Mendelian laws of inheritance





Mendelian Laws Of Inheritance 2808
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Mendelian laws of inheritance are statements about the way certain characteristics are transmitted from one generation to another in an organism. The laws were derived by the Austrian monk Gregor Mendel (1822–1884) based on experiments he conducted in the period from about 1857 to 1865. For his experiments, Mendel used ordinary pea plants. Among the traits that Mendel studied were the color of a plant's flowers, their location on the plant, the shape and color of pea pods, the shape and color of seeds, and the length of plant stems.

Mendel's approach was to transfer pollen (which contains male sex cells) from the stamen (the male reproductive organ) of one pea plant to the pistil (female reproductive organ) of a second pea plant. As a simple example of this kind of experiment, suppose that one takes pollen from a pea plant with red flowers and uses it to fertilize a pea plant with white flowers. What Mendel wanted to know is what color the flowers would be in the offspring of these two plants. In a second series of experiments, Mendel studied the changes that occurred in the second generation. That is, suppose two offspring of the red/white mating ("cross") are themselves mated. What color will the flowers be in this second generation of plants? As a result of these experiments, Mendel was able to state three generalizations about the way characteristics are transmitted from one generation to the next in pea plants.

Words to Know

Allele: One of two or more forms a gene may take.

Dominant: An allele whose expression overpowers the effect of a second form of the same gene.

Gamete: A reproductive cell.

Heterozygous: A condition in which two alleles for a given gene are different from each other.

Homozygous: A condition in which two alleles for a given gene are the same.

Recessive: An allele whose effects are concealed in offspring by the dominant allele in the pair.

Terminology

Before reviewing these three laws, it will be helpful to define some of the terms used in talking about Mendel's laws of inheritance. Most of these terms were invented not by Mendel, but by biologists some years after his research was originally published.

Genes are the units in which characteristics are passed from one generation to the next. For example, a plant with red flowers must carry a gene for that characteristic.

A gene for any given characteristic may occur in one of two forms, called the alleles (pronounced uh-LEELZ) of that gene. For example, the gene for color in pea plants can occur in the form (allele) for a white flower or in the form (allele) for a red color.

The first step that takes place in reproduction is for the sex cells in plants to divide into two halves, called gametes. The next step is for the gametes from the male plant to combine with the gametes of the female plant to produce a fertilized egg. That fertilized egg is called a zygote. A zygote contains genetic information from both parents.

For example, a zygote might contain one allele for white flowers and one allele for red flowers. The plant that develops from that zygote would said to be heterozygous for that trait since its gene for flower color has two different alleles. If the zygote contains a gene with two identical alleles, it is said to be homozygous.

Mendel's Law of Segregation. (Reproduced by permission of The Gale Group.)
Mendel's Law of Segregation. (Reproduced by permission of
The Gale Group
.)

Mendel's laws

Mendel's law of segregation describes what happens to the alleles that make up a gene during formation of gametes. For example, suppose that a pea plant contains a gene for flower color in which both alleles code for red. One way to represent that condition is to write RR, which indicates that both alleles (R and R) code for the color red. Another gene might have a different combination of alleles, as in Rr. In this case, the symbol R stands for red color and the r for "not red" or, in this case, white. Mendel's law of segregation says that the alleles that make up a gene separate from each other, or segregate, during the formation of gametes. That fact can be represented by simple equations, such as:

RR → R + R or Rr → R + r

Mendel's second law is called the law of independent assortment. That law refers to the fact that any plant contains many different kinds of genes. One gene determines flower color, a second gene determines length of stem, a third gene determines shape of pea pods, and so on. Mendel discovered that the way in which alleles from different genes separate and then recombine is unconnected to other genes. That is, suppose that a plant contains genes for color (RR) and for shape of pod (TT). Then Mendel's second law says that the two genes will segregate independently, as:

RR → R + R and TT → T + T

Mendel's third law deals with the matter of dominance. Suppose that a gene contains an allele for red color (R) and an allele for white color (r). What will be the color of the flowers produced on this plant? Mendel's answer was that in every pair of alleles, one is more likely to be expressed than the other. In other words, one allele is dominant and the other allele is recessive. In the example of an Rr gene, the flowers produced will be red because the allele R is dominant over the allele r.

Predicting traits

The application of Mendel's three laws makes it possible to predict the characteristics of offspring produced by parents of known genetic composition. The picture on page 1248, for example, shows the cross between a sweet pea plant with red flowers (RR) and one with white flowers (rr). Notice that the genes from the two parents will segregate to produce the corresponding alleles:

RR → R + R and rr → r + r

There are, then, four ways in which those alleles can recombine, as shown in the same picture. However, all four combinations produce the same result: R + r → Rr. In every case, the gene formed will consist of an allele for red (R) and an allele for "not red" (r).

The drawing at the right in the picture on page 1248 shows what happens when two plants from the first generation are crossed with each other. Again, the alleles of each plant separate from each other:

Rr → R + r

Again, the alleles can recombine in four ways. In this case, however, the results are different from those in the first generation. The possible results of these combinations are two Rr combinations, one RR combination, and one rr combination. Since R is dominant over r, three of the four combinations will produce plants with red flowers and one (the rr option) will product plants with non-red (white) flowers.

Biologists have discovered that Mendel's laws are simplifications of processes that are sometimes much more complex than the examples given here. However, those laws still form an important foundation for the science of genetics.

[ See also Chromosome ; Genetics ]



User Contributions:

Angelica
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Mar 10, 2007 @ 9:09 am
i need help on this question its my homework please help!!!! one trait in pea plants is the color of their pea pods.the gene for the trait has two alleles. the green allele (g) is dominant , and the yellow allele is recessive (g). the punnett square shows a cross between two parents each with a dominant allele and recessive allele. filling in the punnent squre will show you what traits their offspring will have.
joseph dorcas
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Sep 17, 2008 @ 4:04 am
this article really assisted me especially in carrying out my lesson effectively. it has guided my students and i on the easiest way to understanding the topic.Thanks alot!
brittlyn
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Oct 19, 2009 @ 6:18 pm
HELP PLEASE! ok 1. For Mednel's law of segregation to occur, the alleles must be where? a. at different spots on the same chromosome b. at the same spot on homologous chromoseomes. c. on nonhomolgous chromosomes. do at the same spot on the same chromosome. or e. none of that above
nida
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Nov 10, 2009 @ 12:12 pm
its very easy to understand i prepared myself for presentation on this topic
angelica
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Dec 21, 2009 @ 4:04 am
what were the characteristics or traits used in mendel's experiment?
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Apr 5, 2010 @ 10:10 am
very informative it really helpful for the students of genetic.
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Jun 24, 2010 @ 9:21 pm
what are those gene interaction,linkage and quantitative trait loci(QTLs)?
hope you can help me...
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Jun 27, 2010 @ 12:00 am
I am stuck on some homework qestions!!! I'm trying to figure out if a woman who isn't colorblind but has an allele for color blindness reproduces with a man who has normal vision. What is the chance that they will have a colorblind daughter??? I have looked all in my books but I'm so desprate.

I have another question too. I have never used this site. I dont know if you can ask two questions.

But my other ? is: A recessive allele t is responsible for a condition called distonia. A man who has this condition marries a woman who doesn't. One of their 4 children has a condition. What are the possible genotypes of the man and the woman?

PLEASE HELP!
sarah
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Jul 2, 2010 @ 2:02 am
this article gave me the easy way out. its so simple and easy to understand. thanks.
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Sep 11, 2010 @ 10:10 am
The article gives an overview of mendels laws..thanks for this..
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Nov 13, 2010 @ 9:21 pm
Law of ihdependent assortment has not been properly explained.there is need of relation ship between mendels law &linkagr
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Dec 10, 2010 @ 2:02 am
Please tell me the application of the mendelism to man with an example.. i want to make my notes on this main tnpic.
alam
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Dec 23, 2010 @ 9:09 am
this article is very beautifull and very good points have.
I have question that why in some cases there are in complete
dominane.
ms. biology lover
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Jan 3, 2011 @ 4:04 am
.THIS IS INDEED A BIG HELP FOR A STUDENT LIKE ME!THANKS A LOT!
Wan
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Feb 12, 2011 @ 3:03 am
This article really helps. Easy for me to explain to my students. Thank you very much
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Feb 14, 2011 @ 7:07 am
I want to have more information about things in school and more. THIS IS A GREAT HELP TO ME SINCE I DON'T WANT SCIENCE SO MUCH!
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Feb 23, 2011 @ 9:21 pm
Love this website! It gave me a lot of important information that I needed to know about Mendel's law. It answered all my questions.
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Feb 28, 2011 @ 1:01 am
THIS ARTICLE IS NICE, THANKS FOR THE GOOD WORK YOUR HAVE DONE TO ENSURE THAT MOST PEOPLE GET INFORMATION WHEN DEALING WITH THE GENETICS ISSUE BUT IF YOU COULD HAVE USED MORE DIAGRAMS IT COULD HAVE BEEN PERFECT. THANKS AND KEEP IT UP.
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Mar 4, 2011 @ 11:11 am
Thanks I really liked this information. Good work.
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Mar 16, 2011 @ 9:09 am
What is eye color gene dominant between blue and brown ?
I understand brown is dominant.
Please tell me if you know
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Nov 7, 2011 @ 4:16 pm
Mom is red-green color blind, and dad has normal color vision. What is the % chance that a son will be color blind? That the daughter will be a carrier?
Animut Tarik
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Nov 11, 2011 @ 8:08 am
I want a clear information on "inheritance of male sterility".I need also its difference between Mendelian and non-Mendelian inheritance. hope i expect a clear information
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Nov 21, 2011 @ 10:22 pm
what are the 22 varieties of pea plant did Mendel used ?
Ankit
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Feb 22, 2012 @ 7:19 pm
Very itisreentng example of a cause of extinction. Do you think this lack of genetic diversity could also lead to mutations? And if so, maybe this could revive the cheetah population.This lack of diversity will prove to be a great complication if there is ever a change in the cheetahs' environment.
WABWILE DAVID
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Sep 24, 2012 @ 11:11 am
How did people think about inheritance before mendels law?
pooja kharkwal
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Oct 12, 2012 @ 11:23 pm
Hello, i want to know more information about low of segregation. THANK YOU
amma qayyom
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Oct 26, 2012 @ 3:03 am
this is very good artical and it helps me alot in my project
omeje chinweike ibenedum
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Jan 14, 2013 @ 1:01 am
please I need at least five pages on mendelian laws of inheritance and with examples
ene
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Jan 24, 2013 @ 7:07 am
i need help on my assignment,am to write a four page work on mendels laws of inhritaance.HELP
Buck
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Jan 29, 2013 @ 9:09 am
This was very helpful it was easy to find the info i needed.
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Feb 24, 2013 @ 10:22 pm
this was very helpful it was easy to find the info i needed?
Grace Rahua
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Apr 10, 2013 @ 5:17 pm
Hi! This is a very good article. It helps me understand Mendel's Law of Inheritance but there is not much on the second law(Law of Independent Assortment). Can you please ellaborate more on the secod law this.
Fatmata Drammeh
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Feb 25, 2014 @ 3:03 am
He use characteristis for .e.g. colour, shape and texture of the offspring. He used the larg sampling size wgich gave him credibility to his data. He also look through several successive generations of his pea plant.
Fatmata Drammeh
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Feb 25, 2014 @ 3:03 am
This is the most intresting article I have ever read because it help me a lots to learn much about Gregor Mendel's Laws. Thank you for writing this article for students to learn and understanding more Mendel's Laws of Inheritance.
vinod
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May 27, 2014 @ 2:02 am
I understood very well...but we need more information in second law
ogada billy
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Jul 31, 2014 @ 11:11 am
please make appear some calculations on runnette square

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