Unit 2BHuman Diversity & Change

Inheritance

Mutations

A mutation

• A mutation is a permanent structural alteration in DNA.

• In most cases, DNA changes either have no effect or cause harm - occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.

NIH - National Human Genome Research Institute

Somatic mutations

• A somatic mutation is a DNA change that occurs in body cells, other than the egg or sperm (germ cells) – they cannot be passed on to offspring.

Germ line mutations

• A germ line mutation is a DNA change that occurs in the egg or sperm. Germ line mutations can be passed on to offspring.

Point mutations

• The simplest type of mutation is a point mutation. This involves a change in a single base pair.

An analogy

The following short sentence represents a sequence of nucleotides in a fragment of DNA:

THEBOYATETHEBUN-----

This is read in codons (3-letter segments)

THE BOY ATE THE BUN------

Deletion

THEBOYATETHEBUN------

THEBOYTETHEBUN------

THE BOY TET HEB UN------

A

Duplication

• THEBOYATETHEBUN

• THEBOYATETOYATETHEBUN

• THE BOY ATE TOY ATE THE BUN

Inversion

• THEBOYATETHEBUN

• THEBTETAYOHEBUN

• THE BTE TAY OHE BUN

Insertion

THEBOYATETHEBUN------

THEBOYAXTETHEBUN-----

THE BOY AXT ETH EBU N-----

In the above cases part, or all, of the sentence is no longer readable.

X

Substitution

THEBOYATETHEBUN------

THEBOYATETHEBUG------

THE BOY ATE THE BUG------

In this case, the meaning is changed but sentence is still readable.

G

Non-disjunction

• A non-disjunction occurs if homologous chromosomes fail to separate correctly and migrate to opposite poles during meiosis.This results in too many chromosomes in some daughter cells and too few in others.The most common disease resulting from a non-disjunction is Down’s syndrome, or trisomy 21, where there are three copies of chromosome 21.

Cytogenic map

This is how chromosomes look when stained and examined under a microscope. The light and dark bands give each chromosome a unique appearance. This feature is useful for the study of karyotypes, which allows scientists to look for chromosomal alterations.

Normal karyotypes

Normal female - XX Normal male - XY

Wessex Reg, Genetics Centre, Wellcome Images

Autosomal mutations

Down’s syndrome – trisomy 21 Edward’s syndrome – trisomy 18

Wessex Reg, Genetics Centre, Wellcome Images

Down’s syndrome – trisomy 21

• People with Down’s syndrome have an extra copy of chromosome 21.

• Affected people have varying degrees of mental retardation, short stature, and a flattened facial profile.

Wellcome Photo Library

Sex chromosome mutations

Klinefelter’s syndrome - XXY Turner’s syndrome - XO

Wessex Reg, Genetics Centre, Wellcome Images

Turner’s syndrome - XO

• The Turner’s syndrome karyotype has only one X chromosome and no Y chromosome.

• Women with Turner’s syndrome are usually short and have a number of abnormal physical features. Their ovaries never develop so they are infertile, and never menstruate.

Wellcome Photo Library

Klinefelter’s syndrome - XXY• The Klinefelter’s syndrome

karyotype has an extra X chromosome (XXY).

• Men with Klinefelter’s syndrome do not produce enough testosterone. This results in underdeveloped secondary sex characteristics, breast enlargement, small testes and infertility due to lack of sperm.

Wellcome Photo Library

Factors affecting the outcome of a mutation

• The type of cell affected (somatic or germ line)

• The type of mutation (e.g. substitution, insertion)

• The extent of the mutation (e.g. point, non-disjunction)

• The area of the gene affected (coding or non-coding)

Mutagens

• A mutagen is anything that causes a mutation (i.e. causes a permanent change in a cell’s DNA).

• Most mutagens are either chemicals or physical agents such as radiation.

• Some mutations are spontaneous, occurring as a result of errors during DNA replication or repair.