Mendel’s Law of Segregation is one of the fundamental principles of genetics, proposed by Gregor Mendel in the mid-1800s. It describes how alleles for a given trait are inherited from one generation to the next. Here’s a clear explanation of the law:
Mendel’s Law of Segregation
State:
The Law of Segregation states that each individual has two alleles for each gene, one inherited from each parent. These two alleles separate (or segregate) during the formation of gametes (sperm and eggs), so that each gamete carries only one allele for each gene. When two gametes fuse during fertilization, the resulting offspring will inherit one allele from each parent.
Explanation
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Alleles and Genes:
- An organism inherits two copies (alleles) for each gene—one from the mother and one from the father. These alleles can be the same (homozygous) or different (heterozygous).
- For example, a gene for flower color might have two alleles: one for red flowers (R) and one for white flowers (r). In this case, the organism could inherit RR (homozygous dominant), rr (homozygous recessive), or Rr (heterozygous).
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Separation of Alleles:
- During gamete formation (through a process called meiosis), the two alleles for a given gene segregate, so that each gamete only carries one allele.
- This means that, if an organism is heterozygous (e.g., Rr), the two alleles will separate during gamete formation. One gamete will get the R allele, and the other will get the r allele. These alleles are randomly distributed to the offspring during fertilization.
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Fertilization and Recombination:
- When two gametes (one from each parent) fuse during fertilization, the resulting offspring will receive one allele from each parent.
- This process restores the diploid number of alleles (two alleles for each gene) in the zygote, and the next generation can inherit a combination of alleles from both parents.
Mendel’s Experiments and Evidence:
Mendel discovered the Law of Segregation through his experiments with pea plants. He worked with traits that showed clear dominant and recessive inheritance patterns (such as flower color, seed shape, and seed color). For instance:
- He crossed pea plants that were homozygous dominant (RR) for red flowers with plants that were homozygous recessive (rr) for white flowers.
- In the first generation (F1), all the plants had red flowers because the red allele (R) is dominant.
- However, when the F1 plants were crossed with each other, the second generation (F2) showed a 3:1 ratio—three red-flowered plants to one white-flowered plant. This result indicated that the F1 plants must have carried one R allele and one r allele (heterozygous, Rr), and that during gamete formation, these alleles segregated and recombined randomly.
Key Points of Mendel’s Law of Segregation:
- Each organism has two alleles for each gene (one from each parent).
- The alleles segregate during gamete formation, so each gamete receives only one allele.
- Fertilization restores the two alleles in the offspring, one from each parent.
- This law applies to all sexually reproducing organisms.
Example:
Let’s take a simple example of flower color in pea plants:
- The gene for flower color has two alleles: R (dominant for red) and r (recessive for white).
- If a plant is heterozygous (Rr), it will produce two types of gametes: one with the R allele and one with the r allele.
- When two heterozygous plants (Rr x Rr) are crossed, the offspring’s genotypes will be:
- RR (homozygous dominant): Red flowers.
- Rr (heterozygous): Red flowers.
- rr (homozygous recessive): White flowers.
In this cross, the expected ratio of red to white flowers in the offspring would be 3:1, showing how the alleles segregate and recombine to produce different genotypes.