Dominant and recessive traits Knowledge-Based Study

Study notes

1. Inherited Characteristics and Genes

   In living organisms, physical characteristics such as plant height, seed shape, or flower color are known as traits. The instructions for developing these traits are carried on segments of DNA called genes.

   Organisms that reproduce sexually inherit two copies of each gene—one from each parent. Alternative versions of the same gene are called alleles. For example, a single gene controlling pea plant height may exist as an allele for tallness or an allele for shortness.

   Understanding Dominance

   When an organism carries two different alleles for a single gene, one allele often masks the presence of the other in determining the organism's observable appearance.

   • Dominant Allele: An allele that expresses its trait whenever at least one copy is present. In genetic notation, dominant alleles are represented by an uppercase letter (for example, T for tall).
   • Recessive Allele: An allele whose effect is masked in the presence of a dominant allele. It only expresses its trait when two copies are present. Recessive alleles are represented by the corresponding lowercase letter (for example, t for short).

   A common misconception is that dominant traits are 'stronger', more beneficial, or more common in a population. In genetics, dominance refers exclusively to the expression relationship between alleles in a heterozygote, not reproductive fitness or population frequency.

   Distinguishing Genetic Makeup from Physical Appearance

   To accurately describe how traits are inherited and displayed, geneticists distinguish between the genetic constitution and the observable features:

   • Genotype: The specific combination of alleles an organism carries for a given gene.
   • Phenotype: The physical, biochemical, or behavioral manifestation of that genotype.

   Genotypic Combinations

   Because an individual inherits two alleles per gene, three distinct genotypes are possible:

   1. Homozygous Dominant (TT): Carries two dominant alleles. The resulting phenotype is tall.
   2. Heterozygous (Tt): Carries one dominant and one recessive allele. The dominant allele masks the recessive allele, so the resulting phenotype is tall.
   3. Homozygous Recessive (tt): Carries two recessive alleles. The resulting phenotype is short.

   Using Punnett Squares to Predict Outcomes

   A Punnett square is a visual tool used to determine the probability of offspring inheriting particular genotypes and phenotypes from parental gametes.

   Consider a cross between two heterozygous (Tt) tall pea plants:

   • Each parent produces two types of gametes in equal proportions: T and t.
   • Combining these alleles in a 2x2 grid yields the following offspring genotypes:
     • 1 TT (Homozygous dominant)
     • 2 Tt (Heterozygous)
     • 1 tt (Homozygous recessive)

   This cross yields a classic genotypic ratio of 1:2:1 (TT : Tt : tt) and a phenotypic ratio of 3:1 (3 tall : 1 short), illustrating how recessive traits can reappear in subsequent generations even when masked in both parents.