12.12: Law of Segregation

When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F₁ generation, and could reappear in the next generation (F₂). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F₁ generation. When he crossed the F₁ plants, he found that 75% of the offspring in the F₂ generation had the dominant phenotype, while 25% had the recessive phenotype.

Mendel’s model to explain this result had four parts. First, alternative versions of genes, called alleles, account for differences in traits. Second, an organism inherits two copies of each gene, one from each parent. Third, the presence of a dominant allele masks the recessive allele. Fourth, the two alleles for a trait are separated during gamete formation. This last part of the model is called the Law of Segregation. If a parent has two different alleles, or is heterozygous, these alleles will be equally and randomly separated during gamete formation. Scientists now understand that the separation of chromosomes during meiosis accounts for the segregation of parental alleles.

Let us think about the cross between a purple flowering plant (genotype PP) and a white flowering plant (pp). The resulting F₁ generation has purple flowers (Pp). This phenotype is accounted for by part (3) of Mendel’s model, as the dominant purple allele masks the recessive white allele. However, when the F₁ plants are crossed (Pp x Pp), the offspring can be either purple or white with a ratio of 3 purple to 1 white. The corresponding genotypic ratio is 1 PP : 2 Pp : 1 pp. This result is supported by Mendel’s Law of Segregation, as each plant received one allele from each parent.

When Gregor Mendel performed monohybrid crosses of pea plants differing by single traits, such as pod color, he only saw one parental phenotype disappear in the first filial, or F-one, generation. And then, reappear in the second filial, or F-two generation, meaning that one phenotype was dominant over the other. The offspring with the dominant phenotype, comprised 75% of the F-two generation, while those with the recessive phenotype comprised 25%.

From his experiments, Mendel concluded that there must be a pair of particles, now known to be genes, that are responsible for transmitting each trait from one generation to the next. Mendel summarized one of the key concepts underlying heredity in his Law of Segregation, which states that every diploid organism carries two copies of each gene, called alleles, that are segregated into the gametes at random during meiosis. Thus, when organisms reproduce, all offspring receive one allele from each parent.

Because segregation is random, heterozygous parents, those bearing two different alleles, are equally likely to pass on a dominant or recessive allele, and the 3:1 phenotypic ratio observed in a cross between two heterozygotes, illustrates the Law of Segregation. 25% of the offspring will have a homozygous dominant genotype and display the dominant phenotype, 50% will be heterozygous and display the dominant phenotype, and 25% will be homozygous-recessive and display the recessive phenotype.

Law of Segregation - Mendel’s principle stating that the two alleles for a trait separate during gamete formation. Allele - An alternative version of a gene that accounts for variations in inherited traits. Dominant - A type of allele that masks the effect of a recessive allele when both are present in an organism (heterozygous). Recessive - An allele whose effect is hidden when paired with a dominant allele; only expressed when both alleles are recessive. Genotypic Ratio - The proportion of different genetic combinations (genotypes) that appear in offspring

Define the Law of Segregation – Explain how alleles separate during gamete formation. (e.g., pea plants) Differentiate between dominant and recessive alleles – Understand how traits are expressed in offspring. (e.g., flower color) Explore Mendel’s monohybrid crosses – Discover how a 3:1 phenotypic ratio supports segregation. (e.g., Pp × Pp) Explain how meiosis enables segregation – Describe how homologous chromosomes separate. (e.g., Anaphase I) Apply the Law of Segregation to inheritance – Predict genotypes and phenotypes in genetic crosses. (e.g., Punnett square)

Questions that this Law of Segregation video will help you answer: What is the Law of Segregation? How do alleles separate during gamete formation? Why do traits reappear in the F₂ generation?

Students – Understand how the Law of Segregation explains allele inheritance and trait expression across generations. Educators – Use Mendel’s principles to build lessons on genetic inheritance, dominance, and monohybrid crosses. Researchers – Revisit foundational genetic laws to interpret allele behavior in experimental breeding and meiosis studies. Science Enthusiasts – Explore how simple pea plant experiments revealed universal rules of heredity.