Patterns of Inheritance Lesson

Lesson Overview

• Key Genetic Terminology
• Mendel's Experiments
• Mendel's Laws of Inheritance
• Dominance Relationships
• Beyond Mendelian Genetics
• Punnett Squares and Genetic Crosses
• Test Cross
• Problem-Based Examples
• Memory Aids and Tips
• Concepts Reinforced
• Key Takeaway

Patterns of inheritance explain how traits are passed from parents to offspring. This topic builds on Gregor Mendel's foundational work and explores how genes, alleles, and chromosomal behavior shape the diversity seen across living organisms. This lesson provides a structured breakdown of key genetic principles, highlights important exceptions, and prepares students to confidently approach related assessments.

Key Genetic Terminology

Understanding genetics begins with core terms:

A genotype like Tt yields a tall phenotype, as the dominant allele masks the recessive.

Mendel's Experiments

Gregor Mendel's work with pea plants laid the groundwork for modern genetics. He selected peas due to their:

• Many observable traits (e.g., seed color, plant height)
  
• Ability to self- and cross-pollinate
  
• Rapid generation time and high offspring count
  

He observed that traits are inherited as discrete units (genes) rather than blending between parents.

Mendel's Laws of Inheritance

Law of Segregation

Each individual has two alleles for a trait. These alleles separate during gamete formation so each gamete receives one allele.

Example: In a monohybrid cross (Yy × Yy), the F2 generation exhibits:

• Genotype ratio: 1 YY : 2 Yy : 1 yy
  
• Phenotype ratio: 3 yellow : 1 green
  

Law of Independent Assortment

Genes for different traits assort independently during gamete formation if located on different chromosomes.

Example: In a dihybrid cross (YyRr × YyRr):

• Phenotypic ratio = 9:3:3:1
  
  • 9 yellow-round
    
  • 3 yellow-wrinkled
    
  • 3 green-round
    
  • 1 green-wrinkled
    

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Dominance Relationships

Complete Dominance

Dominant allele masks the recessive allele.

• Example: Tt (tall) displays the dominant trait.
  

Incomplete Dominance

Heterozygotes show a blend of traits.

• Example: Red (RR) × White (WW) = Pink (RW)
  

Codominance

Both alleles express equally in heterozygotes.

• Example: Blood type AB (IAIB)
  

Beyond Mendelian Genetics

Multiple Alleles

A gene with more than two alleles in the population.

• Example: Blood types – IA, IB, i
  

Epistasis

One gene affects the expression of another.

• Example: In Labrador retrievers, ee genotype masks coat color gene.
  

Pleiotropy

One gene affects multiple traits.

• Example: Marfan syndrome affects eyes, bones, and heart.
  

Polygenic Traits

Traits controlled by multiple genes; show continuous variation.

• Examples: Human height, skin color
  

Sex-Linked Inheritance

Traits linked to sex chromosomes, especially X chromosome.

• More common in males (XY)
  
• Examples: Color blindness, hemophilia
  

Punnett Squares and Genetic Crosses

Monohybrid Cross (Tt × Tt)

• 
  Genotypic ratio: 1 TT : 2 Tt : 1 tt
  
• Phenotypic ratio: 3 tall : 1 dwarf
  

Dihybrid Cross (YyRr × YyRr)

Possible gametes: YR, Yr, yR, yr

Use a 4x4 Punnett square to get all 16 combinations.

• Ratio: 9:3:3:1 as explained in Mendel's second law.
  

Test Cross

Used to determine if an organism showing a dominant trait is homozygous or heterozygous.

• Cross with homozygous recessive
  
• If any offspring show recessive traits → parent is heterozygous
  

Problem-Based Examples

Example 1: Incomplete Dominance

Cross: Pink (RW) × White (WW)

Gametes: R or W × W

Offspring:

• 50% Pink (RW)
  
• 50% White (WW)
  

Example 2: Codominance

Cross: IAIB (AB) × ii (O)

Offspring:

• 50% IAi (Type A)
  
• 50% IBi (Type B)
  

Example 3: Sex-Linked Inheritance

Cross: Carrier mother (XNXc) × Normal father (XNY)

• Sons: 50% color blind (XcY), 50% normal (XNY)
  
• Daughters: 50% carriers (XNXc), 50% normal (XNXN)
  

Memory Aids and Tips

Concepts Reinforced

Key Takeaway

Patterns of inheritance help us understand how traits pass through generations. While Mendel's principles laid a foundation, modern genetics includes more complex relationships such as codominance, multiple alleles, and epistasis. Mastering these patterns enhances our understanding of biology and equips students to tackle real-world genetic problems with confidence.

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