Law of Independent Assortment

Genetics

Law of Independent Assortment Definition

The Law of Independent Assortment is a principle of inheritance that states that each heritable allele (form of a gene) is inherited independently of one another. It is the second of three principles put forth by Gregor Mendel, an Austrian monk who lived in the 19th Century and whose experiments on pea plant heritability make him known today as the father of modern genetics. Mendel’s other laws are the Law of Segregation and the Law of Dominance.

Mendel’s Experiments

Gregor Mendel performed many experiments involving breeding pea plants. In doing so, he gleaned information about how “units of heredity” work, which would later on become known as genes after DNA was discovered and determined to be the material that encodes genetic information.

Mendel developed the Law of Independent Assortment after breeding two different pea plants with two different characteristics; he bred plants with yellow, round peas with plants that had wrinkled, green peas. Since yellow and round were dominant over wrinkled and green, all the offspring had yellow, round peas. But when this first generation was crossbred with each other in what is called a dihybrid cross, there was a lot of variation in the second generation. Peas were no longer either just yellow and round or green and wrinkled; some were green and round, while some were yellow and wrinkled. Furthermore, the offspring showed their characteristics in a ratio of 9:3:3:1. Nine were round and yellow, three were round and green, three were wrinkled and yellow, and one was wrinkled and green. This ratio stayed the same even when hundreds of dihybrids were crossed.

This occurred because each of the parent plants only gave their offspring one allele and because yellow and round were dominant traits and masked the green and/or wrinkled traits in certain individual plants. In genetics, alleles are represented by letters, and dominant and recessive alleles for the same gene are represented by the same capital and lowercase letters. The diagram below depicts Mendel’s dihybrid cross. In this diagram, R represents the allele for round, while r represents the allele for wrinkled. Y represents the allele for yellow, while y represents the allele for green. The alleles that an organism has are collectively known as its genotype, while the organism’s actual physical appearance is its phenotype.

Mendel’s dihybrid cross

Mendel’s experiment showed that the alleles for round or wrinkled peas were inherited separately from the alleles for yellow or green peas since the plants were not just round and yellow or green and wrinkled.

Mendel’s Other Laws

Law of Segregation

The Law of Segregation is also known as Mendel’s First Law. It states that when a gamete (sperm or egg) is made, the allele pairs separate. Sperm and eggs are haploid, meaning that they only have one of each chromosome. When they unite, the fertilized egg and subsequent organism that develops is diploid and has two sets of chromosomes.

Of course, when Mendel developed the Law of Segregation, he did not know about DNA, genes, or alleles since they had not been discovered yet. He developed this law through his experiments with pea plants, through which he could see that some offspring inherited a certain form of a trait while others inherited a different form. For example, a pea plant bred from a plant that has only alleles for yellow color (YY) and a plant that has only alleles for green color (yy) will be yellow, but some of its offspring could be green since it has the alleles Yy, both yellow and green. Its offspring can only be green if it is bred with another plant that has at least one y allele.

Law of Dominance

The Law of Dominance is Mendel’s Third Law. It states that dominant alleles always mask recessive alleles. (There are exceptions, such as incomplete dominance and co-dominance, but the law is referring to simple Mendelian inheritance, such as the results of the pea plant experiments.) We can see the Law of Dominance in action when looking at Mendel’s experiments. Plants that were true-breeding yellow and true-breeding green, when crossed, produced only yellow offspring because yellow was dominant. In the next generation, when the offspring were crossed with each other, some plants were green even though both parents produced only yellow peas. We know now that these green plants inherited one y allele from each parent, while plants that inherited two Y alleles or one Y and one y were yellow because the yellow allele is dominant.

Related Biology Terms

  • Gene – A region of DNA; genes are the basic unit of heredity.
  • Allele – A form of a gene.
  • Dihybrid cross – A cross between individuals that have two different traits, such as crossing pea plants that produce round, yellow peas with pea plants that produce wrinkled, green peas.
  • Diploid – Having two sets of chromosomes, one inherited from the mother and one inherited from the father.

Quiz

1. Which is NOT one of Mendel’s Laws?
A. Law of Dominance
B. Law of Genetics
C. Law of Independent Assortment
D. Law of Segregation

Answer to Question #1

2. A pea plant with the genotype YYRR is crossed with a pea plant with the genotype yyrr. What will the offspring’s genotype be?
A. YYrr
B. yyRR
C. yyrr
D. YyRr

Answer to Question #2

3. What will be the phenotype of the offspring of the YYRR and yyrr plants from question #2?
A. Round, green peas
B. Round, yellow peas
C. Wrinkled, green peas
D. Wrinkled, yellow peas

Answer to Question #3
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