Biology 100/101
Lecture 12
Chromosomes and Traits
(Print Version)

Announcements &
Assignments

Lecture Objectives

Gender
Determination

Sex Linkage

Linked Genes

Abnormal
Chromosome
Numbers

Prenatal
Diagnosis

Lecture Syllabus

IB 100/101 Home Page

Text readings in Life by Ricki Lewis, et. al.

Chapter 11, Transmission of Inherited Traits

Chapter 12, Chromosomes

The "Reviewing Concepts" boxes are valuable summaries of the main ideas in these sections of the text.

You have open access (no log-in or password needed) to instructional materials on the Text web site. Select the text chapter you want and use the links to the e-learning modules or other available materials. There is also a collection of study materials called the "Essential Study Partner" that you may find useful.

Web Crossing

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Ed" discussion.

Objectives:

The content of this lecture will help you complete these assignments:

The content of this lecture will help you complete these assignments:

Take-Home Assignment #4 - Meiosis and Genetics due at Lecture Tuesday March 6

Take-Home Assignment #5 - Human Genetics due at Lecture Tuesday March 13

Second Web Crossing Assignment, due by 8:00 AM, Tuesday March 27

After studying this material you should be able to:

1. Use drawings or models of chromosomes to explain why genes are considered to be linked and demonstrate how linked genes are recombined by crossing over during meiosis.
2. Explain why linkage of two gene loci produces a genetic ratio of offspring different from a situation in which two gene loci are located on different numbered chromosomes.
3. Describe the effect that spacing of linked gene loci on a chromosome has on the expected genetic ratios in the offspring of a cross between ane individual homozygous recessive for both loci and one who is heterozygous for both loci.
4. Explain how sex is determined in humans.
5. Explain why a trait is considered to be sex-linked (X chromosome) and illustrate why males are more likely to suffer from a sex-linked trait.
6. Describe and give examples of normal, aneuploid, and polyploid chromosome numbers in humans.
7. Use common objects such as paper clips to model the abnormal movement of chromosomes during meiosis that would result in aneuploidy.
8. Explain the relationship between genotype and phenotype.

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Determination of Gender in Humans

(Text, Pg. 214, fig. 12.9)

• XX = female
• XY = male
• Photograph of X and Y Chromosomes (Text, pg. 213, fig. 12.8)
• Interactive Chromosome/Gene Poster from Gene Gateway - Exploring Genes and Genetic Disorders
• Human X Chromosome
• Human Y Chromosome
• The "Real" Y Chromosome
• SRY gene (Sex-determining Region of the Y chromosome)

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Sex-Linked Inheritance:

• X-linked Recessive Inheritance (mother to son)
• Inheritance of hemophilia (Text, pg. 215, fig. 12.10 A)
• A pedigree of hemophilia in the Royal Families of Europe (See also - text, pg. 215, fig. 12.10 B)
• Can a women get hemophilia?
  Can a man be a carrier for hemophilia?
• Color blindness from The Biology Project
• X-linked dominant inheritance
• Woman with allele has associated trait or illness. Males with allele more severly affected.
• Incontinentia Pigmenti
• Y-linked inheritance (father to son)
• Hairy ears?

Linked Genes: Gene loci located close to each other on the same chromosome are usually inherited together.

• Closely linked gene locations do not assort independently.
• Crossing over between the loci separates linked genes
• Crossing Over Animation from Tokyo Medical University Genetics Study Group
• Crossing over is more likely to separate alleles at two gene loci if they are widely spaced on the same chromosome than if they are located close to each other (closely linked). Closely linked alleles tend to be inherited together more often.
• See how this works

Abnormal chromosome numbers (when meiosis goes wrong)

See this site on abnormal chromosomes created by Cindy Kim (Bio100 Fall '98) for extra credit.

• Polyploidy - Extra SETS of chromosomes (triploid = 69 - tetraploid = 92, etc.)
• Aneuploidy - Extra or missing chromosomes resulting from nondisjunction.
• Nondisjunction is the failure of a homologous chromosme pair to separate (become disjunct) during meiosis results in aneuploidy.
• Nondisjunction can occur in either meiosis I or meiosis II. (Text, pg. 220, fig. 12.13)
• Autosomal aneuploids:
• Down syndrome Trisomy 21.
• Nondisjunction in Down syndrome - animation from Hironao NUMABE, M.D.
• See how nondisjunction during meiosis produces an egg cell with two #21 chromosomes.
• See how fertilization with a sperm with the normal number of chromosomes produces an aneuploid zygote with three #21 chromosomes.
• Sex chromosome aneuploids :
• XO=Turner Syndrome
• YO
• XXY=Klinefelter Syndrome
• XXX= Triplo-X Syndrome
• XYY=Jacob Syndrome

Prenatal diagnosis techniques

• Obstetric ultrasound
• Amniocentesis
• Chorionic Villus Sampling from webmd.com