Fig. 15-6a (a) The X-Y system 46 XY 46 XX Parents 46 XY 46 XX 23 X 23 X 23 Y or Sperm Egg + Zygotes (offspring) HELP! I need genes!

Fig Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase IIAnaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow Sister chromatids separate Haploid daughter cells forming

Fig. 13-UN1 Prophase I : Each homologous pair undergoes synapsis and crossing over between nonsister chromatids. 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 separate homologous 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Meiosis I (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Nondisjunction

Fig Meiosis I Nondisjunction (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Meiosis II Nondisjunction

Fig 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 nn

Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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)

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

Fig

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 Deletion A B C D E F G HA B C E F G H (a) (b) (c) (d) Duplication Inversion Reciprocal translocation 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 RA B P Q R

Fig Normal chromosome 9 Normal chromosome 22 Reciprocal translocation 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 a

Fig b Autosomes are Pairs 1-22 Sex chromosomes- XX Click on photo: Click on microscope

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

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 (4 th metacarpal)

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