Chapter 8: Variation in Chromosome Structure and Number

What is Genetic Variation?

Refers to differences between members of the same species or those of different species

What are 2 characteristics of Genetic Variation?

1. Allelic variations are due to differences in particular genes

2. Chromosomes also vary in their structure and number

What are 2 characteristics of Chromosomes?

1. Structure

2. Number

What is the Structure of Chromosomes?

Segments of chromosomes may be deleted, duplicated, and/or rearranged

What is the Number of Chromosomes?

The number of individual chromosomes may vary (e.g., 3 instead of the usual 2) or numbers of chromosome sets may vary (4 sets instead of the usual 2 sets)

What are 2 such changes of Chromosomes?

1. Widespread medical relevance

2. Agriculturally important

What is Cytogenetics?

The field of genetics that involves the microscopic examination of chromosomes

What does a Cytogeneticist typically examine?

The chromosomal composition of a particular cell or organism

Why is the benefit of Cytogeneticist examining chromosomal composition?

1. Allows the detection of individuals with abnormal chromosome number or structure

2. Provides a way to distinguish between species

True or false, Each species has a characteristic number of chromosomes?

True

What does a Micrograph of metaphase chromosomes for Human species look like?

What does a Micrograph of metaphase chromosomes for Fruit Fly species look like?

What does a Micrograph of metaphase chromosomes for Corn species look like?

What are 3 main features Cytogeneticist used to identify and classify chromosomes?

1. Location of the centromere

2. Size

3. Banding patterns

The location of the centromere, size and banding patterns are all seen in what?

Karyotype

What is a karyotype?

A micrograph in which all of the chromosomes within a single cell are arranged in a standard fashion

What happens when different chromosomes are very similar in size and centromere position?

Chromosomes are treated with stains to produce characteristic banding patterns

What is an example of Chromosomes treated with Stains?

G-banding

What are characteristics of G-banding?

1. Chromosomes are exposed to the dye Giemsa

2. Some regions bind the dye heavily

3. Some regions do not bind the dye well

4. Happens In humans

What is an example of Some regions bind the dye heavily?

Dark bands

What is an example of Some regions do not bind the dye well?

Light bands

How many G bands are seen in metaphase in humans?

300

How many G bands are seen in Prometaphase in humans?

800

What are 3 ways that banding pattern is useful in several ways?

1. It distinguishes individual chromosomes from each other

2. It detects changes in chromosome structure

3. It reveals evolutionary relationships among the chromosomes of closely related species

What does Metacentric centromic location look like?

The centromere is near the middle

What does Submetacentric centromic location look like?

The centromere is slightly off center

What does Acrocentric centromic location look like?

The centromere is significantly off center but not at the end

What does Telocentric centromic location look like?

Centromere is at one end

What does the Giemsa staining of human chromosomes look like?

What does the Conventional numbering system of G bands in human chromosomes look like?

What are the 2 primary ways in which the structure of chromosomes can be altered?

1. The total amount of genetic material in the chromosome can change

2. The total amount of genetic material remains the same, but is rearranged

What are 2 examples of The total amount of genetic material in the chromosome can change?

1. Deletions

2. Duplications

What are 2 examples of the total amount of genetic material remains the same, but is rearranged?

1. Inversions

2. Translocations

What is another name for Deletion?

A deficiency (used to describe the condition of a chromosome that is missing a region)

What is Deletion?

1. The loss of a chromosomal segment

2. The affected chromosome is deficient in a significant amount of genetic material

What is Duplication?

The repetition of a chromosomal segment compared to a normal chromosome

What is Inversion?

A change in the direction of part of the genetic material along a single chromosome

What is Translocation?

A segment of one chromosome becomes attached to a different chromosome

What are 2 examples of Translocations?

1. Simple Translocations

2. Reciprocal Translocations

What is Simple Translocations?

A piece of a chromosome is attached to another chromosome

What are Reciprocal Translocations?

Two different types of chromosomes exchange pieces, producing two abnormal chromosomes with translocations

What does a Deletion look like?

What does Duplication look like?

What does Inversion look like?

What does Simple Translocation look like?

What does Reciprocal Translocation look like?

When does a chromosomal deletion occur?

When a chromosome breaks and a fragment is lost

What happens when a chromosome is broken into two pieces?

The part without the centromere is eventually lost

What does the remaining chromosome have?

A terminal deletion

What happens when a chromosome breaks into two places?

The central fragment is lost, and the two outer pieces reattach

When the chromosome breaks into 2 places, the chromosome has what?

An interstital deletion

What does a Terminal Deletion look like?

What does a Interstitial Deletion look like?

What are 2 items that phenotypic consequences depend on?

1. Size of deletion

2. Chromosomal material deleted

What happens when deletions have a phenotypic effect?

They are usually detrimental

What is an example of when deletions have a phenotypic effect, they are usually detrimental?

Cri-du-chat syndrome in humans

What causes the disease Cri-du-chat?

Caused by a deletion in the short arm of chromosome 5

What does Chromosome 5 in cri-du-chat syndrome look like?

What does a child with cri-du-chat syndrome look like?

What is a chromosomal duplication usually caused by?

1. By abnormal events during recombination

2. May be caused by abnormal crossover events

What do Repetitive sequences (in red) cause?

Cause misalignment between homologous chromosomes.

What is the result of a crossover?

Nonallelic homologous recombination

What do Cross over, Repetitive sequences, Duplication and Deletion look like?

How are Duplications similar to Deletions?

The phenotypic consequences of duplications tend to be correlated to size

When do Duplications more likely to have phenotypic effects?

If they involve a large piece of the chromosome

True or false, duplications tend to have less harmful effects than deletions of comparable size?

True

In humans, relatively few well-defined syndromes are caused by?

Small chromosomal duplications

What do the majority of small chromosomal duplications have?

No phenotypic effect

Why are small chromosomal duplications vital?

They provide the raw material for the addition of genes to a species

What do small chromosomal duplications ultimately lead to?

The formation of gene families

What is a Gene Family?

Consists of two or more genes in a single species that are derived from the same ancestral gene

What happens over time when duplicated genes may accumulate mutations?

Alters their function

What is a result of duplicated genes accumulating mutations and having altered function?

They may have similar but distinct functions

Duplicated genes are now members of?

A gene family

What are Paralogs?

Homologous genes within a single species and constiute a gene family

What does Duplication genes, altered functions and Paralogs look like?

What do Globin Genes encode?

All encode subunits of proteins that bind oxygen

What happen over 500 million years to the Globin gene?

1. Duplicated and altered

2. Now 14 paralogs in this gene family on three different chromosomes

Different paralogs carry out what?

Similar but distinct functions

What does the evolution of the ancestral globin look like?

What is Copy Number Variation (CNV)?

A segment of DNA that varies in copy number among members of same species

What are some characteristics of CNV?

1. May be missing a particular gene

2. May be a duplication

3. Surprisingly common in animal and plants

4. 0.1%-10% of a genome may show CNV

5. Human rate is approximately 0.4%

6. Structural variation in a segment of DNA that is 1000 bp or more

7. Commonly exhibits copy # differences among members of the same species

8. CNV occurs at population level

Is CNV relatively common?

Yes

What are possible mechanisms of CNV?

1. Nonallelic homologous recombination

2. Proliferation of transposable elements

3. Errors in DNA replication

CNV is associated with which human diseases?

1. Schizophrenia

2. Autism and certain learning disabilities

3. Susceptibility to infectious disease

4. Cancer

What does CNV look like?

A chromosome with an inversion has what?

A segment that has been flipped to the opposite orientation

What does a Normal Chromosome look like with Inversions and Translocations?

What does a Pericentric inversion look like?

What does a Paracentric inversion look like?

What happens in an inversion?

The total amount of genetic information stays the same

The great majority of inversions have?

No phenotypic consequences

What happens in rare cases of inversions?

Inversions can alter the phenotype of an individual

What are 2 effects of Inversions?

1. Break point effect

2. Position effect

What is the Break Point Effect

An inversion break point occurs within a vital gene, thereby separating it into 2 nonfunctional parts

What is the position effect?

A gene is repositioned in a way that alters its gene expression

What is an example of the position effect?

May be placed close to regulatory elements for another gene

What percent of the human population carries inversions that are detectable with a light microscope?

2%

People who carries inversions that are detectable with a light microscope are?

Most of these individuals are phenotypically normal

What is a con of Some people who carries inversions that are detectable with a light microscope may?

Produce offspring with phenotypic abnormalities