Fundamental Principles of Genetics: Mendelian Inheritance, Blood Types, and Genetic Crosses

Mendelian Genetics

Monohybrid Crosses

Monohybrid crosses examine the inheritance of a single trait controlled by two alleles. This foundational concept in genetics was first described by Gregor Mendel.

• One trait with two alleles (e.g., T = tall, t = short).

• Cross: Tt × Tt

• Genotypes: TT, Tt, tt

• Phenotypic ratio: 3 tall : 1 short

Example: Crossing two heterozygous pea plants for height results in three tall plants for every one short plant.

Dihybrid Crosses

Dihybrid crosses involve two traits, each controlled by two alleles. These crosses help illustrate the principle of independent assortment.

• Two traits (e.g., seed color Y and shape S).

• Cross: YySs × YySs

• Punnett Square: Used to determine genetic combinations.

• Phenotypic ratio:

  • 9 yellow smooth

  • 3 yellow wrinkled

  • 3 white smooth

  • 1 white wrinkled

Example: Crossing two heterozygous plants for seed color and shape yields a 9:3:3:1 ratio in the offspring.

Patterns of Inheritance

Dominant vs Recessive

Alleles can be classified as dominant or recessive, determining how traits are expressed in offspring.

• Dominant: Expressed even if only one allele is present (TT or Tt).

• Recessive: Only expressed if both alleles are recessive (tt).

Example: Free earlobes (dominant) vs attached earlobes (recessive).

Codominance

In codominance, both alleles are fully expressed in the phenotype of the heterozygote.

• Both alleles are fully expressed.

• Example: Black chicken (CB) × White chicken (CW) → CBCW = black and white speckled offspring.

Incomplete Dominance

Incomplete dominance occurs when the heterozygote displays a phenotype intermediate between the two homozygotes.

• Alleles blend in heterozygous form.

• Example: Red (CR) × White (CW) → CRCW = pink flowers.

X-Linked Recessive Traits

X-linked traits are found on the X chromosome and often show different patterns of inheritance in males and females.

• Found on the X chromosome.

• Pedigree analysis: Males (XY) more likely to express trait; females (XX) can be carriers.

• Example: Color blindness, hemophilia.

Blood Type Inheritance

Genotypes and Phenotypes

Human blood types are determined by three alleles: A, B (codominant), and O (recessive). The combination of these alleles produces four possible blood types.

• Alleles: A, B (codominant), O (recessive)

• Genotypes and Phenotypes:

Blood Type	Genotype(s)
A	AA, AO
B	BB, BO
AB	AB
O	OO

Example: A child with parents of blood types A and B can have any of the four blood types, depending on parental genotypes.

Practice Problems and Answers

Monohybrid Cross

• Question: Heterozygous parents mate. What is the phenotypic ratio?

• Answer: 3 tall : 1 short

Dihybrid Cross

• Question: Heterozygous parents mate. What is the phenotypic ratio?

• Answer: 9:3:3:1

Dominant/Recessive

• Question: Child has attached earlobes (recessive). Parents have free earlobes. What are the genotypes of the parents?

• Answer: Ee × Ee

Codominance

• Question: Black and white chicken phenotype?

• Answer: Black and white speckled

Incomplete Dominance

• Question: Red × white flower. What is the phenotype?

• Answer: 100% pink

X-Linked Recessive

• Question: Carrier mom × normal dad. What is the chance son is color blind?

• Answer: 50%

Blood Type

• Question: Heterozygous A mom × heterozygous B dad. What are the possible blood types of children?

• Answer: A, B, AB, O

Additional Topics for Review

• Mitosis

• Meiosis

• Nondisjunction and disorders, karyotype

• DNA structure, component

• Alleles and inheritance:

  • Dominant vs recessive

  • Monohybrid cross

  • Dihybrid cross

  • Codominance

  • Incomplete dominance

  • Multiple alleles

  • Blood typing

  • Sex-linked

  • Pedigree

Additional info: For mitosis and meiosis, students should review the stages and outcomes of each process, as well as their roles in genetic variation. Nondisjunction leads to chromosomal disorders such as Down syndrome. DNA structure includes the double helix, nucleotide components, and base pairing rules.