Biology 100
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 Home Page

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

Answers to many of these questions can be found at the Text On-Line Learning Center

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

Objectives:

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.

Determination of Gender in Humans

(Text, Pg. 208, fig. 1.9)

• XX = female
• XY = male
• Photograph of X and Y Chromosomes (Text, pg. 207, fig. 11.8)
• Human X Chromosome
• Human Y Chromosome
• The "Real" Y Chromosome
• SRY gene (Sex-determining Region of the Y chromosome)

Sex-Linked Inheritance:

• X-linked Recessive Inheritance (mother to son)
• Inheritance of hemophilia (Text, pg. 209, fig. 11.10 A)
• A pedigree of hemophilia in the Royal Families of Europe and see Fig. 14.8, text.
• Common pedigree symbols
• Can a women get hemophilia?
  Can a man be a carrier for hemophilia?
• Color blindness
• 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.

• Linked genes do not assort independently.
• Crossing over between the loci separates linked genes
• Crossing Over Animation
• Crossing over is more likely to separate 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 genes tend to be inherited together more often.

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.
• Aneuploidy - Extra or missing chromosomes.
• Nondisjunction
• The failure of a chromosomal pair to separate during meiosis resulting in aneuploidy.
• Nondisjunction can occur in either meiosis I or meiosis II. (Text, pg. 214, fig. 11.13)
• How does this affect the final number of chromosomes in a gamete? (Fig. 14.15 text).
• Autosomal aneuploids:
• Down syndrome Trisomy 21.
• Patau syndrome. Trisomy 13.
• Sex chromosome aneuploids :
• XO=Turner Syndrome
• YO
• XXY=Klinefelter Syndrome
• XXX=Triplo-X Syndrome
• XYY=Jacob Syndrome

Prenatal diagnosis techniques

• Obstetric ultrasound
• Chorionic Villus Sampling
• Fetal Cell Sorting
• See page 202, Life, by Lewis, et.al.