Name: mendelian traits and exceptions
Uploaded: 2019-01-29T11:04:44.000Z
Duration: 6 min 6 s
Channel: JoVE

K12 Standards

Next Generation Science Standards

High School

Life Science

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

Genetics of Organisms

Una herencia es algo que se transmite de una generación a otra. En algunos contextos, esto significa cosas como casas y dinero. Sin embargo, en el contexto de la biología, el estudio de los genes y cómo se heredan se llama genética. A Gregor Mendel se le atribuye ser el padre de la genética moderna. Su trabajo es responsable de nuestra comprensión de cómo los rasgos físicos notables, o fenotipos, se transmiten de una generación a la siguiente. Como es sabido, estudió estos rasgos en las plantas de guisantes. 
Las piezas de información que controlan esos fenotipos se llaman genes. Como hay dos copias de cada gen, conocidas como alelos, podemos representarlas como letras. Aquí, usaremos la letra P. Este es el primer paso del famoso experimento de Mendel sobre la coloración de la flor del guisante, representado aquí, utilizando una herramienta llamada cuadrado de Punnett. Mendel descubrió que cuando cruzaba flores moradas con blancas, toda la progenie, o las plantas de primera generación, tenían flores púrpuras. Esto se debe a que el color púrpura es dominante, que se muestra con una P mayúscula. Y llevar incluso un alelo dominante significa que el fenotipo se expresará. Pero curiosamente, cuando se volvieron a cruzar estas flores moradas, 1/4 de ellas eran blancas. ¿Dónde estaban las flores blancas en la primera generación? Todas las plantas de la primera generación de flores moradas eran heterocigóticas, lo que significa que tenían una P púrpura mayúscula y una P minúscula o blanca. Cuando transmitieron sus alelos en el cruce de la generación F2, esto significó que 1/4 de la descendencia recibió dos pequeños alelos P y, por lo tanto, expresó el fenotipo blanco. Mendel no tenía la herramienta del cuadrado de Punnett para usar, tuvo que resolver todo esto haciendo un seguimiento de miles de plantas y luego notando patrones en sus números. 
A partir de esta evidencia, y mucho más, ahora sabemos que los genes también están presentes en dos copias en otros organismos, como los humanos y las moscas. Al seguir el trabajo de Mendel, varios científicos descubrieron que no todos los patrones de herencia seguían el modelo simple y básico que Mendel propuso. Por ejemplo, en la hemofilia, un trastorno genético de la coagulación, las madres no afectadas eran capaces de transmitir la enfermedad a sus hijos varones. La razón detrás de esto radica en los cromosomas, que fueron estudiados por Thomas Hunt Morgan, utilizando sus famosas moscas de la fruta, Drosophila. Gracias a Morgan y otros, ahora sabemos que los cromosomas son largas hebras de ADN que normalmente existen en pares. Aquí, podemos ver que Drosophila tiene cuatro de ellos. Estos cromosomas tienen genes para diferentes rasgos, al igual que un libro de cocina contiene muchas recetas diferentes. Hoy en día, utilizando la microscopía moderna, podemos ver estos cromosomas e incluso organizarlos. El producto de este proceso se denomina cariotipo. Aquí, puedes ver uno humano. Tanto en los humanos como en las moscas, hay autosomas y cromosomas sexuales. Los humanos, al igual que las moscas, tienen cromosomas X e Y que controlan su sexo. Sin embargo, la mayoría de los genes de estos cromosomas controlan cosas que no tienen nada que ver con el sexo. En la rara forma de hemofilia que mencionamos anteriormente, la razón por la que ocurre con mayor frecuencia en los hombres es porque el fenotipo está controlado por un gen que se encuentra en el cromosoma X, en una sección que no tiene compañero en el cromosoma Y. Si una mujer tiene una mala copia del gen, y su hijo varón hereda esta copia, tendrá el trastorno, no tiene una copia de seguridad del cromosoma X. Dado que una copia del gen es suficiente para que una persona coagule normalmente, una mujer debe heredar dos alelos malos del gen, uno de cada padre, para exhibir la enfermedad. Lo cual, en este caso, no es posible, porque el padre no está afectado. Como resultado, este tipo de hemofilia afecta más a los hombres que a las mujeres. 
En este laboratorio, analizaremos la herencia en Drosophila. El color de los ojos de las moscas está controlado por una serie de genes, algunos de los cuales controlan qué tipos de pigmentos se producen, y un gen particularmente importante, llamado transportador ABC, que controla el transporte de pigmentos a gránulos en el ojo. Si ese gen se rompe, incluso si la mosca está produciendo pigmento, ese pigmento será invisible y la mosca tendrá ojos blancos. El ejercicio de laboratorio consiste en recrear uno de los experimentos más famosos de Thomas Hunt Morgan y explorar el patrón de herencia genética del gen que codifica el transportador de pigmentos. ¿Se hereda como el color púrpura de las flores de guisante de Mendel, o está ligada al sexo, como la hemofilia?

genética de los organismos

<h4>Genética mendeliana</h4>
La evolución es causada por cambios en la composición genética de las poblaciones. En el campo de la genética de poblaciones, los científicos modelan este proceso como cambios en la frecuencia de alelos en loci genéticos individuales. Esta sencilla representación de cómo se produce la evolución se remonta al análisis de Gregor Mendel de los patrones de herencia de rasgos en las plantas de guisantes, presentado por primera vez en 1865. Mendel determinó, utilizando una rigurosa recopilación de datos, que los rasgos notables están controlados por dos alelos de cada gen (los llamó factores), uno de cada padre. Además, concluyó que algunos alelos son dominantes sobre otros (considerados recesivos). Curiosamente, los datos de Mendel no fueron apreciados hasta 1900, probablemente en parte porque su enfoque del estudio de la herencia era muy divergente de sus predecesores. Incluso después de que sus brillantes experimentos fueran redescubiertos, tuvieron que pasar 30 años más para que la herencia mendeliana se integrara en la teoría de la evolución y ganara la aceptación universal.
¿Por qué un proceso aparentemente tan claro y sencillo estuvo inactivo durante tanto tiempo y luego fue el centro de la controversia durante décadas? Una de las claves de la controversia es que rara vez vemos los patrones claros de herencia descritos por Mendel. Los criadores de animales y plantas a menudo ven evidencia de lo que se describió como herencia mezclada, o descendencia cuyos fenotipos son intermedios entre los de sus padres, en lugar de la segregación discreta de rasgos en categorías bien definidas, como guisantes lisos frente a arrugados o flores rojas frente a blancas. Por lo tanto, la herencia mendeliana no se alineó bien con la experiencia práctica, y se necesitaron décadas para reconciliar las dos. Además, ahora se sabe que hay modos de herencia que, a primera vista, parecen violar las leyes de Mendel. Investigaciones más profundas en genética utilizando la biología molecular han caracterizado eficazmente estos nuevos modos de herencia. La nueva información no invalida los hallazgos de Mendel, sin embargo, simplemente mejora su modelo.
<h4>Fenómenos que violan las reglas mendelianas</h4>
Hay tres fenómenos principales: la epistasis, la pleiotropía y la vinculación sexual que parecen violar las reglas básicas de dominancia y recesividad de la herencia mendeliana. La epistasis ocurre cuando dos o más genes o alelos interactúan para afectar un fenotipo. Una función de un producto génico puede ser necesaria para permitir que otro gen se exprese o para funcionar normalmente. Por otro lado, la pleiotropía ocurre cuando un gen controla la expresión de múltiples fenotipos en un individuo. Por ejemplo, una gran proporción de gatos con pelaje blanco y ojos azules son sordos 1. La pleiotropía es a menudo antagónica, lo que significa que el mismo gen causa cambios beneficiosos en una faceta del fenotipo de un individuo y, al mismo tiempo, causa cambios dañinos en algún otro aspecto del fenotipo de un individuo. La pleiotropía antagónica es vista como una compensación o restricción en la evolución, como lo propuso George C. Williams. Por ejemplo, se cree que la disminución del rendimiento específica de la edad, o senescencia, es un rasgo pleiotrópico controlado por los mismos genes que aumentan la fecundidad en los primeros años de vida.
<h4>Vinculación sexual</h4>
La vinculación sexual es otro fenómeno descubierto como contraejemplo de la herencia mendeliana básica. En muchos organismos, los pares de cromosomas sexuales determinan el sexo. En los seres humanos y muchos mamíferos, los individuos XX son hembras, mientras que los individuos XY son machos. La descendencia masculina hereda una X de su madre y una Y de su padre; las hembras heredan una X de su madre y una X de su padre. En la mayoría de las especies, la proporción de machos y hembras al nacer en condiciones ambientales "normales" es de 1:1. Los cromosomas Y sufren poca recombinación con los cromosomas X durante la meiosis, debido al hecho de que, a lo largo del tiempo evolutivo, han perdido un número sustancial de genes. Curiosamente, solo hay 16 genes compartidos por los cromosomas X e Yhumanos 4.
En el caso de los rasgos ligados al cromosoma X (rasgos codificados por genes en el cromosoma X), las mujeres pueden ser homocigóticas (con dos copias del mismo alelo) o heterocigóticas (con dos alelos diferentes). Si el rasgo es recesivo, las hembras solo mostrarán el fenotipo del rasgo si son homocigóticas. Las hembras heterocigóticas "portan" el alelo recesivo, pero no expresan el fenotipo de ese alelo. Estadísticamente hablando, la mitad de sus hijos heredarán este alelo recesivo y todos expresarán el fenotipo porque los varones tienen un solo cromosoma X.
Un ejemplo comúnmente utilizado de un fenotipo ligado al cromosoma X es la hemofilia, una enfermedad que impide la síntesis de proteínas de coagulación de la sangre. Si una hembra es heterocigótica (es portadora, genotipo XH X h) se aparea con un macho normal (genotipo XH Y), el 50% de los hijos se verán afectados. Si, en cambio, la hembra fuera homocigota para el rasgo (genotipo X,h,X, h), ninguna de sus hijas sería hemofílica, pero el 100% de sus hijos sí lo serían. Las proporciones fenotípicas diferenciales observadas en hijos e hijas parecen violar las leyes de Mendel, sin embargo, la primera ley de Mendel aún se aplica; cada descendencia tiene un 50% de probabilidades de heredar un cromosoma X portador del alelo de hemofilia de una madre heterocigota5.
Al igual que los humanos, la mosca de la fruta Drosophila melanogaster tiene un sistema de determinación del sexo XY. D. melanogaster son excelentes organismos de laboratorio porque son fáciles de mantener, criar y manipular. Las moscas de tipo salvaje tienen lo que se considera una morfología corporal normal y ojos rojos. Muchas líneas están disponibles para su compra, incluidas algunas con mutaciones que dan como resultado moscas sin ojos, con ojos de diferentes colores o sin alas. Estos organismos se pueden utilizar para estudiar o demostrar los patrones de herencia mendelianos y no mendelianos. De hecho, el ganador del Premio Nobel de 1933, Thomas Hunt Morgan, notó por primera vez que algunos genes están ligados al sexo mediante este sistema.
<h4>Referencias</h4>
<ol class="referencias">
	<li>Strain, George M., 'Los gatos se reproducen con sordera congénita', [en línea]. Disponible: https://www.lsu.edu/deafness/catbreeds.htm. [Consultado el 29 de agosto de 2018].</li>
	<li>W. GC., 'Pleiotropía, selección natural y la evolución de la senescencia', Evolución, vol. 11, p. 398-411, 1957.</li>
	<li>T. & S. P. S. Flatt, 'Integración de la genética evolutiva y molecular del envejecimiento', Biochimica et Biophysica Acta, vol. 1790, no. 10, p. 951-962, 2009.</li>
	<li>D. Bachtrog, 'Evolución del cromosoma Y: perspectivas emergentes sobre los procesos de degeneración del cromosoma Y', Nature Reviews. Genética, Vols. 14(2), 113–124, 2013.</li>
	<li>Biblioteca Nacional de Medicina de EE. UU., 'Hemofilia', Genetics Home Reference, [en línea]. Disponible: https://ghr.nlm.nih.gov/condition/hemophilia. [Consultado el 31 de agosto de 2018].</li>
	<li>'El Premio Nobel de Fisiología o Medicina 1933', [en línea]. Disponible: https://www.nobelprize.org/prizes/medicine/1933/summary/. [Consultado el 31 de agosto de 2018].</li>
</ol>

Genética de los Organismos

Genética mendeliana
La evolución es causada por cambios en la composición genética de las poblaciones. En el campo de la genética de poblaciones, los científicos modelan esto ...

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Educación

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Biology

Biología

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Genética

mendelian traits and exceptions

Mendelian genetics explains how traits move from parents to offspring. Gregor Mendel studied pea plants in 1865 and showed that many visible traits are controlled by two alleles, or gene versions, with one inherited from each parent. He also found that some alleles are dominant and others are recessive.

Mendel’s work was not widely accepted until 1900, and it took about 30 more years for Mendelian inheritance to fit into evolutionary theory. One reason for the delay was that real breeding results often looked like blending inheritance, where offspring have intermediate traits instead of clear categories. That pattern seemed different from Mendel’s smooth-versus-wrinkled peas or red-versus-white flowers.

Molecular genetics has since shown that some inheritance patterns can seem to break Mendel’s rules without making his model wrong. The transcript highlights three major examples: epistasis, pleiotropy, and sex linkage. Epistasis happens when two or more genes interact to shape a trait. Pleiotropy happens when one gene affects more than one phenotype, such as white cats with blue eyes that are often deaf.

Pleiotropy can also create tradeoffs. The same gene may help in one part of life and cause harm in another. This kind of antagonistic pleiotropy has been linked to senescence, or age-related decline, where genes that improve early-life reproduction may also contribute to later-life aging effects.

Sex linkage is another case that looks non-Mendelian at first. In humans and many mammals, XX individuals are female and XY individuals are male. Males inherit an X from their mother and a Y from their father, while females inherit an X from each parent. Because Y chromosomes recombine very little with X chromosomes and have lost many genes over time, only 16 genes are shared by the human X and Y chromosomes.

X-linked traits are encoded on the X chromosome. Females can be homozygous or heterozygous for these traits. If a trait is recessive, a female shows it only when she has two copies. A heterozygous female can carry the allele without showing the phenotype, but about half of her sons may inherit it and express the trait because males have only one X chromosome. Hemophilia is a common example. If a carrier female with genotype XH Xh mates with a normal male XH Y, 50% of the sons will be affected. If the female is Xh Xh, none of her daughters will be hemophiliacs, but 100% of her sons will be.

Fruit flies, Drosophila melanogaster, also use an XY system and are useful lab organisms because they are easy to keep, breed, and manipulate. Wild-type flies have normal body shape and red eyes. Researchers can also study flies with no eyes, different eye colors, or missing wings. Thomas Hunt Morgan used this system to notice sex-linked genes, and it remains a useful model for both Mendelian and non-Mendelian inheritance.

Mendelian Traits and Exceptions

genetics of organisms

Mendelian genetics explains how traits move from parents to offspring. Gregor Mendel studied pea plants in 1865 and showed that many visible traits are controlled by two alleles, or gene versions, with one inherited from each parent. He also found that some alleles are dominant and others are recessive. Mendel’s work was not widely accepted until 1900, and it took about 30 more years for Mendelian inheritance to fit into evolutionary theory. One reason for the delay was that real breeding...

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genetics

Explore Mendelian inheritance, dominant and recessive alleles, and key exceptions like epistasis, pleiotropy, and X-linked traits in humans and fruit flies.

Mendelian Traits and Exceptions | JoVE K12

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New York State Science Learning Standards

Inheritance and Variation of Traits

HS-LS3-1.

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Many human diseases are associated with mutations or variations in genetic sequences. Some of these genetic variants are heritable, passed down from ...

Make and defend a claim based on evidence that inheritable genetic variations may result from (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and or (3) mutations caused by environmental factors. [Clarification Statement: Emphasis is on using data to support arguments for the way variation occurs.] [Assessment Boundary: Assessment does not include the phases of meiosis or the biochemical mechanism of specific steps in the process.]

HS-LS3-2

Introduction to Statistics

The science of statistics involves collecting, analyzing, interpreting, and presenting data. The method of collecting, organizing, and summarizing data is ...

Probability Distributions

&#160;The probability of a random variable x&#160; is the likelihood of its occurrence. A probability distribution represents the probabilities of a ...

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Distributions to Estimate Population Parameter

The accurate values of population parameters such as population proportion, population mean, and population standard deviation (or variance) are usually ...

What is Population Genetics?

A population is composed of members of the same species that simultaneously live and interact in the same area. When individuals in a population breed, ...

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Variation: Normal Distribution, Range, and Standard Deviation

In the field of psychology, there are&nbsp;several ways to organize measurements of a trait, feature, or characteristic (i.e., variables). Qualitative ...

Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population. [Clarification Statement: Emphasis is on the use of mathematics to describe the probability of traits as it relates to genetic and environmental factors in the expression of traits.] [Assessment Boundary: Assessment does not include Hardy-Weinberg calculations.]

HS-LS3-3

Mendelian Traits and Exceptions

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