Beyond Mendel

Rediscovery of Mendel’s Work Carl Correns Erich von Tschermak Hugo De Vries

Chromosomal theory of inheritance Walter SuttonTheodor 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 15.2 P Generation F 1 Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R rr r r rr r r r r r r YY Y Y Y Y Y Y Y Y Y Y y y y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR F 2 Generation 3 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 r

Figure 15.2a P Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r r

Figure 15.2b F 1 Generation All F 1 plants produce yellow-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 YY Y Y Y Y Y Y Y Y Y Y yy y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR

Figure 15.2c F 2 Generation 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT 3

Thomas Hunt Morgan

The common fruit fly – Drosophila melanogaster

Red eye – the “wild type”White eye – a mutant Drosophila melanogaster

All offspring had red eyes. P Generation F 1 Generation F 2 Generation F 1 Generation P Generation Eggs Sperm X X X Y ww ww ww ww ww ww ww ww ww ww ww w w w w ww RESULTS EXPERIMENT CONCLUSION

All offspring had red eyes. P Generation F 1 Generation F 2 Generation RESULTS EXPERIMENT

F 2 Generation P Generation Eggs Sperm X ww CONCLUSION X X Y ww ww ww ww ww ww ww ww ww ww w w w w w w F 1 Generation

X Y Human x and y chromosomes

Parents or Sperm 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 Sperm (a) (b) (c) XNXNXNXN XnYXnY XNXnXNXn XNYXNY XNXnXNXn XnYXnY XnXn Y XNXN Y Y XnXn XnXn XnXn XNXN XNXN XNXN XNXN XNXnXNXn XNYXNY XNYXNY XNYXNYXNYXNY XnYXnY XnYXnY XNXnXNXn XNXnXNXn XNXnXNXn XNXNXNXN XnXnXnXn Transmission of sex-linked recessive traits: a)Father with trait passes trait to all daughters - carriers b)Female carrier passes trait to half her sons and daughters c)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