Genes: Modes of Inheritance

Genes: Modes of Inheritance

DOI: 10.4018/978-1-5225-8066-9.ch005
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Charles Darwin proposed a mechanism that explained how organisms evolved, but nothing was known on the form those heritable changes took, what they were, and how they were passed from one generation to another. Gregor Mendel started experiments using the common pea studying easily observable characteristics. Genes are units of DNA carrying information about specific traits. Any given trait has two alleles that may be the same (homozygous) or different (heterozygous). Mendel's first theory studied one trait (monohybrid) in peas. Mendel's second theory studied two traits (dihybrid). Two examples of inheritance do not conform to Mendel's theories. In incomplete dominance, the expression of both alleles results in an intermediate phenotype. In codominance, both alleles are expressed resulting in a new phenotype comprised of both alleles. Mothers who are Rh- but bearing a Rh+ baby will need a Rh immunoglobulin shot to counteract the formation of antibodies against a future foetus. This chapter explores genes.
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Chapter Outline

  • 5.1 Heritable Traits

  • 5.2 Genetics Terminology

  • 5.3 The Work of Gregor Mendel

  • 5.3.1 1st Theory

  • 5.3.2 2nd Theory

  • 5.4 Beyond Mendel’s Work

  • 5.4.1 Incomplete Dominance

  • 5.4.2 Codominance

  • Chapter Summary


Learning Outcomes

  • Explain heritable traits and give examples in humans.

  • With examples, be able to explain the terminology used in genetics.

  • Summarize the experimental work of Gregor Mendel.

  • With examples, ability to explain genetic inheritance not consistent with Mendel’s work.


5.2 Genetics Terminology

Before we explore the experiments Mendel did on garden peas, we will define some genetics terminology.

  • Genes: Units of information on DNA about specific traits, e.g., flower colour.

  • Locus (Plural Loci): The actual location or address of a gene on a chromosome.

  • Allele: Alternative forms of the same gene responsible for a given trait.

  • Dominant: The allele whose effects completely mask the expression of the other.

  • Recessive: The allele whose effects show up only when dominant allele is absent.

    • o

      Note: Upper-case are letters used for dominant traits; while the recessive traits assume the lower-case letters used for dominant traits.

  • Homozygous: Alleles same, e.g. AA or aa

  • Heterozygous: Alleles different, e.g. Aa

    • o

      Homozygous Dominant would be AA

    • o

      Homozygous Recessive aa

    • o

      Heterozygous Aa

  • Genotype: The actual alleles present in an individual, e.g. Aa or aa

  • Phenotype: The physically observable traits in the individual like height or flower colour.

  • Generations:

    • o

      P (Parental)

    • o

      F1 (First Filial)

    • o

      F2 (Second Filial)

Key Terms in this Chapter

Dihybrid Inheritance: Inheritance of two different characteristic traits.

Heterozygous: Terminology used to describe different alleles.

Phenotypic Ratio: The ratio used to describe differences in observable traits from a cross or breeding event.

Monohybrid Inheritance: Inheritance of one characteristic trait.

Allele: Alternative forms of the same gene responsible for a given trait.

Genotype: The actual alleles present in an individual.

Recombinants: Novel genotypes that result from the reshuffling of genes and the crossover of chromosomes during meiosis.

Genes: Units of information about specific traits on DNA.

Codominance: A genetic effect in which both alleles of are phenotypically expressed in an organism.

Phenotype: The physically observable traits in an individual.

Homozygous: Terminology used to describe the same alleles.

Genotypic Ratio: The ratio used to describe differences in allele frequency from a cross or breeding event.

Punnett Square: Visual diagram used to predict the offspring, genotype, and phenotype ratio of a particular cross or breeding event.

Testcross: A genetic cross between a known homozygous recessive parent and a suspected heterozygous parent to determine specific genotypes (Pp or PP).

Locus: The location of a gene on a chromosome.

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