Genes: Inheritance Patterns in Humans

Genes: Inheritance Patterns in Humans

DOI: 10.4018/978-1-5225-8066-9.ch006

Abstract

There are several commonly observed human traits. Chromosomes are composed of DNA tightly packed and coiled around blocks of protein known as histones. Humans have a total of 46 chromosomes made up of 1 pair of sex chromosomes and 22 pairs of autosomes. There are several minimally invasive procedures for detecting birth defects in humans during early pregnancy (prenatal diagnosis). Gene linkage occurs when genes that are close together are inherited as a set in autosomes or sex chromosomes. The interrelationships of parents and their children across many generations is called a family tree or pedigree and can be derived by tracing traits across several generations. Sex-linked disorders are genetic defects carried by the X chromosome. Autosomal disorders can be recessive or dominant. Disorders can result from the alterations in chromosome number in autosomes or sex chromosomes. When the integrity of chromosomes is affected, other disorders occur. This chapter explores this aspect of genes.
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Chapter Outline

  • 6.1 Common Human Traits

  • 6.2 Nature of Chromosomes

  • 6.3 Human Karyotyping and Detection of Birth Defects

  • 6.4 Sex Determination

  • 6.5 Gene Linkage and Linkage Maps

  • 6.6 Sexually-Linked Genes

  • 6.7 When Genes and Chromosomes Change

  • 6.7.1 Sex-linked disorders

  • 6.7.2 Autosomal Disorders (Recessive)

  • 6.7.3 Autosomal Disorders (Dominant)

  • 6.7.4 Chromosomal Abnormalities

  • 6.7.4.1 Alterations in Chromosome Number (Autosomes)

  • 6.7.4.2 Alterations in Chromosome Number (Sex Chromosomes)

  • 6.7.4.3 Alterations in Chromosome Structure

  • Chapter Summary

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Learning Outcomes

  • Compare observable genetic traits in humans

  • Explain the structure, nature and composition of chromosomes

  • Summarize the human blood karyotyping process

  • Illustrate the process of sex determination in humans

  • With examples demonstrate gene linkage, sex-linked genes and linkage maps

  • Outline the process of gene mutation with examples

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6.1 Common Human Traits

Let’s examine some examples of common human traits. Check these traits and how they apply to you and your family.

  • 1.

    Hand Clasping: Clasp your hands together without effort (that is naturally without thinking about the process). Check which thumb is lying on top of the other. Is it the right or left thumb? Interlacing fingers is consistent throughout life in humans (right fingers over left or vice versa). Several genes are involved in the expression of this phenotype.

  • 2.

    Eye Color: Examine the eye of your colleague in class (the colored portion or iris). Is it blue, grey, amber, green, brown, hazel or other color? Eye color is determined by multiple genes.

  • 3.

    Dimples: When you make a facial expression as in smiling, do your cheeks or those of your colleagues have a small part where skin is indented? Dimples are controlled by a single gene locus but other genes may have influence.

  • 4.

    Earlobes: Check or feel your ears at lower lobe. Is it free or connected to the side of face? Free earlobes are determined by multiple genes with the phenotype being a continuum from free earlobes to attached earlobes and everything in-between.

  • 5.

    Tongue Rolling: Try to roll your tongue at both sides upwards. Can you? This trait is determined by a single gene locus with the roller being dominant to non-roller.

  • 6.

    Mid-Digit Hair: Check your middle finger segments for hair. Is there any? Research shows mid-digit hair is determined by recessive alleles although environmental factors may influence the trait.

  • 7.

    Freckles: Most common on the face, freckles represent overproduction of melanin pigment in those cells. Freckles have been attributed to allele variants of one specific gene, the MC1R.

  • 8.

    Widows Peak: Now look at your colleague’s forehead. Does the hairline have an “M” shape or the hairline runs straight across the face? Researchers think several genes may be involved in determining this trait.

  • 9.

    Hitch-Hiker’s Thumb: Bend your thumb backwards at the top segment. How far does it go? The hitch-hiker’s thumb is a phenotype where the top segment of the thumb bends significantly lower creating a large angle between the two segments. Studies indicate this trait varies in a continuum from straight to hitch-hiker’s thumb and is thought to be controlled by multiple genes.

Key Terms in this Chapter

“p” Arm: The shorter section of the chromosome when it is divided by the centromere.

“q” Arm: The longer section of the chromosome when it is divided by the centromere.

Personal Wellness Profile: A questionnaire that is used to assess an individual’s health lifestyle.

Sex Chromosomes: The 23rd pair of chromosomes, X and Y, that determines sex and sexual characteristics of an organism.

Karyotype Analysis: A process used to observe the number and appearance of chromosomes in order to detect birth disorders.

Pedigree: A diagram used to determine the lineage and probability of inheriting genes in humans and direct ancestors.

Autosomes: Twenty-two pairs of chromosomes that exist in the human genome and are not sex chromosomes.

Linkage: Genes that are closer together on the same chromosome are more likely inherited together.

Chromosomes: Units that contain chromatin specific for each species (e.g., humans have 46 chromosomes).

Sex Determining Region: Also known as testis-determining factor, it is a gene on the Y chromosome that gives rise to male phenotypes, such as the testes.

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