Chromosomes and Inheritance

Since genes are carried on chromosomes, knowledge of chromosome number and structure has far-reaching implications for basic genetics, human health, and evolution.

A normal human male karyotype.

X-linked Inheritance – When Men and Woman Play by Different Rules

Behind the 8-ball? Colorblindness is an X-linked recessive trait.

X-linked Inheritance

There are many X-linked recessive traits.

Pedigree Analysis is a Key Tool in Human Genetics

Analyzing a pedigree is like puzzle-building – you try things (assigning potential genotypes) until the pieces fit (you’re as certain as you can be about genotypes and modes of transmission (autosomal vs. X-linked; dominant vs. recessive).

A Pedigree of a Recessive Human Trait

Note that the trait can appear in offspring of parents without the trait.

Heterozygotes who do not show the trait are termed carriers.

A Pedigree of a Dominant Human Trait

Note that the trait appears in every generation and ½ the offspring of an affected heterozygote are expected to show the trait.

Changes in Chromosome Number and Structure

Changes in chromosome number and structure are important for health and evolution.

Down syndrome is caused by a change in chromosome number.

Aneuploidy occurs when one of the chromosomes is present in an abnormal number of copies.

Aneuploidy

Trisomy and monosomy are two forms of aneuploidy.

Down Syndrome is Caused by Trisomy for Chromosome 21

Aneuploidy is remarkably common, causing termination of at least 25% of human conceptions.

Aneuploidy is also a driving force in cancer progression (virtually all cancer cells are aneuploid).

Chromosome Non-Disjunction in Meiosis Causes Aneuploidy

The Frequency of Chromosome Non-Disjunction And Down Syndrome Rises Sharply with Maternal Age

The phenomenon is clear – the explanation isn’t.

Sex Chromosome Aneuploid Conditions are Common

Klinefelter syndrome

Polyploidy

Polyploidy occurs when all the chromosomes are present in three or more copies.

Polyploidy is common in plants and rare in animals.

Polyploids Are Created When Chromosome Number Doubles

A common way for this to occur is for the mitotic spindle to fail, leaving all chromosomes in one cell.

Polyploidy is a Major Force in Plant Evolution

Roughly 35% of flowering plants (the most familiar plant species) arose through polyploidization.

Most Crop Species are Polyploid

Polyploids, like the one on the left, are larger than their diploid progenitors (strawberry on right).

There are 4 types of chromosome structural change – all of them associated with human disorders

Chromosome Structural Changes

A Boy with Cri-du-Chat Syndrome – a Debilitating Disorder Caused by Chromosome Deletion

Cri-du-Chat is Caused by the Loss of the Short Arm of One Copy of Chromosome 5

Translocations Lead to a Number of Human Cancers

In Burkitt’s lymphoma, a chromosome translocation

causes a cell cycle-promoting gene to always be active.

Searching for Chromosomal Defects - Amniocentesis and Chorionic Villus Sampling

Many new techniques for learning about individual genes rather than whole chromosomes are available or under development.

Searching for Chromosome and Gene Defects – Pre-Implantation Genetic Diagnosis (PGD)

Removing a cell for diagnosis from a human embryo.

The diagnosis: trisomy 21 (Down syndrome).