Beyond Mendel

Rediscovery of Mendel’s Work

Carl Correns Erich von Tschermak Hugo De Vries

Chromosomal theory of inheritance

Walter Sutton Theodor Boveri

Chromosomal theory of inheritance

• Mendelian genes have specific loci (positions) along chromosomes and it is the chromosomes that undergo segregation and independent assortment.

Chromosomal theory of inheritance

Figure 15.2P Generation

F1 Generation

Yellow-roundseeds (YYRR)

Green-wrinkledseeds (yyrr)

Meiosis

Fertilization

Gametes

Y

YR R

YR

y

yr

y r

All F1 plants produceyellow-round seeds (YyRr).

Meiosis

Metaphase I

Anaphase I

Metaphase II

R R

R R

R R

R R

R R R R

r r

r r

r r

r r

r r r r

Y Y

Y Y

Y Y

Y Y

Y Y Y Y

y y

y y

y y

y y

yy y y

Gametes

LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.

LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologous chromosomesassort independently duringgamete formation.

1

2 2

1

1/41/4

1/41/4YR yr Yr yR

F2 Generation

3 3Fertilization recombinesthe R and r alleles at random.

Fertilization results in the 9:3:3:1 phenotypic ratioin the F2 generation.

An F1 F1 cross-fertilization

9 : 3 : 3 : 1

r

Figure 15.2a

P Generation Yellow-roundseeds (YYRR)

Green-wrinkledseeds (yyrr)

Meiosis

Fertilization

Gametes

Y

YR R

YR

y

yr

y r

r

Figure 15.2b

F1 Generation

All F1 plants produceyellow-round seeds (YyRr).

Meiosis

Metaphase I

Anaphase I

Metaphase II

R R

R R

R R

R R

R R R R

r r

r r

r r

r r

r r r r

Y Y

Y Y

Y Y

Y Y

Y Y Y Y

y y

y y

y y

y y

yy y y

Gametes

LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

1

2 2

1

1/41/4

1/41/4YR yr Yr yR

Figure 15.2c

F2 Generation

3Fertilization recombines the R and r alleles at random.

Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.

An F1 F1 cross-fertilization

9 : 3 : 3 : 1

LAW OF SEGREGATION LAW OF INDEPENDENTASSORTMENT

3

ThomasHuntMorgan

The common fruit fly – Drosophila melanogaster

Red eye – the “wild type” White eye – a mutantDrosophila melanogaster

All offspringhad red eyes.

PGeneration

F1

Generation

F2

Generation

F2

Generation

F1

Generation

PGeneration

Eggs

Eggs

Sperm

Sperm

XX

XY

w

w

ww w

w

ww w

w

w

w

w

w

ww w

RESULTS

EXPERIMENT

CONCLUSION

All offspringhad red eyes.

PGeneration

F1

Generation

F2

Generation

RESULTS

EXPERIMENT

F2

Generation

PGeneration

Eggs

Eggs

Sperm

Sperm

Xw

CONCLUSION

XX Y

w

ww w

w

ww w

w

w

w

w

w

ww w

F1

Generation

X

Y

Human x and y chromosomes

Parents

orSperm

or

Egg

Zygotes (offspring)

44 XY

44 XX

22 X

22 Y

22 X

44 XX

44 XY

22 XX

22 X

76 ZW

76 ZZ

32 (Diploid)

16 (Haploid)

(a) The X-Y system

(b) The X-0 system

(c) The Z-W system

(d) The haplo-diploid system

Sex determination in Humans

• In humans, the anatomical signs of sex first appear when the embryo is about two months old.

• In individuals with the SRY gene (sex-determining region of the Y chromosome), the generic embryonic gonads are modified into testes.– Activity of the SRY gene triggers a cascade of

biochemical, physiological, and anatomical features because it regulates many other genes.

– In addition, other genes on the Y chromosome are necessary for the production of functional sperm.

• In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.

Eggs Eggs Eggs

Sperm Sperm Sperm

(a) (b) (c)

XNXN XnY XNXn XNY XNXn XnY

Xn Y XN Y YXn

Xn Xn

XN

XN

XN XNXNXn XNY

XNY

XNY XNY

XnY XnYXNXn XNXn

XNXnXNXN

XnXn

Transmission of sex-linked recessive traits:a) Father with trait passes trait to all daughters - carriersb) Female carrier passes trait to half her sons and daughtersc) Female carrier mates with male with trait – half of offspring will have trait, half of

daughters will be carriers, half of males will be free of trait

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy

• Duchenne muscular dystrophy affects one in 3,500 males born in the United States. – Affected individuals rarely live past their early

20s.– This disorder is due to the absence of an X-linked

gene for a key muscle protein, called dystrophin. – The disease is characterized by a progressive

weakening of the muscles and a loss of coordination.

Dystrophin muscle complex

X Inactivation

• Although female mammals inherit two X chromosomes, only one X chromosome is active.

• Therefore, males and females have the same effective dose (one copy ) of genes on the X chromosome.– During female development, one X chromosome per cell

condenses into a compact object, a Barr body.– This inactivates most of its genes.

• The condensed Barr body chromosome is reactivated in ovarian cells that produce ova.

• Mary Lyon, a British geneticist, has demonstrated that the selection of which X chromosome to form the Barr body occurs randomly and independently in embryonic cells at the time of X inactivation.

• As a consequence, females consist of a mosaic of cells, some with an active paternal X, others with an active maternal X.

Mary Lyon

X Inactivation Mosaic

X inactivation and coat color in tortoiseshell cats