13 Heredity

Martha Lally; Suzanne Valentine-French; and Dinesh Ramoo

Learning Objectives

  • Define genes
  • Distinguish between mitosis and meiosis, genotype and phenotype, homozygous and heterozygous, and dominant and recessive
  • Describe some genetic and chromosomal disorders
  • Define polygenic and incomplete dominance
  • Describe the function of genetic counselling and why individuals may seek genetic counselling
  • Define behavioural genetics, describe genotype-environment correlations and interactions, and define epigenetics
As your recall from chapter one, the nature perspective refers to the contribution of genetics to one’s development. The basic building block of the nature perspective is the gene. Genes are recipes for making proteins, while proteins influence the structure and functions of cells. Genes are located on the chromosomes and there are an estimated 20,500 genes for humans, according to the Human Genome Project (NIH, 2015). See Box 2.2 at the end of this section for more details on the Human Genome Project.
A diagram showing DNA located in the chromosome.
Figure 2.1 Eukaryote DNA

Normal human cells contain forty-six chromosomes (or twenty-three pairs; one from each parent) in the nucleus of the cells. After conception, most cells of the body are created by a process called mitosis. Mitosis is defined as the cell’s nucleus making an exact copy of all the chromosomes and splitting into two new cells. However, the cells used in sexual reproduction, called the gametes (sperm or ova), are formed in a process called meiosis. In meiosis the gamete’s chromosomes duplicate and then divide twice, resulting in four cells containing only half the genetic material of the original gamete. Thus, each sperm and egg possesses only twenty-three chromosomes and combine to produce the normal forty-six. See Figure 2.2 for details on both mitosis and meiosis. Given the amount of genes present and the unpredictability of the meiosis process, the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould and Keeton, 1997).

Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.
Figure 2.2 Three cell-growth types

Of the twenty-three pairs of chromosomes created at conception, twenty-two pairs are similar in length. These are called autosomes. The remaining pair – sex chromosomes – may differ in length. If a child receives the combination of XY the child will be genetically male. If the child receives the combination XX the child will be genetically female.

Media Attributions

  • Eukaryote DNA

About the authors


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Heredity Copyright © 2022 by Martha Lally; Suzanne Valentine-French; and Dinesh Ramoo is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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