Fig. 15-6a

(a) The X-Y system

46 XY

46 XXParents

46 XY

46 XX

23 X

23 X

23 Y

or

or

Sperm Egg

+

Zygotes (offspring)

HELP! I need genes!

Meiosis & Fertilization

Fig. 13-8

Prophase I Metaphase I Anaphase I Telophase I andCytokinesis

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis

Centrosome(with centriole pair)

Sisterchromatids Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Sister chromatidsremain attached

Homologouschromosomesseparate

Cleavagefurrow

Sister chromatidsseparate Haploid daughter cells

forming

Phases of Meiosis

Fig. 13-UN1

Prophase I: Each homologous pair undergoessynapsis and crossing over between nonsisterchromatids.

Metaphase I: Chromosomes line up as homolo-gous pairs on the metaphase plate.

Anaphase I: Homologs separate from each other;sister chromatids remain joined at the centromere.

Fig. 13-UN2

F

H

The point of Meiosis I

is to separatehomologous pairs

Fig. 13-UN4

Meiosis Video

Changes of chromosome number or structure cause some genetic disorders

• Large-scale chromosomal alterations often lead to Spontaneous abortions (miscarriages) or cause a variety of developmental disorders

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Abnormal Chromosome Number

• 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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-13-1

Meiosis I

(a) Nondisjunction of homologous chromosomes in meiosis I

(b) Nondisjunction of sister chromatids in meiosis II

Nondisjunction

Fig. 15-13-2

Meiosis I

Nondisjunction

(a) Nondisjunction of homologous chromosomes in meiosis I

(b) Nondisjunction of sister chromatids in meiosis II

Meiosis II

Nondisjunction

Fig. 15-13-3

Meiosis I

Nondisjunction

(a) Nondisjunction of homologous chromosomes in meiosis I

(b) Nondisjunction of sister chromatids in meiosis II

Meiosis II

Nondisjunction

Gametes

Number of chromosomes

n + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n

• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred

• Offspring with this condition have an abnormal number of a particular chromosome

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• A monosomic zygote has only one copy of a particular chromosome

• A trisomic zygote has three copies of a particular chromosome

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Patau Syndrome (Trisomy 13)

Patau Syndrome (Trisomy 13)• cleft palate• fetuses with this condition rarely to go to term• so it occurs in only 1 in 6000 live births • It is rare for babies to survive for very long if

liveborn because of the multitude of anomalies • atrial septal defect • inguinal hernia• polydactyly of hands and feet• Sometimes cyclopia or absence of eyes

Edward Syndrome (Trisomy 18)• Sometimes due to translocation• Survival rate of Edwards Syndrome is very low • Approximately 95% die in utero. • Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive

their first year of life. • The median life span is five to fifteen days• One percent of children born with this syndrome live to age ten, typically in

cases of the less severe Edwards syndrome.• heart defects at birth • intestines protruding outside the body• Feeding and breathing difficulties• Microcephaly• Clenched fists

• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes– Triploidy (3n) is three sets of chromosomes– Tetraploidy (4n) is four sets of chromosomes

• Polyploidy is common in plants, but not animals• Polyploids are more normal in appearance than

aneuploids

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polyploidy

Fig. 15-14

Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes in chromosome structure:– Deletion removes a chromosomal segment– Duplication repeats a segment– Inversion reverses a segment within a chromosome– Translocation moves a segment from one

chromosome to another

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-15

DeletionA B C D E F G H A B C E F G H(a)

(b)

(c)

(d)

Duplication

Inversion

Reciprocaltranslocation

A B C D E F G H

A B C D E F G H

A B C D E F G H

A B C B C D E F G H

A D C B E F G H

M N O C D E F G H

M N O P Q R A B P Q R

Fig. 15-17

Normal chromosome 9

Normal chromosome 22

Reciprocaltranslocation Translocated chromosome 9

Translocated chromosome 22(Philadelphia chromosome)

Disorders Caused by Structurally Altered Chromosomes

• The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5

• A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood

• Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes

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Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number and structure are associated with some serious disorders

• Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond

• These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Down Syndrome (Trisomy 21)

• Down syndrome is an aneuploid condition that results from three copies of chromosome 21

• It affects about one out of every 700 children born in the United States

• The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-16a

Fig. 15-16b

Autosomes arePairs 1-22

Sex chromosomes-XX

Click on photo:

Click on microscope

click on diagram

Aneuploidy of Sex Chromosomes

• Nondisjunction of sex chromosomes produces a variety of aneuploid (or abnormal # of chromosomes) conditions

• Klinefelter syndrome 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

• TOTAL # of chromosomes ????

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•1 out of every 2,000-2,500 female live births •Almost all (95%) have

•Short stature •Loss of ovarian function

•Caused by the absence of a set of genes from the short arm of one X chromosome •Single X chromosome

•75-80% single X is contributed by mother •X-O = Female (Turner Syndrome) •X-X-Y=Male (Klinefelter Syndrome)

•Hearing loss due to otosclerosis is common in adults

•Appearance •Short stature

•Absent adolescent growth spurt •Average adult height is 4 feet 8 inches •Loss of SHOX gene necessary for growth of long bones

•Short fingers (4th metacarpal)

Turner’s Syndrome

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