Mendelian Genetics Laws to Know for AP Biology

Mendelian genetics explains how traits are inherited through laws governing alleles and their interactions. Understanding these principles, like segregation and independent assortment, is key to grasping genetic variation in organisms, a fundamental concept in biology courses.

1. Law of Segregation

   • Each individual has two alleles for each gene, one inherited from each parent.
   • During gamete formation, the alleles segregate so that each gamete carries only one allele for each gene.
   • This law explains the 3:1 phenotypic ratio observed in the offspring of a monohybrid cross.

2. Law of Independent Assortment

   • Genes for different traits assort independently of one another during gamete formation.
   • This law applies only to genes located on different chromosomes or those far apart on the same chromosome.
   • It leads to the formation of new combinations of traits in the offspring, exemplified in a dihybrid cross.

3. Law of Dominance

   • In a heterozygous individual, one allele may mask the expression of another allele.
   • The dominant allele is expressed in the phenotype, while the recessive allele is not.
   • This principle helps explain why certain traits appear more frequently in offspring.

4. Law of Uniformity

   • When two homozygous parents (one dominant and one recessive) are crossed, all offspring in the first generation (F1) will display the dominant trait.
   • This law illustrates the predictability of genetic outcomes in monohybrid crosses.
   • It emphasizes the concept of uniformity in the F1 generation.

5. Law of Reciprocal Crosses

   • The results of a genetic cross remain the same regardless of which parent contributes the dominant or recessive allele.
   • This law reinforces the idea that the inheritance of traits is not influenced by the sex of the parent.
   • It is useful for confirming the consistency of genetic ratios.

6. Law of Unit Characters

   • Traits are determined by discrete units (genes) that exist in pairs.
   • Each gene can have different forms (alleles) that contribute to the organism's phenotype.
   • This law lays the foundation for understanding genetic inheritance and variation.

7. Principle of Probability

   • The likelihood of inheriting a particular trait can be calculated using probability.
   • The outcomes of genetic crosses can be predicted using Punnett squares.
   • This principle helps in understanding the ratios of phenotypes and genotypes in offspring.

8. Concept of Alleles

   • Alleles are different versions of a gene that can produce variations in a trait.
   • An individual inherits one allele from each parent, resulting in homozygous or heterozygous genotypes.
   • The interaction between alleles determines the phenotype expressed.

9. Monohybrid Cross

   • A genetic cross that examines the inheritance of a single trait.
   • Typically involves one trait with two alleles (dominant and recessive).
   • The expected phenotypic ratio in the F2 generation is 3:1.

10. Dihybrid Cross

    • A genetic cross that examines the inheritance of two traits simultaneously.
    • Involves two traits, each with two alleles, leading to a 9:3:3:1 phenotypic ratio in the F2 generation.
    • Demonstrates the Law of Independent Assortment through the combination of traits.