Inheritance of One Gene
Inheritance of One Gene
Mendel's Hybridization Experiments:
- Mendel conducted experiments on garden peas, crossing tall and dwarf plants to study the inheritance of one gene.
- He observed that in the first hybrid generation (F1), all offspring were tall, resembling one parent, with no dwarfs.
- Mendel noted that F1 progeny always resembled one parent, and the trait of the other parent was not visible.
- When self-pollinating the tall F1 plants, he found that in the second hybrid generation (F2), some offspring were dwarf, in a 3:1 ratio (3 tall: 1 dwarf).
- Mendel proposed the existence of "factors" (now known as genes) that are stably passed from parents to offspring through gametes.
- Genes are the units of inheritance and contain information for expressing specific traits.
- Pairs of contrasting traits are called alleles; for example, T and t for tallness and dwarfness.
- Homozygous plants have identical alleles (TT or tt), while heterozygous plants have different alleles (Tt).
- Mendel observed that one allele often dominated the other. The dominant allele's trait was expressed in the phenotype.
- The dominant allele is denoted with a capital letter, while the recessive allele uses a lowercase letter.
- For example, T (tall) is dominant over t (dwarf).
- Reginald C. Punnett developed the Punnett Square, a graphical representation to calculate the probability of offspring genotypes in genetic crosses.
- It shows how alleles from parents combine to form genotypes in offspring.
- Mendel's cross between TT and tt is called a monohybrid cross, involving a single trait (height).
- During meiosis, alleles segregate, with each gamete receiving one allele randomly.
- Phenotype refers to the outward appearance or trait expressed in an organism.
- Genotype represents the genetic makeup of an organism concerning its alleles.
- To determine the genotype of a tall plant (e.g., TT or Tt), Mendel performed a test cross by crossing it with a dwarf plant (tt).
- The resulting offspring reveal the genotype of the tall plant.
Mendel's experiments laid the foundation for understanding inheritance patterns and introduced the concept of genes, alleles, dominance, and the segregation of traits.
- Characters (traits) are controlled by discrete units known as factors or genes.
- These factors occur in pairs, one from each parent.
- In a dissimilar pair of factors, one member of the pair dominates (is dominant) over the other, which is called recessive.
- The dominant allele determines the phenotype in heterozygous individuals (e.g., TT or Tt), while the recessive allele's trait is not visibly expressed.
- Law of Segregation:
- Alleles (different forms of a gene) do not blend together but are recovered intact in the F2 generation, even if one allele is not observed in the F1 stage.
- During gamete formation, alleles of a gene segregate from each other so that a gamete receives only one of the two alleles.
- Homozygous parents produce gametes that are all identical, while heterozygous parents produce two kinds of gametes, each with one allele.
- Incomplete Dominance:
- Mendel's laws of dominance and segregation explain the expression of only one parental trait in F1 and the 3:1 ratio observed in F2.
- However, in some cases, the F1 phenotype is intermediate between the two parents, as seen in incomplete dominance.
- An example is the pink color of flowers in a cross between red (RR) and white (rr) flowered plants.
- Dominance is related to the function of alleles. Genes contain information for expressing traits.
- In a diploid organism, two alleles of a gene may not always be identical.
- Dominance depends on whether the alleles produce equivalent or modified gene products.
- When one allele produces a non-functional product or no product at all, the phenotype depends on the functioning allele, which is dominant.
- Co-dominance occurs when both alleles in the heterozygous condition express their traits equally, and neither dominates the other.
- A classic example is the ABO blood group system in humans, where alleles IA and IB produce distinct sugars on red blood cells, while allele i does not produce any sugar.
- ABO blood group demonstrates multiple alleles with three different variants ( IA, IB, and i) governing the same trait.
- In individuals, only two alleles are present, but population studies reveal multiple alleles.
- Sometimes, a single gene product can have multiple effects on different traits.
- An example is starch synthesis in pea seeds controlled by the B and b alleles, affecting both seed shape and starch grain size.
Incomplete dominance, co-dominance, and multiple alleles illustrate the complexity and diversity of genetic inheritance.