12.5
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…
Gregor Mendel performed monohybrid crosses between pea plants that differed in a single trait, such as pod color. He saw one parental phenotype disappear in the first filial, or F1, generation.
The F1 pea plants contain both male and female reproductive structures, allowing them to self-pollinate. When these plants were self-crossed, the missing parental phenotype, which was not expressed in the F1 generation, reappeared in the F2 generation.
In the F2 generation, offspring with the dominant phenotype made up about 75% of the population, while offspring with the recessive phenotype made up about 25%.
From these experiments, Mendel concluded that traits pass from one generation to the next through pairs of heritable factors, now known as genes.
Mendel proposed the Law of Segregation. This law states that every diploid organism carries two variants of each gene, called alleles. These alleles separate randomly into gametes during meiosis. So, when organisms reproduce, each offspring receives one allele from each parent.
Because allele segregation is random, heterozygous parents carrying two different alleles, like the plants of the F1 generation, are equally likely to pass on either the dominant allele or the recessive allele. This explains the three-to-one phenotypic ratio seen in a monohybrid cross between two heterozygotes.
Based on this ratio, 25% of the offspring are expected to have a homozygous dominant genotype and show the dominant phenotype. Another 50% are expected to be heterozygous and show the dominant phenotype. The remaining 25% are expected to have a homozygous recessive genotype and show the recessive phenotype.
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Q1: What is the Law of Segregation and why is it important in genetics?
The Law of Segregation states that every diploid organism carries two copies of each gene, called alleles, which are separated into gametes randomly during meiosis. This principle explains how traits pass from parents to offspring and why offspring receive one allele from each parent. It accounts for the predictable inheritance patterns Mendel observed in his pea plant experiments.
Q2: Why do heterozygous parents produce a 3:1 phenotypic ratio in offspring?
When two heterozygous parents (each carrying one dominant and one recessive allele) reproduce, their gametes randomly combine. This random segregation produces offspring with a 1:2:1 genotypic ratio, resulting in 75% displaying the dominant phenotype and 25% displaying the recessive phenotype. The dominant allele masks the recessive allele in heterozygous offspring, creating the characteristic 3:1 ratio.
Q3: How did Mendel's monohybrid crosses demonstrate the Law of Segregation?
Mendel crossed pea plants differing by single traits and observed that one parental phenotype disappeared in the F1 generation, then reappeared in the F2 generation. The F2 generation showed a 3:1 phenotypic ratio: 75% dominant and 25% recessive. This pattern revealed that traits are controlled by segregating alleles that separate during gamete formation and recombine randomly in offspring.
Q4: What is the relationship between alleles and the segregation of chromosomes during meiosis?
Alleles segregate during meiosis because they are located on homologous chromosomes that separate into different gametes. Scientists now understand that the separation of chromosomes during meiosis accounts for the segregation of parental alleles. This chromosomal basis explains how the Law of Segregation operates at the molecular level.
Q5: How do dominant and recessive alleles interact to produce phenotypes?
The presence of a dominant allele masks the recessive allele, determining the organism's phenotype. A homozygous dominant individual (two dominant alleles) and a heterozygous individual (one dominant, one recessive allele) both display the dominant phenotype. Only homozygous recessive individuals (two recessive alleles) display the recessive phenotype.
Q6: What are the four parts of Mendel's model for explaining inheritance patterns?
Mendel's model consists of four parts: first, alternative gene versions called alleles account for trait differences; second, organisms inherit two gene copies, one from each parent; third, dominant alleles mask recessive alleles; and fourth, the two alleles separate during gamete formation. This last principle is the Law of Segregation, which explains how traits segregate across generations.
Q7: What genotypic and phenotypic ratios result from crossing two heterozygous organisms?
Crossing two heterozygotes (Pp × Pp) produces a 1:2:1 genotypic ratio: 25% homozygous dominant (PP), 50% heterozygous (Pp), and 25% homozygous recessive (pp). The phenotypic ratio is 3:1, with 75% displaying the dominant phenotype and 25% displaying the recessive phenotype, demonstrating the Law of Segregation in action.