LOS EXPERIMENTOS QUE PERMITIERON A MENDEL DESCUBRIR LAS LEYES SOBRE LA HERENCIA GENÉTICA
Gregor Mendel
INTRODUCCIÓN
En este
último punto del blog vamos a estudiar y analizar el perfil personal y profesional
de Gregor
Mendel (1822-1884), su entorno, su personalidad, su biografía y todo cuanto contribuyó a diseñar
su personalidad religiosa y sus experimentos en genética hereditaria.
Con la
presentación del siguiente documento gráfico podemos aproximarnos a
la comprensión del trabajo realizado por Gregor Mendel y sus implicaciones en
la construcción de la genética como disciplina científica.
a) Historical context
Gregor Mendel was an Austrian monk who
discovered the basic principles of heredity through experiments in his garden.
Mendel's observations became the foundation of modern genetics and the study of
heredity, and he is widely considered a pioneer in the field of genetics.
Casa natal de Juan Gregorio Mendel
Early Life
Following his graduation, Mendel enrolled in a
two-year program at the Philosophical Institute of the University of Olmütz.
There, he again distinguished himself academically, particularly in the
subjects of physics and math, and tutored in his spare time to make ends meet.
Despite suffering from deep bouts of depression that, more than once, caused
him to temporarily abandon his studies, Mendel graduated from the program in
1843.
That same year, against the wishes of his
father, who expected him to take over the family farm, Mendel began studying to
be a monk: He joined the Augustinian order at the St. Thomas Monastery in Brno,
and was given the name Gregor. At that time, the monastery was a cultural
center for the region, and Mendel was immediately exposed to the research and
teaching of its members, and also gained access to the monastery’s extensive
library and experimental facilities.
Monasterio de Agustinos de
Brno
In 1849, when his work in the community in Brno
exhausted him to the point of illness, Mendel was sent to fill a temporary
teaching position in Znaim. However, he failed a teaching-certification exam
the following year, and in 1851, he was sent to the University of Vienna, at
the monastery’s expense, to continue his studies in the sciences. While there, Mendel
studied mathematics and physics under Christian Doppler, after whom the Doppler
effect of wave frequency is named; he studied botany under Franz Unger, who had
begun using a microscope in his studies, and who was a proponent of a
pre-Darwinian version of evolutionary theory.
In 1853, upon completing his studies at the
University of Vienna, Mendel returned to the monastery in Brno and was given a
teaching position at a secondary school, where he would stay for more than a
decade. It was during this time that he began the experiments for which he is
best known.
b) Descriptions
of the experiments step by step, using images to illustrate the main ideas.
A continuación se pone este original documento donde unos
alumnos resumen de forma animada los cruces que Mendel llevó a cabo en sus
experimentos para posteriormente convertir estos experimentos en sus leyes de
la genética.
This video offers a description of the
monohybrid (single trait) controlled cross developed by Gregor Mendel and used
by geneticists ever since. It is the most basic type of cross and often the
first learned when studying genetics at any level.
Around 1854, Mendel began to research
the transmission of hereditary traits in plant hybrids. At the time of Mendel’s
studies, it was a generally accepted fact that the hereditary traits of the
offspring of any species were merely the diluted blending of whatever traits
were present in the “parents.” It was also commonly accepted that, over
generations, a hybrid would revert to its original form, the implication of
which suggested that a hybrid could not create new forms. However, the results
of such studies were often skewed by the relatively short period of time during
which the experiments were conducted, whereas Mendel’s research continued over
as many as eight years (between 1856 and 1863), and involved tens of thousands
of individual plants.
Mendel chose to use peas for his
experiments due to their many distinct varieties, and because offspring could
be quickly and easily produced. He cross-fertilized pea plants that had clearly
opposite characteristics—tall with short, smooth with wrinkled, those
containing green seeds with those containing yellow seeds, etc.—and, after
analyzing his results, reached two of his most important conclusions: the Law
of Segregation, which established that there are dominant and recessive traits
passed on randomly from parents to offspring (and provided an alternative to
blending inheritance, the dominant theory of the time), and the Law of
Independent Assortment, which established that traits were passed on independently
of other traits from parent to offspring. He also proposed that this heredity
followed basic statistical laws. Though Mendel’s experiments had been conducted
with pea plants, he put forth the theory that all living things had such
traits.
Condiciones experimentales de Mendel
Una vez que Mendel había establecido líneas de guisantes
genéticamente puras con diferentes rasgos para una o más características de
interés (como altura alta vs. baja), comenzó a investigar cómo se heredaban los
rasgos realizando una serie de cruzamientos.
Primero, cruzó un
progenitor genéticamente puro con otro. Las plantas usadas en este cruzamiento
inicial son llamadas generación P o
generación parental.
Mendel recolectó las
semillas del cruzamiento de la generación P y las cultivó. Estos descendientes
fueron llamados generación F1,
abreviatura para primera generación filial. (Filius significa "hijo" en latín, ¡así que este nombre es
un poco menos raro de lo que parece!)
Los experimentos de Mendel se
extendieron más allá de la generación F2, a las generaciones F3, F4 y
posteriores, pero su modelo de la herencia se basó principalmente en las
primeras tres generaciones (P, F1 y F2).
c) Diagramas para resumir las ideas
principales
Primera Ley De Mendel
La primera ley de Mendel, también llamada Ley de
la uniformidad de los híbridos de la primera generación, o simplemente Ley de la Uniformidad.
Segunda Ley De Mendel
La segunda ley de Mendel, también conocida como
la Ley de la Segregación, Ley de la
Separación Equitativa, o hasta Ley de Disyunción de los Alelos.
Tercera Ley De Mendel
La tercera ley de Mendel, también llamada Ley de la Herencia Independiente.
d) Preguntas relacionadas con las
ideas anteriores
¿En
qué consistieron los experimentos de Mendel?
Los
experimentos de Mendel sobre la herencia se realizaron con guisantes. Mendel
eligió está planta porque se reproduce con rapidez, permitiendo obtener varias
generaciones en un poco tiempo. Además, tiene rasgos que solo admiten dos
formas (los guisantes son lisos o rugosos, verdes o amarillos…) y son capaces
tanto de auto-polinizarse como de fertilizarse de forma cruzada.
En
sus experimentos, Mendel estudió siete características de la planta de
guisante: color de la semilla, forma de la semilla, posición de la flor, color
de la flor, forma de la vaina, color de la vaina y longitud del tallo.
Por
ejemplo, en una de sus pruebas Mendel cruzó dos variedades de guisantes: una
con flores purpuras y otra con flores blancas.
Esta era la generación P o parental, y su descendencia fue la F1 (primera
generación filial). La generación F1 luego se reprodujo por autopolinización,
Dando lugar a la generación F2.
El
resultado de la prueba fue bastante clarificador. Si en la generación parental
había el mismo número de guisantes con flores blancas que con flores purpuras,
en la F1 solo aparecieron flores purpura. Sin embargo en la generación F2
reaparecieron los guisantes con flores blancas, que representaron
aproximadamente ¼ de la descendencia.
¿Qué
concluyó Mendel con sus experimentos?
A
la vista de los resultados Mendel dedujo que el color purpura en la flor del
guisante era un rasgo dominante (A) y la flor blanca un rasgo recesivo (a).
Mendel observó el mismo patrón de herencia en otros seis personajes, cada uno
representado por dos rasgos diferentes. A partir de ahí llegó a las siguientes
conclusiones:
● Los
organismos tienen factores discretos que determinan sus características (estos
"factores" ahora se reconocen como genes)
●
Además,
los organismos poseen dos versiones de cada factor (estas 'versiones' ahora se
conocen como alelos)
● Cada
gameto contiene solo una versión de cada factor (ahora se sabe que las células
sexuales son haploides, es decir, solo tienen n cromosomas y no 2n, como el
resto de células del organismo)
●
Los
padres contribuyen igualmente a la herencia de la descendencia como resultado
de la fusión entre el óvulo y los espermatozoides seleccionados al azar.
●
Para
cada factor, una versión es dominante sobre otra y se expresará completamente
si está presente.
Later Life and Legacy
In 1868, Mendel was elected abbot of the
school where he had been teaching for the previous 14 years, and both his
resulting administrative duties and his gradually failing eyesight kept him
from continuing any extensive scientific work. He traveled little during this
time, and was further isolated from his contemporaries as the result of his
public opposition to an 1874 taxation law that increased the tax on the
monasteries to cover Church expenses.
Gregor Mendel died on January 6, 1884,
at the age of 61. He was laid to rest in the monastery’s burial plot and his
funeral was well attended. His work, however, was still largely unknown.
It was not until decades later, when Mendel’s research
informed the work of several noted geneticists, botanists and biologists
conducting research on heredity, that its significance was more fully
appreciated, and his studies began to be referred to as Mendel’s Laws. Hugo de
Vries, Carl Correns and Erich von Tschermak-Seysenegg each independently
duplicated Mendel's experiments and results in 1900, finding out after the
fact, allegedly, that both the data and the general theory had been published
in 1866 by Mendel. Questions arose about the validity of the claims that the
trio of botanists were not aware of Mendel's previous results, but they soon
did credit Mendel with priority. Even then, however, his work was often
marginalized by Darwinians, who claimed that his findings were irrelevant to a
theory of evolution. As genetic theory continued to develop, the relevance of
Mendel’s work fell in and out of favor, but his research and theories are
considered fundamental to any understanding of the field, and he is thus
considered the "father of modern genetics."
e) Referencias
bibliográficas
d) Preguntas relacionadas con las
ideas anteriores
¿En qué consistieron los experimentos de Mendel?
¿En qué consistieron los experimentos de Mendel?
Los
experimentos de Mendel sobre la herencia se realizaron con guisantes. Mendel
eligió está planta porque se reproduce con rapidez, permitiendo obtener varias
generaciones en un poco tiempo. Además, tiene rasgos que solo admiten dos
formas (los guisantes son lisos o rugosos, verdes o amarillos…) y son capaces
tanto de auto-polinizarse como de fertilizarse de forma cruzada.
En
sus experimentos, Mendel estudió siete características de la planta de
guisante: color de la semilla, forma de la semilla, posición de la flor, color
de la flor, forma de la vaina, color de la vaina y longitud del tallo.
Por
ejemplo, en una de sus pruebas Mendel cruzó dos variedades de guisantes: una
con flores purpuras y otra con flores blancas.
Esta era la generación P o parental, y su descendencia fue la F1 (primera
generación filial). La generación F1 luego se reprodujo por autopolinización,
Dando lugar a la generación F2.
El
resultado de la prueba fue bastante clarificador. Si en la generación parental
había el mismo número de guisantes con flores blancas que con flores purpuras,
en la F1 solo aparecieron flores purpura. Sin embargo en la generación F2
reaparecieron los guisantes con flores blancas, que representaron
aproximadamente ¼ de la descendencia.
¿Qué
concluyó Mendel con sus experimentos?
A
la vista de los resultados Mendel dedujo que el color purpura en la flor del
guisante era un rasgo dominante (A) y la flor blanca un rasgo recesivo (a).
Mendel observó el mismo patrón de herencia en otros seis personajes, cada uno
representado por dos rasgos diferentes. A partir de ahí llegó a las siguientes
conclusiones:
● Los
organismos tienen factores discretos que determinan sus características (estos
"factores" ahora se reconocen como genes)
●
Además,
los organismos poseen dos versiones de cada factor (estas 'versiones' ahora se
conocen como alelos)
● Cada
gameto contiene solo una versión de cada factor (ahora se sabe que las células
sexuales son haploides, es decir, solo tienen n cromosomas y no 2n, como el
resto de células del organismo)
●
Los
padres contribuyen igualmente a la herencia de la descendencia como resultado
de la fusión entre el óvulo y los espermatozoides seleccionados al azar.
●
Para
cada factor, una versión es dominante sobre otra y se expresará completamente
si está presente.
Later Life and Legacy
In 1868, Mendel was elected abbot of the
school where he had been teaching for the previous 14 years, and both his
resulting administrative duties and his gradually failing eyesight kept him
from continuing any extensive scientific work. He traveled little during this
time, and was further isolated from his contemporaries as the result of his
public opposition to an 1874 taxation law that increased the tax on the
monasteries to cover Church expenses.
Gregor Mendel died on January 6, 1884,
at the age of 61. He was laid to rest in the monastery’s burial plot and his
funeral was well attended. His work, however, was still largely unknown.
It was not until decades later, when Mendel’s research
informed the work of several noted geneticists, botanists and biologists
conducting research on heredity, that its significance was more fully
appreciated, and his studies began to be referred to as Mendel’s Laws. Hugo de
Vries, Carl Correns and Erich von Tschermak-Seysenegg each independently
duplicated Mendel's experiments and results in 1900, finding out after the
fact, allegedly, that both the data and the general theory had been published
in 1866 by Mendel. Questions arose about the validity of the claims that the
trio of botanists were not aware of Mendel's previous results, but they soon
did credit Mendel with priority. Even then, however, his work was often
marginalized by Darwinians, who claimed that his findings were irrelevant to a
theory of evolution. As genetic theory continued to develop, the relevance of
Mendel’s work fell in and out of favor, but his research and theories are
considered fundamental to any understanding of the field, and he is thus
considered the "father of modern genetics."
e) Referencias
bibliográficas
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