Chapter 3

LAB: Inheritance

Introduction

  • Family resemblance often prompts questions about traits passed down through generations.
  • Key questions:
    • Is familial resemblance real or perceived?
    • How do traits from parents manifest in offspring?
    • Reasons behind similarities between siblings.
  • The lab addresses these inquiries and broader population changes over time.

Lab Learning Objectives

By the end of this lab, students should be able to:

  • Describe the work of Gregor Mendel, the data he collected, and the derived laws: the law of segregation and the law of independent assortment.
  • Explain the relationship between dominant and recessive alleles and the connection between genotypes and phenotypes.
  • Draw and interpret Punnett squares and pedigree diagrams.
  • Differentiate between Mendelian and polygenic traits using human examples.
  • Discuss complexities of inheritance using sex-linked traits and the ABO blood group as examples.

GREGOR MENDEL

Contributions

  • Mid-1800s, conducted experiments in a monastery in what is now the Czech Republic.
  • Crossbred pea plants, observing multiple generations.
  • Studied traits:
    • Flower Color: purple or white.
    • Plant Height: tall or short.
    • Pea Color: yellow or green.
    • Other traits included flower position, pea shape, pod shape, and pod color.
  • Observed regular patterns of trait outcomes, leading to key principles of inheritance.
Key Principles Identified

  1. Law of Segregation

    • Refutes blending theory of inheritance. Traits from parents do not blend.
    • Traits controlled by distinct units (now known as genes), which occur in pairs, one from each parent.
    • During gamete formation (meiosis), alleles segregate into sex cells independently.
    • Example scenarios: Tall (T) and short (t) plants demonstrate random assortment of alleles during reproduction.

  2. Law of Independent Assortment

    • Traits are inherited independently unless located on the same chromosome.
    • Observations showed that traits like plant height and seed color segregate independently during gamete formation.
    • Application to Meiosis: Alleles of different traits sort into gametes independently, resulting generally in a mix of dominant and recessive phenotypes.
  • Mendel's work was overlooked; rediscovered in the early 1900s, paving the way for modern genetics.

DOMINANCE AND RECESSIVENESS

  • Dominance refers to certain alleles appearing more prominently in offspring (e.g., purple flowers over white).
  • **Definitions:
    • Dominant allele:** Masks effects of recessive alleles.
    • Recessive allele: Only manifests when paired with another recessive allele.
  • Example: In pea plants, a cross between purple (P) and white (p) yields mostly purple offspring (P being dominant).

GENOTYPE AND PHENOTYPE

  • Genotype: Genetic makeup (alleles), written as:
    • Dominant: represented by a capital letter (e.g., T).
    • Recessive: represented by a lowercase letter (e.g., t).
  • Phenotype: Physical expression of the genotype.
    • Tall phenotype: TT or Tt (dominant).
    • Short phenotype: tt (homozygous recessive).
  • Homozygous vs. Heterozygous:
    • TT (homozygous dominant), tt (homozygous recessive), Tt (heterozygous).
  • Determining phenotype from genotype:
    • If recessive phenotype, genotype is definitely homozygous recessive.
    • If dominant phenotype, genotype could be homozygous dominant or heterozygous.

PUNNETT SQUARES

  • Purpose: Tool to diagram possible genotypic outcomes from a mating.
  • Created by Reginald Punnett in the early 1900s.
  • Structure:
    • Square divided into boxes, each representing possible inheritance outcomes based on parental genotypes.
  • Example: For TT (homozygous dominant) and Tt (heterozygous) parent mating:
    • Possible outcomes: 50% TT, 50% Tt; all offspring will appear phenotypically tall.
  • Emphasizes randomness in allele assortment during reproduction, upholding the Law of Segregation.

PEDIGREE DIAGRAMS

  • Function: Shows family trees to represent inheritance across generations.
  • Represents phenotypes rather than genotypes but can infer genotypes.
  • Symbols used: Squares (males), Circles (females), Shaded (affected by trait), Unshaded (not affected).
  • Informative for understanding inheritance patterns of traits like freckles (dominant) or attached earlobes (recessive).

MENDELIAN TRAITS AND POLYGENIC TRAITS

  • Mendelian Traits: Traits governed by a single gene, demonstrating simple inheritance.
    • Examples: Hitchhiker's thumb (dominant straight/non-hitchhiker's), freckles.
  • Polygenic Traits: Traits influenced by multiple genes, resulting in continuous variation.
    • Examples: Human height, skin and eye color.

SEX-LINKED TRAITS

  • Traits linked to sex chromosomes exhibit different inheritance patterns.
  • X chromosomes carry many influential genes; Y chromosome has fewer traits linked to maleness.
  • In XX females, both X chromosomes can exert effects, whereas XY males only express traits linked to the X chromosome because the Y lacks an alternative allele.

ABO BLOOD GROUP SYSTEM

  • Exemplifies complexity in Mendelian traits through blood types A, B, AB, and O.
  • Antigens: Molecules on the surface of red blood cells signaling blood type.
  • **Blood Genotypes:
    • AA, AO  Blood type A
    • BB, BO  Blood type B
    • AB  Blood type AB
    • OO  Blood type O**
  • Dominance relationships:
    • A and B are codominant; O is recessive.
    • Universal donor (O) has no antigens, universal recipient (AB) has both.
  • Importance in transfusions due to antibodies produced against foreign antigens. Risk of hemolytic reactions when incompatible blood types are mixed.