lesson 5 monohybrid crosses

Monohybrid Crosses

Understanding Inheritance

  • Historical Understanding of Inheritance:
    • The traditional beliefs around inheritance were limited and have since evolved.
  • Gregor Mendel's Contributions:
    • Recognized as "The Father of Genetics" for his pioneering research.
    • Conducted experiments using pea plants to explore inheritance patterns.
  • Genetic Mechanisms:
    • Chromosomes contain genes, which are responsible for traits.
    • Parents pass different versions of genes, termed alleles, to their offspring through the process of meiosis.

Quick Recap of Mendel’s Work

  • Dominance of Traits:
    • By crossing pea plants with distinct traits, Mendel discovered:
    • Dominant Traits: Purple flower allele (P) is dominant over the white flower allele (p).
    • When a plant has one purple (P) allele and one white (p) allele, the plant exhibits purple flowers due to dominance.
    • Recessive Traits: A flower will only be white if it possesses two copies of the white allele (pp).

How Do Two Purple Plants Make a White Flower?

  • Example Cross of Two Purple Plants:
    • Parental Genotypes:
    • P (for purple) and p (for white)
    • Cross Result:
    • The potential genotypes formed (P/P, P/p, p/p).

Important Terms to Learn

  • Genotype:
    • Definition: The combination of alleles for a specific trait.
    • Examples: PP, Pp, AA, aa.
  • Phenotype:
    • Definition: The observable physical traits expressed by the genotype.
  • Homozygous:
    • Definition: An organism with two identical alleles for a trait.
    • Examples: BB or bb.
  • Heterozygous:
    • Definition: An organism with two different alleles for a trait.
    • Examples: Bb.

Practice Questions

  • Identify Genotype or Phenotype:
    • Is curly hair a genotype or phenotype? (Phenotype)
    • Is Mm a genotype or phenotype? (Genotype)
    • Is Mm heterozygous or homozygous? (Heterozygous)
    • Is AA heterozygous or homozygous? (Homozygous)
    • Individuals with sickle-cell anemia possess misshaped red blood cells. Does this describe a genotype or phenotype? (Phenotype)

Genetics and Meiosis

  • Allele Pairing in Gametes:
    • Since each cell contains two alleles for each gene, two potential outcomes arise during meiosis.

Predicting Genetic Outcomes

  • Punnett Squares:
    • Utilized to visualize potential genetic combinations resulting from crosses.
    • They provide the probabilities for specific outcomes but do not guarantee results due to the randomness of meiosis.
    • Example Scenario: The chances of getting “tails” when flipping a coin is 50%; however, it’s possible to flip a coin four times and only get “heads.”

Using Punnett Squares for Monohybrid Crosses

  • Focus: A monohybrid cross studies one particular trait's inheritance.
  • Example Trait in Pea Plants: Yellow seed color being dominant to green.
    • When crossing a homozygous recessive plant with a heterozygous plant:
    • Definitions of alleles:
    • Let Y = Yellow seed allele
    • Let y = Green seed allele
  • Parent Generation (P Generation):
    • P1 = yy (homozygous recessive)
    • P2 = Yy (heterozygous)

Steps to Complete a Punnett Square

  • Step 1: Let statement explaining alleles used.
  • Step 2: List genotypes of the P generation (P1 = yy, P2 = Yy).
  • Step 3: Fill in the Punnett Square:
    • Gamete combinations:
    • P1: y, y
    • P2: Y, y
    • Resultant Square:
      • y | y
      • Y | Yy | Yy
      • y | yy | yy
  • Step 4: Ratios:
    • Genetic ratio: 2:2 (1:1) of Yy:yy
    • Phenotypic ratio: 2:2 (1:1) Yellow peas: Green peas.
  • Interpretation: 50% of offspring will yield yellow seeds, and 50% will yield green seeds, labelled as F1 generation.

Example 1: Dwarfism in Alaskan Malamutes

  • Genetic Context:
    • Dwarfism is the recessive trait compared to normal size.
    • Two heterozygous dogs are mated:
    • Alleles defined:
    • Let N = normal-sized dogs
    • Let n = allele for dwarfism
  • Cross Results:
    • Punnett square setup:
    • P1 = Nn
    • P2 = Nn
    • Resulting Genotypes:
    • NN, Nn, Nn, nn
    • Genotypic Ratio: 1:2:1 (NN:Nn:nn)
    • Phenotypic Ratio: 3:1 (Normal size:Dwarf)

Example 2: Thorns in Plant Species

  • Genetic Context:
    • A dominant gene for thorns compared to a smooth stem.
    • Crossing a homozygous dominant plant with a smooth stem plant:
    • Alleles defined:
    • Let T = stem with thorns
    • Let t = smooth stem
  • Cross Results:
    • P1 = TT (homozygous dominant)
    • P2 = tt (homozygous recessive)
    • Punnett square results:
    • Resulting Genotypes:
    • T, T | t, Tt, Tt
    • Genotypic Ratio: 100% Tt
    • Phenotypic Ratio: 100% (All have stems with thorns).