Chromosomes and Traits

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

Announcements

Text readings in Hoefnagels

Chapter 10, Patterns of Inheritance

Chapter 11, Chromosomes and Human Inheritance Patterns

You have open access (no log-in or password needed) to instructional materials on the Text web site. Select "Resources" from the upper left of the page and select the text chapter you want.

Moodle

You may also ask questions and see answers to your classmates' questions in Moodle in the "Talk to Ed and Mike" forum.

Objectives:

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

Take-Home Assignment #4 - Meiosis and Genetics Due at Lecture Wednesday October 21.

Take-Home Assignment #5 - Human Genetics Due at Lecture Monday October 26.

Second Moodle Assignment, due in your TA's Moodle Forum by 8 AM Wednesday October 28.

Take-Home Assignment #6 - Gene Expression and Mutation Due at Lecture Wednesday November 4.

After studying this material you should be able to:

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.
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.
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.
Explain how sex is determined in humans.
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.
Describe and give examples of normal, aneuploid, and polyploid chromosome numbers in humans.
Use common objects such as paper clips to model the abnormal movement of chromosomes during meiosis that would result in aneuploidy.
Explain the relationship between genotype and phenotype.

Determination of Gender in Humans

(Hoefnagels, Pg. 225, fig. 11.11)

XX = female
XY = male
Photograph of X and Y Chromosomes (Hoefnagels, Pg. 222, fig. 11.8)
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)

Sex-Linked Inheritance:

X-linked Recessive Inheritance (mother to son)

Inheritance of hemophilia (Hoefnagels, pg. 226, fig. 11.12 A)

Can a women get hemophilia?
Can a man be a carrier for hemophilia?

Color blindness from Wayne's Word: An On-Line Textbook of Natural History

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 sex linked on Y? --- Study says not.

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 . (Hoefnagels, pg. 185, fig. 9.11)
Autosomal aneuploids:

Down syndrome Trisomy 21 (Cytogenetics Gallery). From your text: Trisomy 21 . (Hoefnagels, pg. 186, fig. 9.12)
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 :

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