Genes: Inheritance Patterns in Humans

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

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

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.