9.2 Experimental genetics began in an abbey garden Gregor Mendel Augustinian monk who taught natural science to high school students Mendel's brilliant

9.2 Experimental genetics began in an abbey garden

Gregor Mendel Augustinian monk who taught natural

science to high school students

Mendel's brilliant performance at school as a youngster encouraged his family to support his pursuit of a higher education, but their resources were limited, so Mendel entered an Augustinian monastery

Copyright © 2009 Pearson Education, Inc.

Gregor Mendel discovered principles of genetics in experiments with the garden pea

• Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes)

9.2 Experimental genetics began in an abbey garden

Crosses meticulously documented

Crosses numerically/statistically analyzed

Scientists of 1860s did not understand and appreciate

Work lost in journals for 50 years

Rediscovered in 1900s independently by 3 scientists

Terms to know and understand

• The phenotype is the appearance or expression of a trait• Genes are found in alternative versions called alleles; a

genotype is the listing of alleles an individual carries for a specific gene

• If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect

• Homozygous individuals have the same allele on both homologues

• Heterozygous individuals have a different allele on each homologue

• Mating of plants can be controlled• Each pea plant has sperm-

producing organs (stamens) and egg-producing organs (carpels)

• Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Transferredpollen from stamens of whiteflower to carpel of purple flower

StamensCarpel

Parents(P)

Purple

2

White

Removedstamens frompurple flower

1

Pollinated carpelmatured into pod

3

Offspring(F1)

Planted seedsfrom pod

4

Mendel’s crosses led to questions

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple


Fig. 14-3-1

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

Fig. 14-3-2

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

Mendel’s crosses led to questions

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation


Fig. 14-3-3

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Questions are . . .

Questions that arise

– Why does one trait seemed to disappear in the F1 generation?

– Why does that trait reappear in one quarter of the F2 offspring?

Answers?

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• These inherited factors are what we now call a genes


Table 14-1

Mendel’s Model

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• Four related concepts make up this model

• These concepts can be related to what we now know about genes and chromosomes


1. The first concept is that alternative versions of genes account for variations in inherited characters

• For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers


2. The second concept is that for each character an organism inherits two alleles, one from each parent

• Mendel made this deduction without knowing about the role of chromosomes

• The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation - homozygous

• Alternatively, the two alleles at a locus may differ, as in the F1 hybrids - heterozygous


3. The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

• In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant


4. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism


Let’s look at this example again Before we were only

looking at phenotype just like Gregor Mendel

Let’s apply his hypotheses and look at genotype as well

Draw a Punnett square that would represent true breeding purple flowers crossed with true breeding white flowers

To make a Punnett Square . . .

• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele


Let’s look at this example again

Before we were only looking at phenotype just like Gregor Mendel


PP pp

Pp

P P

p

p

Pp Pp

Pp Pp

Flowers will all be purple because of dominance

Let’s look at this example again Before we were only

looking at phenotype just like Gregor Mendel


Draw the Punnett square that would represent two of the F1 generation breedingt

PP pp

Pp

P P

p

p

Pp Pp

Pp Pp

This cross

gives us t

he F 1 generatio

n


Allow F1 generation to self-pollinate and fill out the next Punnett square PP pp

Pp

PP Pp Pp pp

P p

p

P


Allow F1 generation to self-pollinate and fill out the next Punnett square PP pp

Pp

PP Pp Pp pp

P p

p

PP Pp

Pp pp

¾ of flowers will all be purple because of dominance. ¼ will be white.

P


PP pp

Pp

PP Pp Pp pp

P p

p

PP Pp

Pp pp

P

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

Single trait cross

9.3 Mendel’s law of segregation describes the inheritance of a single character

Four Hypotheses

1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene

2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different

– A homozygous genotype has identical alleles

– A heterozygous genotype has two different alleles


9.3 Mendel’s law of segregation describes the inheritance of a single character

Four Hypotheses

3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect

– The phenotype is the appearance or expression of a trait

– The same phenotype may be determined by more than one genotype

4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene


Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

Three traits that are determined by single genes with alternate alleles

The dominant allele determines the phenotype when at least one copy is present

Humans show dominant and recessive traits

PTC taste strips

Some Genetic Disorders/Anomalies

Polydactyly - dominant

Tay Sachs - recessive

Cystic Fibrosis - recessive

Huntington’s Disease - dominant

Sickle Cell Anemia - recessive

– Cause?

– Single substitution of an amino acid in hemoglobin

Polydactyly

Alfredo Alfonseca "The Six Shooter", former Chicago Cubs relief pitcher

Six fingers and toes on each hand, all functional

9.8 Genetic traits in humans can be tracked through family pedigrees

A pedigree

– Shows the inheritance of a trait in a family through multiple generations

– Demonstrates dominant or recessive inheritance

– Can also be used to deduce genotypes of family members


Ff

Female Male

Marriage

Unaffected

First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

Ff Ff

Ff

Ff Ff

Ff

ff

ff ff ff

ff

FF

FF

or

or

Siblings

Affected

Pedigree for a family inheritance of a recessive trait such as deafness

Recessive disease

Rr

?

rr

rr

rr

Dominant disease

Dominant disease

dd

dd

dd

dd

Dd

Huntington’s Disease - handout

http://www.youtube.com/watch?v=OveGZdZ_sVs&feature=related

Huntington’s Disease

Questions What are the chances of a parent with HD passing with HD

passing the gene on to any one of his or her children?

Thinking of all the ways in which the number of repeats may be passed on to children, discuss the possibility that John may get an HD gene

– from his mother

– from his father

From which parent did Dink get the gene for HD? Explain how you can tell.

Do you expect John's mother to show symptoms of HD? Why or why not.

Huntington’s Disease

Questions John's Aunt Barbara refused to have the test for HD.

Discuss some reasons that she may have had for coming to that decision.

When she applied for a job as an airline pilot, the company, knowing of the occurrence of HD in the family, insisted that she be tested. Does the company have the right to require the test? Explain why or why not.

When John reaches the age of 18, he may make the decision of whether or not to be tested for the HD gene. Consider this question from the perspective of John, John's future children, John's future wife, John's employer, John's medical insurer.

– Explain to John why he should have the test done.

– Explain to John why he should not have the test done.

VARIATIONS ON MENDEL’S LAWS


9.11 Incomplete dominance results in intermediate phenotypes

Incomplete dominance

– Neither allele is dominant over the other

– Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual


P generation

1–2

1–2

1–2

1–2

1–2

1–2

F1 generation

F2 generation

RedRR

Gametes

Gametes

Eggs

Sperm

RR rR

Rr rr

R

r

R r

R r

PinkRr

R r

Whiterr

HHHomozygous

for ability to makeLDL receptors

hhHomozygous

for inability to makeLDL receptors

HhHeterozygous

LDL receptor

LDL

CellNormal Mild disease Severe disease

Genotypes:

Phenotypes:

9.12 Many genes have more than two alleles in the population

Multiple alleles

– More than two alleles are found in the population

– A diploid individual can carry any two of these alleles

– The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood


9.12 Many genes have more than two alleles in the population

Codominance

– Neither allele is dominant over the other

– Expression of both alleles is observed as a distinct phenotype in the heterozygous individual

– Observed for type AB blood


BloodGroup(Phenotype) Genotypes

O

A

ii

IAIA

orIAi

Red Blood Cells

Carbohydrate A

BIBIB

orIBi

Carbohydrate B

AB IAIB

Blood Type

Phenotype Genotype

A IA IA , IA i

B IB IB , IB i

AB IA IB

O i i

Blood Genetics

IA IB IB i

IA i i i

IA i

IB

i

Given a mom with blood type AB and child with blood type A, what are the possible blood types of the father?

IAIA

IA i

IA IB

? ____

? ____

Blood Typing Questions

IA

iIA IAIA

Dominance Worksheet


9.13 A single gene may affect many phenotypic characters

Pleiotropy

– One gene influencing many characteristics

– The gene for sickle cell disease

– Affects the type of hemoglobin produced

– Affects the shape of red blood cells

– Causes anemia

– Causes organ damage

– Is related to susceptibility to malaria


Clumping of cellsand clogging of

small blood vessels

Pneumoniaand otherinfections

Accumulation ofsickled cells in spleen

Pain andfever

Rheumatism

Heartfailure

Damage toother organs

Braindamage

Spleendamage

Kidneyfailure

Anemia

ParalysisImpairedmental

function

Physicalweakness

Breakdown ofred blood cells

Individual homozygousfor sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

9.14 A single character may be influenced by many genes

Polygenic inheritance

– Many genes influence one trait

– Skin color is affected by at least three genes


P generation

1–8

F1 generation

F2 generation

Fra

ctio

n o

f p

op

ula

tio

n

Skin color

Eggs

Sperm1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

1––64

15––64

6––64

20––64

9.15 The environment affects many characters

Phenotypic variations are influenced by the environment

– Skin color is affected by exposure to sunlight

– Susceptibility to diseases, such as cancer, has hereditary and environmental components


Environment—It can fool you

Environment—It can fool you !

The hydrangea flower color is controlled first by flower color genes similar to those in the pea.

Purple vs. white has complete dominance. But, pink vs. blue is controlled by the acidity of the soil

Environment—It can fool you !

The hydrangea flower color is controlled first by flower color genes similar to those in the pea.

Purple vs. white has complete dominance. But, pink vs. blue is controlled by the acidity of the soil

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9.2 Experimental genetics began in an abbey garden Gregor Mendel Augustinian monk who taught natural science to high school students Mendel's brilliant