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The law of segregation

Learn about the law of segregation, the terms genotype, phenotype, alleles, heterozygous, and homozygous, and how to solve 2 x 2 Punnett squares.

Key points:

  • Gregor Mendel studied inheritance of traits in pea plants. He proposed a model where pairs of "heritable elements," or genes, specified traits.
  • Genes come in different versions, or alleles. A dominant allele hides a recessive allele and determines the organism's appearance.
  • When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.
  • A Punnett square can be used to predict genotypes (allele combinations) and phenotypes (observable traits) of offspring from genetic crosses.
  • A test cross can be used to determine whether an organism with a dominant phenotype is homozygous or heterozygous.


Today, we know that many of people's characteristics, from hair color to height to risk of diabetes, are influenced by genes. We also know that genes are the way parents pass characteristics on to their children (including things like dimples, or—in the case of me and my father—a terrible singing voice). In the last hundred years, we've come to understand that genes are actually pieces of DNA that are found on chromosomes and specify proteins.
But did we always know those things? Not by a long shot! About 150 years ago, a monk named Gregor Mendel published a paper that first proposed the existence of genes and presented a model for how they were inherited. Mendel's work was the first step on a long road, involving many hard-working scientists, that's led to our present understanding of genes and what they do.
In this article, we’ll trace the experiments and reasoning that led Mendel to formulate his model for the inheritance of single genes.

Mendel's model: It started with a 3:1 ratio

Mendel studied the genetics of pea plants, and he traced the inheritance of a variety of characteristics, including flower color, flower position, seed color, and seed shape. To do so, he started by crossing pure-breeding parent plants with different forms of a characteristic, such as violet and white flowers. Pure-breeding just means that the plant will always make more offspring like itself, when self-fertilized over many generations.
What results did Mendel find in his crosses for flower color? In the parental, or P generation, Mendel crossed a pure-breeding violet-flowered plant to a pure-breeding white-flowered plant. When he gathered and planted the seeds produced in this cross, Mendel found that 100 percent of the plants in the next generation, or F1 generation, had violet flowers.
Conventional wisdom at that time would have predicted that the hybrid flowers should be pale violet—that is, that the parents' traits should blend in the offspring. Instead, Mendel’s results showed that the white flower trait had completely disappeared. He called the trait that was visible in the F1 generation (violet flowers) the dominant trait, and the trait that was hidden or lost (white flowers) the recessive trait.
Image credit: "Mendel's experiments: Figure 2," by Robert Bear et al., OpenStax, CC BY 4.0
Importantly, Mendel did not stop his experimentation there. Instead, he let the F1 plants self-fertilize. Among their offspring, called the F2 generation, he found that 705 plants had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1.
This 3:1 ratio was no fluke. For the other six characteristics that Mendel examined, both the F1 and F2 generations behaved in the same way they did for flower color. One of the two traits would disappear completely from the F1 generation, only to reappear in the F2 generation in a ratio of roughly 3:1
Image credit: "Mendel's experiments: Figure 3," by Robert Bear et al., OpenStax, CC BY 4.0
As it turned out, the 3:1 ratio was a crucial clue that let Mendel crack the puzzle of inheritance. Let's take a closer look at what Mendel figured out.

Mendel's model of inheritance

Based on his results (including that magic 3:1 ratio), Mendel came up with a model for the inheritance of individual characteristics, such as flower color.
In Mendel's model, parents pass along “heritable factors," which we now call genes, that determine the traits of the offspring. Each individual has two copies of a given gene, such as the gene for seed color (Y gene) shown below. If these copies represent different versions, or alleles, of the gene, one allele—the dominant one—may hide the other allele—the recessive one. For seed color, the dominant yellow allele Y hides the recessive green allele y.
Image modified from "Laws of inheritance: Figure 1," by Robert Bear et al., OpenStax, CC BY 4.0
The set of alleles carried by an organism is known as its genotype. Genotype determines phenotype, an organism's observable features. When an organism has two copies of the same allele (say, YY or yy), it is said to be homozygous for that gene. If, instead, it has two different copies (like Yy), we can say it is heterozygous. Phenotype can also be affected by the environment in many real-life cases, though this did not have an impact on Mendel's work.

Mendel's model: The law of segregation

So far, so good. But this model alone doesn't explain why Mendel saw the exact patterns of inheritance he did. In particular, it doesn't account for the 3:1 ratio. For that, we need Mendel's law of segregation.
According to the law of segregation, only one of the two gene copies present in an organism is distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is random. When an egg and a sperm join in fertilization, they form a new organism, whose genotype consists of the alleles contained in the gametes. The diagram below illustrates this idea:
Image modified from"Laws of inheritance: Figure 5," by Robert Bear et al., OpenStax, CC BY 4.0
The four-squared box shown for the F2 generation is known as a Punnett square. To prepare a Punnett square, all possible gametes made by the parents are written along the top (for the father) and side (for the mother) of a grid. Here, since it is self-fertilization, the same plant is both mother and father.
The combinations of egg and sperm are then made in the boxes in the table, representing fertilization to make new individuals. Because each square represents an equally likely event, we can determine genotype and phenotype ratios by counting the squares.

The test cross

Mendel also came up with a way to figure out whether an organism with a dominant phenotype (such as a yellow-seeded pea plant) was a heterozygote (Yy) or a homozygote (YY). This technique is called a test cross and is still used by plant and animal breeders today.
In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive (e.g., green-seeded):
Image credit: "Laws of inheritance: Figure 4," by Robert Bear et al., OpenStax, CC BY 4.0
If the organism with the dominant phenotype is homozygous, then all of the F1 offspring will get a dominant allele from that parent, be heterozygous, and show the dominant phenotype. If the organism with the dominant phenotype organism is instead a heterozygote, the F1 offspring will be half heterozygotes (dominant phenotype) and half recessive homozygotes (recessive phenotype).
The fact that we get a 1:1 ratio in this second case is another confirmation of Mendel’s law of segregation.

Is that Mendel's complete model of inheritance?

Not quite! We've seen all of Mendel's model for the inheritance of single genes. However, Mendel's complete model also addressed whether genes for different characteristics (such as flower color and seed shape) influence each other's inheritance. You can learn more about Mendel's model for the inheritance of multiple genes in the law of independent assortment article.
One thing I find pretty amazing is that Mendel was able to figure out his entire model of inheritance simply from his observations of pea plants. This wasn't because he was some kind of crazy super genius, but rather, because he was very careful, persistent, and curious, and also because he thought about his results mathematically (for instance, the 3:1 ratio). These are some of the qualities of a great scientist—ones that anyone, anywhere, can develop!

Check your understanding

  1. Imagine that you are a rabbit breeder with two purebred rabbits, a male with black fur and a female with tan fur. When you cross your rabbits, all of the F1 kits (baby rabbits) have tan fur.
    Which trait is dominant, and which is recessive?
    Choose 1 answer:
  2. Which statement best describes the relationship between genotype and phenotype?
    Choose 1 answer:

Want to join the conversation?

  • mr pants teal style avatar for user Abigail smith
    Who came up with the punnet squares
    (19 votes)
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  • blobby green style avatar for user ngwe forkum
    what is epistasis
    (13 votes)
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    • blobby green style avatar for user emir.emirus
      It is when one gene affects the expression of another gene. For example, mice have a color gene and can have an allele for black (B) fur color and an allele for brown (b) fur color (black being dominant), BUT they also have a gene that determines pigmentation; one allele C produces pigment (fur color shows) and the other allele c does not (fur color is white/mouse is albino).
      So, the possible genotypes would be:
      - CCBB, CCBb, CcBB, CcBb (phenotype: black)
      - CCbb, Ccbb (penotype: brown)
      - ccBB, ccBb, ccbb (phenotype: white, pigment is not produced and therefore fur color cannot be expressed)
      I hope this example clarifies things a bit! (:
      (47 votes)
  • blobby green style avatar for user rishik bagaria
    explain why is it possible for browned eye parents to have a blue eyed child ?which law does it indicate??
    (13 votes)
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    • orange juice squid orange style avatar for user briancsherman
      This demonstrates recession and dominance. Brown eyes are dominant; blue eyes are recessive. The phenotype is what the appearance is - mother and father have brown eyes. The genotype, is what the genes they have code for - in order for mother and father to have a blue eyed child, they must have a genotype that includes both brown and blue. If father and mother each give their individual blue eye gene to their child, the child will have two blue eye genes and no brown eye genes, so eyes will be blue.
      (37 votes)
  • mr pants teal style avatar for user Tikel Taylor
    Why does both Geno and phenotype influence from the environment??Are they talking about the environment that the Gene's are placed or just mean the environment in general? Question No.2
    (10 votes)
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  • blobby green style avatar for user ulhaaskashyap
    what are homologous genes
    (3 votes)
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    • blobby green style avatar for user tracy14
      i don't think the other posted answer is right. The question was "what are homoLOgous genes", but the answer seemed more lined up for "homoZYgous" genes. I am still learning this stuff, but my understanding is
      - Homologous means genes controlling the same inherited character - may have different versions of same gene. For example - flower colour, may be purple, or white, but still homologous because it's flower colour.
      - Homozygous means the genes carry two identical alleles, PP or pp.
      - Heterozygous means the genes carry two different alleles, Pp.
      (25 votes)
  • blobby green style avatar for user Julia Kim
    How did Mendel derive his law of segregation from this monohybrid experiment? (It is not clear to me in the article).
    (8 votes)
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  • blobby green style avatar for user aalnahas
    Doesn't the crossover between the homologous chromosomes mix up the alleles?
    (7 votes)
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  • leaf orange style avatar for user Oyuna Nataa
    if your parents are one brown and blue eyed and the child is brown eyed. in the future can the child's child be able to have blue eyed if he/she marries brown eyed person?
    (5 votes)
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  • stelly blue style avatar for user Morgan
    So would my grandparents be the F2 generation of my great great grandparents?
    (7 votes)
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  • hopper jumping style avatar for user Yuvraj M
    when it says self fertilization, I think I understand the concept of how one of a plant's sperm fertilizes the same plant's seed/ but I also know that usually in human life when like 500 years ago, the kings used to marry his sister to keep the royalty in the same family and when they would have kids, those kids would have serious problems like internal bleeding and many disabilities. so my question is, do the pea plants that come out of self-fertilization have any sort of problem or are they perfectly healthy?
    (6 votes)
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