenic Inheritance Example: Skin Color

ABC/ABC

ABC/ABc

ABC/ABc

ABc/ABc

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2. In some traits there is neither dominant nor recessive genes. These are called incomplete dominance. Example: Red flower RR and a White WW flower both with incomplete dominance will have pink flowers.

Incomplete Dominance Example

X =

Red Carnation WhiteCarnationX =

PinkCarnation

Inco

mpl

ete

Dom

inan

ce w

ith

Sna

pdra

gons

Flo

wer

Col

or

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes 1⁄2 1⁄2

1⁄2

1⁄2

1⁄2Eggs

1⁄2

CR CR CR CW

CW CWCR CW

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 3. When two dominant alleles are expressed at the same time, both forms of the trait are displayed this is called co dominance. Example: Coat colors in a horse.

4. Genes with three or more alleles are said to have multiple alleles. Example blood groups ABO.

 

BW 

BW 

  BW

BW

checkered chicken

WW X BB

B B

W

W

BW

                  

                  

Example of Codominance:In chickens, black feather color (BB) is codominant to white feather color (WW).  Both feather colors show up in a checkered pattern in the heterozygous individual (BW). Cross a checkered chicken with a checkered chicken.

Blood Types Multiple Alleles

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1.      Genotypes PhenotypesAA AAo ABB BBo BAB ABoo O

Multiple Alleles

BA

See page 261

Mendelian Inheritance In Humans

See page 261

Mendelian Inheritance In Humans

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5. An individual’s phenotype often depends on conditions in the environment.

6. Some traits are affected by the temperature, sunlight or seasons.

7. In humans the environment influences height. Height is influenced by nutrition, an internal environment conditions.

Full coat color:CCCCch

Cch

Cc

Chinchilla coat color:cchcch

cchch

cchc

Himalayancoat color:chch

chc**tempSensitive!

Albino coat color:cc

Multiple Alleles

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B. Some traits are caused by mutations

 

Sickle Cell Anemia** NN is Not afflicted; Nn is a carrier; nn is afflicted

**Sickle cell anemia is caused by an abnormal type of hemoglobin and cells become sickle shaped when oxygen is low

**Sickle cells clot give

rise to recurrent painful

episodes called a

“sickle cell pain crisis”

** “n” makes a person immune to malaria

• Results in the absence of an enzyme that normally breaks down a lipid (fat) produced and stored in nervous tissues and brain

A child with Tay-Sachs Syndrome are mentally challenged

• Phenylketonuria (PKU)lacks an enzyme that converts one amino acid, phenylalanine, to a different amino acid, tyrosine.

* Results in severe damage to the central nervous system.

• When detected in infants, dietary adjustments (about 1 in 15k babies born with it)

Phenylketonurics: Contains Phenylalanine

Genetic Methemoglobinemia

*Although oxygen-poor blood is NOT blue, but appears bluish because of the distortion of skin

“Blue People” who have too much methhemoglobin in their blood and oxygen cannot bind due to an enzyme deficiency

**The Fugates of Hazard, Kentucky. Mom was a carrier while Dad had the disease

Picture taken in the 1950’s

• It results in a breakdown of certain areas of the brain.

~Causes dementia due to progressive deterioration of the brain, increase in involuntary movements, eventually bedridden

Achondroplasia - type of dwarfism.

AA genotypes are lethal and result in spontaneous abortion, Aa have dwarfism. 99% of Americans are aa

Polydactyl (extra fingers and/or toes):

PP or Pp = extra digits. 98% of all people in the world are homozygous recessive (pp).

Progeria (very premature aging):

* Most die before age 13

Spontaneous mutation of one gene creates a dominant mutation that rapidly accelerates aging.

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5. There is genetic testing to detect any bad genes.

 

6. Gene technology may soon by replacing defected genes with copies of healthy ones this is called gene therapy.