CHROMOSOME THEORY OF

INHERITANCE

Proposed in 1902 by Walter Sutton and Theodor Boveri

Noticed that transmission of chromosomes closely parallels

Mendelian pattern

Theory states: chromosomes are the carriers of the genes

Theory strongly supported after discovery of sex chromosomes

Female: homogameticMale: heterogametic

Cruzamientos realizados por Morgan

F1: Toda de ojos rojos

Conclusion: Ojo Blanco es recesivo

Generación Parental

Parental femenino Parental masculino

OJOSROJOS

OJOSBLANCOS

Morgan’s eye-color crosses

F1: All red-eyed

Conclusion? White eye is recessive

Resultados de la F2

1/2 Hembras ojo rojo : 1/4 machos ojo rojo : 1/4 machos ojo blanco

Todas las hembras son de ojo rojo

En base a este resultado Morgan propone que el carácter ojo blanco debe estar asociado al cromosoma X

Cruzamientos realizados por Morgan

Generación F1

Fenotipos F1 OjosRojos

OjosRojos

Morgan’s eye-color crosses

What was going on?

All the white-eyed flies were males!

White-eyed flies have lower viability than wild-type

Morgan’s eye-color crosses

So, the ratio was probably really 3 : 1

On the basis of these results from crosses, Morgan proposed that w is on the X

Hypothesized the following model as an explanation:

El macho tiene ojos blancos porque es hemicigoto.

Muestra el fenotipo de un alelo recesivo en simple dosis

Cruzamientos realizados por Morgan

Generación Parental

Genotipo Parentaldiploide

Gametoshaploides

Cruzamientos realizados por Morgan

Gametos masculinos

Gametos femeninos

Generación F1

Genotipos F1

½ w+/w (o X+/Xw) ½ w+/Y (o X+/Y)

Fenotipos F1½ hembras ½ machosTodos de ojos rojos

Morgan’s eye-color crosses

Morgan’s eye-color crosses

Morgan’s eye-color crosses

Morgan’s crosses showed a transmission pattern of:

Male P to Female F1 to Male F2

This pattern is called crisscross inheritance

It is indicative of X-linked recessives

Morgan’s eye-color crossesReciprocal crosses:

Morgan’s eye-color crosses

Morgan’s eye-color crosses

Morgan’s eye-color crosses

Morgan’s eye-color crosses

The difference in the phenotypic ratios in the 2 sets of crosses is because of the sex

linkage

Morgan’s eye-color crosses

When the results of reciprocal crosses are not the same,

Different ratios are seen for the sexes of the offspring

Sex-linkage is strongly suspected

Morgan’s results strongly supported the chromosome theory of inheritance

The transmission pattern of the phenotypes neatly fit the pattern of the X

chromosome

The theory is further supported by the parallel behavior between Mendelian traits and chromosomes in meiosis: