Mutations all

AP Students

need to know

Mutations caused by

whole/part chromosomes

Chromosomal mutations

• In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

Having an abnormal number of

chromosomes is a condition known

as aneuploidy

Having 3 copies of a particular

chromosome is called a trisomy

Having 1 copy of a particular

chromosome is called a monosomy

Polyploidy is a

condition in which

an organism has

more than two

complete sets of

chromosomes

Recent research

has shown that

this Chilean

rodent is a

tetraploid

Very rare among

animals

Common in plants,

some fish, some

amphibians

Polyploidy - having more than two complete chromosome sets The normal state is having 2 of each chromosome

(homologous pairs) When organisms have 3 or 4 of each chromosome, that is

called polyploidy Triploidy (3n), tetraloidy (4n), hexploid (6n), octapolid (8n)

Very uncommon in animals Some fish and amphibians, mostly

Quite common in plants

Down

Syndrome

Caused by a trisomy of chromosome 21

1/700 births in US

The result of nondisjunction during meiosis I

Risk increases with the age of the mother

Mental retardation, increased risk of other diseases, shortened

life span, underdeveloped and likely sterile

Trisomy 18 – Edward’s syndrome low birth

weight, mental retardation, extra fingers and toes

Trisomy of sex

chromosomes

• Klinefeltersyndrome is the result of an extra chromosome in a male, producing XXY individuals

• Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans

Quick Think

Gene dosage, the number of active

copies of a gene, is important to

proper development.

Identify and describe two disorders

that are the result of improper gene

dosage

Alterations of Chromosome Structure Chromosomal mutations

include

Deletion

Inversion

Duplication

Translocation

Often occur during meiosis

Chromosomes break &

rejoin incorrectly

Deletion mutations

A piece of chromosome is lost during meiosis

Cri du chat is one condition that results -distinctive cry, severe retardation, shortened life span

Inversion Mutations

A double break where the piece reattaches, but backwards

Hemophilia is a inversion on the X chromosome

Duplication mutation

A piece breaks off from one chromosome and reattaches to the sister chromatid

Fragile X syndrome is caused by this, 2nd most common form of mental retardation after Downs syndrome

Translocation mutation

A piece breaks off one chromosome and

reattaches to a different chromosome

Burkitt’s lymphoma is caused by this

Sex-linked mutations in

humans

Colorblindness Colorblindness

Recessive

X chromosome

Men are more likely to be colorblind than women…WHY

Hemophilia

Recessive, X-linked

Prolonged bleeding when injured

Clots slow to form

Caused by a mutation for a gene on

the X chromosome that codes for a blood clotting

protein

Can be treated but not cured

Bleeding

(bruising) after an

injection

Duchenne muscular

dystrophy

Recessive, X-linked

1/3500 males in US

Progressive weakening of

muscles

Loss of coordination

Death by early 20s

Mutation in the gene that codes

for a muscle protein (dystrophin)

Point MutationsSmall changes in the DNA

sequence

Point Mutations

point mutation = a change in a single base pair in a gene

They can have catastrophic consequences, or none at all

3 main types:

Substitution

Insertion

Deletion

Substitution mutations

A base pair is replaced with a different base pair

Substitution Think of it like a sentence:

Normal sentence would read THE DOG BIT THE CAT

A point mutation might make the sentence read: THE DOG BIT THE CAR

This changes the meaning of the sentence, but not dramatically.

Example: sickle cell anemia

Advantages of sickle cell AA – no sickle cell, susceptible to the worst effects of

malaria

Aa – no sickle cell, but call pass recessive allele to

offspring, some protection from malaria

aa – sickle cell disease

This is called The Heterozygote Advantage

Changing a single

base may not cause

any change at all:

The changed base may

still code for the same

amino acid

Proline is coded for by

CCC, CCA, CCG, and

CCU,

This is called a silent

mutation

Insertions & Deletions

•Insertion - add extra letters

•Deletions – delete letters

•called frameshift mutations

• The 3 letter reading frame is

shifted

Frameshift Mutations Think of it as a sentence again:

THE DOG BIT THE CAT

Adding an extra letter makes it: THH EDO GBI TTH ECA T

It changes the entire sentence to nonsense. This kind of mutation has a more dramatic effect on the DNA sequence and is usually lethal

1.

2.

3.

What kind of mutation is represented

by 1, 2, and 3?

Will mutation 1 affect the resulting

protein?

Tay-Sachs Frame shift mutation – 4 added nucleotides (insertion)

Autosomal, recessive

Cystic Fibrosis Deletion frameshift mutation – 3 missing letters,

autosomal recessive

Breast/Ovarian Cancer BRCA I and BRCA II – usually a deletion mutation

Autosomal dominant

Huntington’s Disease Autosomal dominant

>37 CAG repeats on the short arm of chromosome 4