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Chapters 6.14-6.18

Meiosis I Review

Meiosis II: Review

Worksheet 3

Formation of Sex Cells in Humans

• The formation of gametes in animals is called gametogenesis

• In humans, gametes are produced in the testes (males) and in the ovaries (female)

Spermatogenesis (Males)

• A primary spermatocyte (diploid sex cell) undergoes meiosis I and II and forms 4 spermatids (haploid), which will develop into sperm

• Males can produce one billion sperm cells every day!

Oogenesis (Females)

• A primary oocyte (diploid sex cell) undergoes meiosis I, but the division of the cytoplasm is unequal, resulting in the formation of a polar body

• The larger cell (ootid) and the polar body both go through meiosis II• However, the polar bodies die and the nutrients are absorbed by the

body• Only one egg (ovum) is produced by meiosis• Women have about 300 000 oocytes, of which only 400 – 500 will be

released during the reproductive years

Worksheet 4

Sex Chromosomes • sex chromosomes: the pair of chromosomes that have a role in

the sex of an individual• autosomes: the chromosomes not involved in sex determination

• Although they are physically different, the X and the Y chromosomes are able to synapse for part of their length and function as homologous chromosomes during meiosis.

Sex Determination in Humans• In humans, xx is female,

xy is male. • Sex determination

depends on the sex chromosome inherited from the father, who is xy.

Sex Determination in Non-Humans• In birds, the mother

determines the sex of the child. – WZ is female, ZZ is male

• In ants, bees, and wasps, sex is determined by the number of chromosome sets an individual has.– males are haploid and females

are diploid

• In turtles, sex is determined by the environment, particularly the temperature at which the eggs are incubated. – HOT=female, COLD=male

Amniocentesis• In Amniocentesis, fetal cells are

obtained from 2 tablespoons of amniotic fluid

• A 3 to 4 inch needle is pushed through the abdomen and into the amniotic fluid.

• Do 3 to 4 months after pregnancy.

• This procedure is done without an anaesthetic.

• The cells are then cultured and a karyotype or other testing can be done.

• This test can be used to detect genetic disorders such as downs syndrome (trisomy 21).

Chorionic Villus Sampling (CVS)• CVS- cells are obtained from

the chorion of the placenta (tissue from the placenta) using a syringe.

• A needle is inserted through the abdomen or through the vagina and cervix, using the ultrasound for guidance.

• Do after 10 or 12 weeks after pregnancy.

• The cells are then cultured and a karyotype or other testing can be done.

• This test can be used to detect genetic disorders such as downs syndrome (trisomy 21).

Karyotype Chart• karyotype chart: a picture of chromosomes

arranged in homologous pairs

• monosomy: the condition where there is a single chromosome in place of a homologous pair

• trisomy: the condition where there are three homologous chromosomes in place of a homologous pair

Abnormal Meiosis: Nondisjunction

• Nondisjunction occurs when two homologous chromosomes move to the same pole during meiosis. The result is that one of the daughter cells will be missing one chromosome while the other will retain an extra chromosome.

• -can occur in any cell during mitosis BUT effects are most devastating during the formation of sex cells in meiosis.

• Can cause Down Syndrome. • Nondisjunction means “doesn’t break apart”.

Down SyndromeDown syndrome: a trisomic disorder in which a zygote receives

three homologous chromosomes for chromosome pair number 21

-regardless of race: a round, full face; enlarged and creased tongue; short height; and a large forehead.

-1 in 1000 babies is born with Down syndrome

Down Syndrome• increased risk as mother gets older

Turner Syndrome (X)

Turner syndrome: a monosomic disorder in which a female has a single X chromosome

-the individual has 45 chromosomes -The only condition where humans can survive without one of a pair of chromosomes. -1 in every 5000 female babies-In 98% of fertilized eggs, the egg is spontaneously aborted.

Turner Syndrome

• short (average is 4 feet and 8 inches)

• web of skin between neck and shoulders

• sterile• breasts and other secondary

sex characteristics develop incompletely

• normal intelligence but individuals do have learning difficulties

Klinefelter Syndrome (XXY)

Klinefelter syndrome: a trisomic disorder in which a male carries an XXY condition

-child appears to be a male at birth-however, as he enters sexual maturity, he

begins producing high levels of female sex hormones.

Males with Klinefelter syndrome are sterile. -1 in 1000 males have genotype XXY

Klinefelter Syndrome (XXY)

• Small testes• Female features including

breast development• Long limbs. Slightly taller

than average. (about 6 feet)• Language impairments,

speak later. • Might have more X

chromosomes: XXXY or XXXXXY. These individuals have mental retardation.

Super Males (XYY)

• occurs in 1 in 1000 males• very tall (about 6 feet 2 inches)• moderate to severe acne• average to slightly lower than average

intelligence• most men who are XYY don’t even know it!

Metafemales (XXX)

• occurs in 1 in 1000 females• sterile• no other physical or mental problems

Review

Worksheet 5

Lab Part 2

Tuesdayhttp://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.htmlHand in your lab at the end of class. WednesdayInternet Search WorksheetHand in at the end of class