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2.2 Inheritance & Variation 👨‍👩‍👧‍👦

inheritance and variation

Introduction to inheritance & variation

Genetics of inheritance & variation is a core JC1 topic in H2 BIO A levels. This topic explores the various ways of passing down genes to the next generation. In addition, how its expression and observation may vary due to the gene behavior, different genders, gene interactions as well as environment. Finally, the topic also covers statistical technique and analysis of the χ2.

Materials for inheritance & variation

YouTube video

Phrasing errors

  • Environment have no influence on discontinuous characters which is coded for by a single gene.
  • A/B/AB/O blood groups are each coded for by a different gene; multiple genes code for the blood group of a person.

Exam tips

  • In a standard dihybrid cross assume no linkage no epistasis.
  • There is a difference between calculated and critical χ2 value.
  • Don’t assume boxes in a darker shade in family pedigrees are always dominant characters.
  • Rule of thumb: whenever questions ask probabilities of different characteristics in a dihybrid cross (or more), multiply instead of adding.
  • Discontinuous characters can also show a range of values, this is sometimes an environment cause e.g. unequal distribution of fertilizer to plants.
  • Incomplete and complete linkage are different. Look for clues (e.g. next to each other vs on the same chromosome) to determine which one best suits the scenario.

Doing χ2 test using TI-84 Calculator

  • Press STAT and after that EDIT.
  • Clear all the values in the L1 as well as L2 columns.
  • Input the observed values into L1 and expected values into L2.
  • Press STAT and scroll down and select TEST.
  • Scroll down and select D: χ2 GOF-Test.
  • The calculator will prompt for the values input, make sure L1 and L2 columns correctly.
  • Input the correct degree of freedom (df) and then select CALCULATE and press ENTER.
  • The calculated χ2 value will appear allowing you to input directly into your exam script.