Mendel's Law
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To determine the inheritance pattern of flower color in a species of plants without knowing the underlying genotypes, let's explore how Mendel performed a monohybrid cross and deduced his laws from this experiment.
Let’s assume a scenario where you are with Mendel, and he want to understand how flower color is determined and inherited, or how a single trait is determined and inherited.
Let’s assume a scenario where you are with Mendel, and he want to understand how flower color is determined and inherited, or how a single trait is determined and inherited.
In order to form a hybrid of a single trait, Mendel will use a monohybrid cross. We all know what a Monohybrid Cross is.
In order to form a hybrid of a single trait, Mendel will use a monohybrid cross. We all know what a Monohybrid Cross is.
Materials
A species of flowering plant with noticeable variation in flower color.
Gardening tools for planting and cross-pollination.
A controlled environment for growing plants.
A notebook for recording observations and results.
Procedure
Select True-Breeding Plants: (Now it might surprise you what True Breeding Plant is: It is defined as the plants that, when self-fertilized, consistently produce offspring with the same traits as the parent plant. This means that the plants are homozygous for the trait in question, carrying two identical alleles.
Identify two distinct flower colors in the plant species, such as red and white.
Ensure that each plant consistently produces the same flower color over multiple generations, suggesting they are true-breeding. Let’s label these plants as:
Red-flowered plant (True-breeding)
White-flowered plant (True-breeding)
Cross True-Breeding Plants:
Cross-pollinate the red-flowered plant with the white-flowered plant.
Collect seeds from this cross and plant them to grow the first generation (F1).
Observation: F1 Generation:
After crossing true-breeding plants with red and white flowers, we have observed that in the F1 generation, all plants display one flower color (e.g., red). You might be surprised and wonder where the white flower color has gone (see in above figure-stage 1).
Mendel wanted to understand why all plants in the F1 generation had red flowers instead of white. To investigate, he allowed these plants to Self Pollinate. This self-pollination was crucial because it helped confirm whether there is reappearance of white color or not.
Self-Pollinate F1 Plants:
Allow the F1 plants to self-pollinate and produce seeds.
Collect these seeds and plant them to grow the second generation (F2) (see in above figure-stage 2).
Observation: F2 Generation:
In the second generation (F2 generation), Mendel observed that the white flower color reappears, in a ratio of 3 (red) to 1 (white) i.e., Phenotypic Ratio.
Mendel's Assumption
After observing the results from two generations, Mendel was surprised by the unexpected outcomes. Therefore, he made several assumptions to explain his observations. He hypothesized that flower color is determined by a gene, which he referred to as the "Color gene," represented by the letter R.
According to Mendel:
True-breeding red plants have a genotype RR, which expresses the dominant trait (red).
True-breeding white plants have a genotype rr, expressing the recessive trait (white).
In the F1 generation, plants inherit one dominant (R) and one recessive (r) allele, resulting in the heterozygous genotype Rr.
Mendel used a test cross to confirm his assumptions about how traits are inherited:
He hypothesized two possible outcomes for the test cross:
Assumption 1st: If the F1 plants were true-breeding (homozygous dominant), then all offspring would display red flowers.
Assumption 2nd: If the F1 plants were hybrids (heterozygous), then the recessive trait (white color) would reappear in some offspring.
This test cross allowed Mendel to distinguish between true-breeding and hybrid plants based on the phenotypic ratios observed in the offspring.
When Mendel crossed hybrid red plants (Rr) from the F1 generation with homozygous recessive white plants (rr), the offspring (F2 generation) displayed a phenotypic ratio of approximately 3 red : 1 white. This confirmed Mendel's second assumption (see above in Mendel's Assumption):
The trait observed in the F1 generation (red color) was due to the dominance of the allele (R) over the recessive allele (r).
The F1 hybrids inherited one dominant allele (R) and one recessive allele (r), demonstrating the segregation of alleles during gamete formation.
These findings led to the formulation of two fundamental laws: the Law of Dominance and the Law of Segregation. These laws explain that traits are determined by dominant and recessive alleles, and they describe how these alleles segregate and recombine during gamete formation.
What laws were formulated from the monohybrid cross (single trait cross)?
Law of Segregation
The law of segregation states that during the formation of gametes i.e., sperm or egg cells, the two alleles for a single trait separate (or segregate) from each other so that each gamete carries only one allele for each trait.
This segregation ensures that offspring acquire one allele for each trait from each parent.
Example: Consider a plant with a genotype Rr, where R is the dominant allele for Red color and "r" is the recessive allele for white color.
According to the law of segregation, during gamete formation, the alleles R and r will segregate such that each gamete will receive either R or r, but not both. This means that the gametes will have an equal probability of carrying either the R or the r allele.
Law of Dominance
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The law of dominance states that "in a heterozygous organism, where two different alleles for a particular trait are present, one allele will mask the expression of the other". The allele that masks the other is termed as dominant, while the allele whose expression is masked is called recessive.
Example: In Mendel's experiments with pea plants, the allele for Red (R) is dominant over the allele for white (r). Thus, a plant with the genotype Rr (heterozygous) will exhibit the Red phenotype, as the dominant R allele masks the expression of the recessive r allele.
Key Terms:
Key Terms:
From the above experiment, we have found many new terms:
From the above experiment, we have found many new terms:
Genotype: The genotype refers to the genetic makeup of an organism, specifically the combination of alleles inherited from its parents. It determines the hereditary traits the organism can pass on to its offspring (e.g., Rr, RR, rr).
Allele: An allele is one of two or more versions of a gene that are found at the same place (locus) on a chromosome. Different alleles can result in traits like flower color or seed shape. Example: In pea plants, the gene for flower color may have two alleles: RR for red flowers and rr for white flowers
Dominant: A dominant allele is an allele that expresses its phenotype even when only one copy is present in the genotype. It masks the expression of a recessive allele when paired together. Example: If R (red) is dominant over r (white), a plant with genotype Rr will have red flowers because the dominant R allele masks the recessive r allele.
Recessive: A recessive allele is an allele that expresses its phenotype only when two copies are present in the genotype. Its effect is masked by a dominant allele in a heterozygous condition. Example: If r (white) is recessive, a plant must have the genotype rr to have white flowers, because the presence of the dominant R allele would result in red flowers.
Homozygote: A homozygote is an organism that has two identical alleles for a specific trait. This can be either two dominant alleles (homozygous dominant) or two recessive alleles (homozygous recessive). Example: For the flower color trait in pea plants, RR is a homozygous dominant genotype, and rr is a homozygous recessive genotype.
Heterozygote: A heterozygote is an organism that has two different alleles for a specific trait. One allele is usually dominant, and the other is recessive. For the flower color trait in pea plants, Rr is a heterozygous genotype, where the presence of the dominant R allele results in red flowers, even though the recessive r allele is also present.
Genotypic Ratio: The genotypic ratio refers to the ratio of different genetic combinations or genotypes that occur in a population or offspring generation resulting from a genetic cross or mating.
Phenotypic Ratio: The phenotypic ratio refers to the ratio of different observable traits or phenotypes that appear in a population or offspring generation resulting from a genetic cross or mating.
Punnett Square: A diagram used to predict the genotype and phenotype ratios of offspring in a genetic cross.
Parental Generation (P): The initial generation in a genetic cross.
First Filial Generation (F1): The offspring of the parental generation.
Second Filial Generation (F2): The offspring resulting from a cross between individuals of the F1 generation.
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