Unit 2b human diversity change
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Unit 2B Human Diversity & Change. Inheritance Mutations. A mutation. A mutation is a permanent structural alteration in DNA.

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Unit 2b human diversity change

Unit 2BHuman Diversity & Change

Inheritance

Mutations


A mutation
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.



Somatic mutations
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
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
Point mutations

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


An analogy
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
Deletion

THEBOYATETHEBUN------

THEBOYTETHEBUN------

THE BOY TET HEB UN------

A


Duplication
Duplication

  • THEBOYATETHEBUN

  • THEBOYATETOYATETHEBUN

  • THE BOY ATE TOY ATE THE BUN


Inversion
Inversion

  • THEBOYATETHEBUN

  • THEBTETAYOHEBUN

  • THE BTE TAY OHE BUN


Insertion
Insertion

X

THEBOYATETHEBUN------

THEBOYAXTETHEBUN-----

THE BOY AXT ETH EBU N-----

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


Substitution
Substitution

THEBOYATETHEBUN------

THEBOYATETHEBUG------

THE BOY ATE THE BUG------

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

G


Non disjunction
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
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 karyotypes

Normal female - XX

Normal male - XY

Wessex Reg, Genetics Centre, Wellcome Images


Autosomal mutations
Autosomal mutations

Down’s syndrome – trisomy 21

Edward’s syndrome – trisomy 18

Wessex Reg, Genetics Centre, Wellcome Images


Down s syndrome trisomy 21
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
Sex chromosome mutations

Turner’s syndrome - XO

Klinefelter’s syndrome - XXY

Wessex Reg, Genetics Centre, Wellcome Images


Turner s syndrome xo
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
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
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
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.


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