1. Independent segregation at metaphase I Each pair of chromosomes independently aligns at the cell...

1. Independent segregation at metaphase I Each pair of chromosomes independently aligns at the

cell equator; equal probability of the maternal or paternal chromosome going to a pole

The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes

2. Random fertilization The combination of each unique sperm with each

unique egg increases genetic variability

3. Genetic recombination (crossing-over)

3 Ways to Achieve Genetic Variation Through Sexual Reproduction

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility 1 Possibility 2

Metaphase II

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility 1 Possibility 2

Metaphase II

Combination 1

Gametes

Combination 2 Combination 3 Combination 4

Homologous chromosomes can carry different versions of genes

Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes

– Homologous chromosomes may have different versions of a gene at the same locus

– One version was inherited from the maternal parent, and the other came from the paternal parent

– Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene

Copyright © 2009 Pearson Education, Inc.

Brown coat (C); black eyes (E) White coat (c); pink eyes (e)

Offspring (next page)

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Coat-colorgenes

Chromosomes ofthe four gametes

Meiosis

PinkWhite

BlackBrown

Eye-colorgenes

C

e

E

c

C

e

E

c

C

e

E

c

Genetic recombination is the production of new combinations of genes due to crossing over

Crossing over involves exchange of genetic material between homologous chromosomes

– Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over

– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids

Crossing over further increases genetic variability

Centromere

ChiasmaTetrad

Breakage of homologous chromatids

Coat-colorgenes

Eye-colorgenes

C

(homologous pair ofchromosomes in synapsis)

E

c e

Tetrad

C E

c e

Joining of homologous chromatids2

C E

c e

Chiasma

1

Separation of homologous chromosomes at anaphase I

C E

c e

Chiasma

Separation of chromatids at anaphase II andcompletion of meiosis

C E

c e

c E

C e

c e

c E

C E

C e

Parental type of chromosome

Gametes of four genetic types

Recombinant chromosome

Parental type of chromosome

Recombinant chromosome

4

3

Changing Chromosome Number or Structure:

Generally not a good thing

A karyotype shows stained and magnified versions of chromosomes

– Karyotypes are produced from dividing white blood cells, stopped at metaphase

– Karyotypes allow observation of

– Homologous chromosome pairs

– Chromosome number

– Chromosome structure

8.19 A karyotype is a photographic inventory of an individual’s chromosomes

Packed redand white bloodcells

CentrifugeBloodculture

Fluid1

Packed redand white bloodcells

CentrifugeBloodculture

Fluid1

Hypotonicsolution

2

Packed redand white bloodcells

CentrifugeBloodculture

Fluid1

Hypotonicsolution

2

3

Fixative

Whitebloodcells

Stain

4

Centromere

Sisterchromatids

Pair of homologouschromosomes

5

http://learn.genetics.utah.edu/content/begin/traits/karyotype/

Trisomy 21 involves the inheritance of three copies of chromosome 21

– Trisomy 21 is the most common human chromosome abnormality

– An imbalance in chromosome number causes Down syndrome, which is characterized by

– Characteristic facial features

– Cardiac defects

– Mental deficits

– Variation in characteristics

– Association with Alzheimer’s Disease

– The incidence increases with the age of the mother

8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome

Infa

nts

wit

h D

ow

n s

ynd

rom

e(p

er 1

,000

bir

ths)

Age of mother

90

70

60

50

40

30

20

10

0

80

20 40353025 5045

Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis

– During Meiosis I

– Both members of a homologous pair go to one pole

– During Meiosis II

– Both sister chromatids go to one pole

Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes

Accidents during meiosis can alter chromosome number

Nondisjunctionin meiosis I

Nondisjunctionin meiosis I

Normalmeiosis II

Nondisjunctionin meiosis I

Normalmeiosis II

n + 1

Gametes

Number of chromosomes

n + 1 n – 1 n – 1

Normalmeiosis I

Nondisjunctionin meiosis II

Normalmeiosis I

Nondisjunctionin meiosis II

Normalmeiosis I

Gametes

Number of chromosomes

n + 1 n – 1 n n

Sex chromosome abnormalities tend to be less severe as a result of

– Small size of the Y chromosome

– X-chromosome inactivation

– In each cell of a human female, one of the two X chromosomes becomes tightly coiled and inactive

– This is a random process that inactivates either the maternal or paternal chromosome

– “Barr-body” formation

Abnormal numbers of sex chromosomes do not usually affect survival

What does a Barr body look like?

Polyploid species have more than two chromosome sets

– Observed in many plant species

– Seen less frequently in animals

Example

– Diploid gametes are produced by failures in meiosis

– Diploid gamete + Diploid gamete Tetraploid offspring

– The tetraploid offspring have four chromosome sets

New species can arise from errors in cell division

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polyploidy.html

Structure changes result from breakage and rejoining of chromosome segments

– Deletion is the loss of a chromosome segment– Duplication is the repeat of a chromosome

segment – Inversion is the reversal of a chromosome

segment– Translocation is the attachment of a segment

to a nonhomologous chromosome; can be reciprocal

Altered chromosomes carried by gametes cause birth defects

Chromosomal alterations in somatic cells can cause cancer

Alterations of chromosome structure can cause birth defects and cancer

Copyright © 2009 Pearson Education, Inc.

Deletion

Inversion

Duplication

Homologouschromosomes

Reciprocaltranslocation

Nonhomologouschromosomes

Chromosome 9

“Philadelphia chromosome”

Activated cancer-causing gene

Reciprocaltranslocation

Chromosome 22