Law of Segregation

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Law of Segregation Definition

Gregor Mendel’s law of segregation states that the two alleles for each trait segregate, or separate, during the formation of gametes, and that during the formation of new zygotes, the alleles will combine at random with other alleles. The law of segregation ensures that a parent, with two copies of each gene, can pass on either allele. Both alleles will have the same chance of ending up in a zygote.

In sexually reproducing organsisms, the genome is carried in two identical copies. A copy was inherited from each parent, in the form of a gamete. These organisms are known as diploid when they have both copies of the genome, and haploid when they are gametes and have only one copy. Though Gregor Mendel was not clear on exactly how the process took place, modern microscopes and molecular techniques have revealed that alleles are separated during the process of meiosis.

Meiosis occurs in specialized cells known as gametocytes, which form haploid cells from diploid cells. In order for the ploidy of the cell to be reduced, the chromosomes in the cell must be equally divided. To start the process, all of the DNA in a cell is duplicated. This creates two copies of each allele. In this cell, there are now 4 alleles for each gene, although 2 of them are simply identical copies of the original 2. As meiosis begins, the chromosomes condense and align with their homologous pairs. Homologous chromosomes are those which contain identical portions of DNA, originally inherited from different parents.

During prophase I of meiosis I, the homologous chromosomes bind together. Special sections of the DNA can overlap, causing breakages in the DNA. Due to the similarity of the DNA, the breaks simply exchange segments in a process called crossing-over. This crossing-over helps establish both the randomness of allele inheritance and also the separation of different genes. The separation of different genes during meiosis is known as the law of independent assortment. During metaphase I of meiosis I, these bonded homologous pairs are aligned in the middle of the cell and separated. In doing this, the different alleles for each gene are affectively separated. During meiosis II, the copies of the alleles will be separated into individual gametes. This insures that each allele makes it to a new gamete, giving it an essentially equal chance of finding a gamete to fuse with and create a new organism.

Due to the law of segregation each allele is its own entity and always has an equal chance of being passed on to the next generation. This means that regardless of whether the allele is dominant or recessive in its relationship with the other allele it will be passed on in the same way, with the same frequency. The law of independent assortment states that while genes may exist on the same chromosomes, they too are inherited independently of each other due to the mechanisms of meiosis.

  • Law of Independent Assortment – Genes for different traits are independently assorted into separate gametes.
  • Meiosis – Cell division that reduces the amount of genetic information to form gametes.
  • Mitosis – The process of cell division that simply replicates cells.
  • Gene – A segment of DNA that carries information for a specific protein, which may have many different versions, or alleles.


1. A pea plant has alleles for both Yellow peas (Y) and green peas(y). On the actual plant, all the peas are yellow because the yellow allele is dominant. This pea plant is crossed with another plant with the genome (Yy) as well. Some of the offspring have green peas. Which law is this an example of?
A. Law of Segregation
B. Law of Independent Assortment
C. Neither

Answer to Question #1
A is correct. This is a clear example of the law of segregation. The law of segregation separates the alleles of the parents, so they can be passed on equally in the following generation. If this were not true, alleles would remain with the pairs they were always with, and the peas would always be yellow, even though the plant carried a green allele. The law of segregation allows for many different combinations of alleles in a population, causing great amounts of variety. The law of independent assortment assures that traits of different genes are inherited separately.

2. Researchers are breeding mice to study two traits, fur color and fur length. The alleles for these traits are contained on separate genes, which exist on different chromosomes. A long-haired black mouse breeds with a short-haired white mouse. Can the mice produce a white offspring with long hair? Why or why not?
A. Yes, the law of segregation insures it.
B. Yes, the law of independent assortment insures it.
C. No, the offspring must resemble one of the parents.

Answer to Question #2
B is correct. The law of independent assortment states that the traits for various genes are inherited independently of one another. Therefore, the alleles for black or white hair are separated from the alleles for short or long hair. While these traits may be linked traits and exist on the same chromosome, they will still be inherited independently due to mechanisms such as crossing-over, which recombine parental genes, and the final separation of different alleles into independent gametes. The law of segregation simply states that the two alleles for each gene have an equal chance of being inherited.

3. An organisms has two copies of the same allele, one from each parent. Since the alleles are the same, can the law of segregation take place in this gene, for this organisms?
A. Yes, although the alleles will produce the same effect in the offspring.
B. No, without a difference in the alleles they aren’t really separated.
C. Yes, but it is different than in an organisms with different alleles.

Answer to Question #3
A is correct. Although the alleles have the same phenotypic effect, they came from different parental sources. The law of segregation still takes place when the alleles are separated during meiosis. Each allele, although it will do the same thing, now has an equal chance of being passed to the next generation. Remember that while one organism can only have two copies of an allele, there can be hundreds, or even thousands of different alleles in a population. By separating these similar alleles, they can be recombined with different alleles in the next generation.

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MLAAPAChicago Editors. "Law of Segregation." Biology Dictionary,, 27 Jan. 2017, Editors. (2017, January 27). Law of Segregation. Retrieved from Editors. "Law of Segregation." Biology Dictionary., January 27, 2017.

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