Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the Gene Idea.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 14Mendel and the Gene Idea


Figure 14.1 Gregor Mendel and his garden peas


Figure 14.2 Research Method Crossing Pea Plants

1

5

4

3

2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower

Parentalgeneration(P)

Pollinated carpelmatured into pod

Carpel(female)

Stamens(male)

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.

TECHNIQUETECHNIQUE

When pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all have purpleflowers. The result is the same for the reciprocal cross, the transferof pollen from purple flowers to white flowers.

TECHNIQUERESULTS


Figure 14.3 When F1 pea plants with purple flowers are allowed to self-pollinate, what flower color appears in the F2 generation

P Generation

(true-breeding parents) Purple

flowersWhiteflowers

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.

RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.


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


Figure 14.4 Alleles, alternative versions of a gene

Allele for purple flowers

Locus for flower-color gene

Homologouspair ofchromosomes

Allele for white flowers


Figure 14.5 Mendel’s law of segregation (layer 1)

P Generation

F1 Generation

P p

Appearance:Genetic makeup:

Purple flowerPP

White flowerspp

Purple flowersPp


Gametes:

Gametes:

Each true-breeding plant of the parental generation has identicalalleles, PP or pp.

Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.

When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.


Figure 14.5 Mendel’s law of segregation (layer 2)

P Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp


Purple flowerPP

White flowerspp

Purple flowersPp


Gametes:

Gametes:

F1 sperm

F1 eggs

1/21/2

Each true-breeding plant of the parental generation has identicalalleles, PP or pp.

Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.

When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.

Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation. 3 : 1


Figure 14.6 Phenotype versus genotype

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1


Figure 14.7 The Testcross

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

If PP,then all offspring

purple:

If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTS


Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently?

YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis ofindependentassortment

F2 Generation(predictedoffspring)

1 ⁄2 YR

YR

yr

1 ⁄2

1 ⁄2

1 ⁄2 yr

YYRR YyRr

yyrrYyRr

3 ⁄4 1 ⁄4

Sperm

Eggs

Phenotypic ratio 3:1

YR1 ⁄4

Yr1 ⁄4

yR1 ⁄4

yr1 ⁄4

9 ⁄163 ⁄16

3 ⁄161 ⁄16

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

315 108 101 32 Phenotypic ratio approximately 9:3:3:1

F1 Generation

Eggs

YR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4

Sperm

RESULTS

CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.

EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.


Figure 14.9 Segregation of alleles and fertilization as chance events

Rr

Segregation ofalleles into eggs

Rr

Segregation ofalleles into sperm

R r

rR

R

R

R1⁄2

1⁄2 1⁄2

1⁄41⁄4

1⁄41⁄4

1⁄2 r

rR r

r

Sperm

Eggs


Figure 14.10 Incomplete dominance in snapdragon color

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes1⁄2 1⁄2

1⁄2

1⁄2

1⁄2

Eggs1⁄2

CR CR CR CW

CW CWCR CW


Table 14.2 Determination of ABO Blood Group by Multiple Alleles


Figure 14.11 An example of epistasis

BC bC Bc bc1⁄41⁄41⁄41⁄4

BC

bC

Bc

bc

1⁄4

1⁄4

1⁄4

1⁄4

BBCc BbCc BBcc Bbcc

Bbcc bbccbbCcBbCc

BbCC bbCC BbCc bbCc

BBCC BbCC BBCc BbCc

9⁄163⁄16

4⁄16

BbCc BbCc

Sperm

Eggs


Figure 14.12 A simplified model for polygenic inheritance of skin color

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

Fra

cti o

n o

f p

rog

en

y


Figure 14.13 The effect of environment on phenotype


Figure 14.14 Pedigree analysis

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)


Figure 14.15 Achondroplasia


Figure 14.16 Large families as excellent case studies of human genetics


Figure 14.17 Testing a fetus for genetic disorders

(a) Amniocentesis

Amnioticfluidwithdrawn

Fetus

Placenta Uterus Cervix

Centrifugation

A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.

(b) Chorionic villus sampling (CVS)

Fluid

Fetalcells

Biochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.

Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.

Severalweeks

Biochemicaltests

Severalhours

Fetalcells

Placenta Chorionic viIIi

A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.

Suction tubeInserted throughcervix

Fetus

Karyotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.

Karyotyping


Unnumbered Figure p.273

George Arlene

Sandra Tom Sam Wilma Ann Michael

Daniel Alan Tina

Christopher

Carla

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the Gene Idea.

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