Meiosis 1

Meiosis 2

Random fertilization Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Michael & Kirk Douglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

Mitosis vs. Meiosis

Changes in Chromosomes Number Euploidy = correct # of chromosomes Aneuploidy = a change in chromosomes number due to non-disjunction during meiosis Monosomy- only 1 copy of an individual chromosome Trisomy- 3 copies of an individual chromosome

Primary nondisjuction = Meiosis I Secondary nondisjuction = Meiosis II

Trisomy 21 : Down Syndrome Delayed mental and social skills Decreased muscle tone at birth Asymmetrical or odd-shaped skull Small skull Small mouth with protruding tongue Broad short hands Increased risk of developing Leukemia and Alzheimer’s later in life

Trisomy 18 : Edward’s Syndrome Most children die in the first year of life, some have lived 10 years Growth deficiency Feeding difficulties Breathing difficulties Developmental delays Mental Retardation Overlapped, flexed fingers Webbing of the second and third toes Clubfeet Structural heart defects at birth

Trisomy 13 : Patau Syndrome Mental retardation, severe Seizures Small head Scalp defects Cleft lip and/or palate Eyes close set (hypotelorism) –may fuse Extra digits (polydactyl) Hernias Undescended testicle Children die in the first year of life

Procedure: Amniocentesis and Karyotyping

Karyotyping

https://youtu.be/BD6h-wDj7bw

Changes in Sex Chromosome # SRY gene (located on short arm of Y chromosome) -hormone= testis-determining factor

Turners Syndrome (XO) – missing Barr Body Kleinfelter’s Syndrome (XXY) Swyer Syndrome (XY female) Y chromosome is missing the SRY gene - The uterus and fallopian tubes are normally-formed, but the gonads (ovaries or testes) are not functional; affected individuals have undeveloped clumps of tissue called streak gonads. Because of the lack of development of the gonads, Swyer syndrome is also called 46,XY complete gonadal dysgenesis.

La Chapelle Syndrome (XX male) XX male syndrome Caused by unequal crossing over between the X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the SRY gene. When this X combines with a normal X from the mother during fertilization, the result is an XX male. Poly-X Females (XXX, XXXX) Triple X syndrome, also called trisomy X or 47,XXX, is characterized by the presence of an additional X chromosome in each of a female's cells. Although females with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most females with triple X syndrome have normal sexual development and are able to conceive children. Triple X syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills. Seizures or kidney abnormalities occur in about 10 percent of affected females. This condition occurs in about 1 in 1,000 newborn girls. Five to 10 girls with triple X syndrome are born in the United States each day.

Jacob’s Syndrome (XYY males) What is 47,XYY syndrome? 47,XYY syndrome is characterized by an extra copy of the Y chromosome in each of a male's cells. Although males with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most males with 47,XYY syndrome have normal sexual development and are able to father children. 47,XYY syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills. A small percentage of males with 47,XYY syndrome are diagnosed with autistic spectrum disorders This condition occurs in about 1 in 1,000 newborn boys.

Chromosomal Mutations

Deletion Syndromes Williams Syndrome (deletion of a piece of chromosome 7) Cri du chat (cat’s cry) (deletion of a piece of chromosome 5)

Spermatogenesis Production of sperm Occurs in the testes Continuous and prolific Each ejaculation contains ~100-650 million sperm Occurs in the testes Seminiferous tubules

Spermatogonia (stem cells that give rise to sperm) are located at the periphery of each seminiferous tubule Spermatogonia are diploid Developing sperm move toward the central opening (lumen) of the tubule as they undergo meiosis and differentiation 4 cells result Develop into mature sperm Haploid

The structure of sperm Campbell

Oogenesis The development of ova Mature, unfertilized eggs cells Happens in the ovary Oogonia (stem cells that give rise to ova) Multiply and begin meiosis STOPS at prophase 1 At this phase, the cells are called primary oocytes Remain in this phase until the onset of puberty, when they are activated by hormones Activated by LH and FSH

Meiosis then STOPS again Beginning at puberty, FSH periodically stimulates a follicle to grow and induces its primary oocyte to complete meiosis 1 and start meiosis 2 Meiosis then STOPS again The secondary oocyte, released during ovulation, does not continue meiosis right away Penetration of the egg cell by sperm triggers the completion of meiosis Then meiosis is complete LH stimulates the completion of meiosis

Campbell Growing follicle

Major difference #1 1. unevenness in the mitotic division/cytokinesis of oogenesis Almost all the cytoplasm  1 daughter cell (secondary oocyte) 3 polar bodies which degenerate Spermatogenesis = four mature sperm

Major difference #2 The cells that produce sperm continue to divide by mitosis throughout life (males) Not the case for women *born with all the primary oocytes

Major difference #3 Oogenesis has long resting periods Sperm is produced in an uninterrupted sequence