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Neurospora - a haploid fungus; products of meiosis occur in a linear array; arrangment can be used to calculate the map distance between a single gene and the centromere. Distance between gene and centromere = (0.5 x (# of second division segregants))/total asci scored) x 100 Sex Determination and Sex Chromosomes Cpt. 7 Asexual Reproduction Parthenogenesis: development of a new individual from an unfertilized egg (Cnemidophorus uniparens whiptail lizard) Adventitious embryony: development of a new individual from a single somatic cell (e.g. millet Pennisetum squamulatum) Vegetative reproduction: development of a new individual from a group of somatic cells (e.g. Iris bulbs) Chlamydomonas - the “cockroach” of the algae world most of life spent in haploid vegetative phase gametes formed under unfavorable nutrient conditions there are two types of isogametes (+ and -). the only types of matings involve opposite types of gametes: + and - or - and + Chlamydomonas - the “cockroach” of the algae world most of life spent in haploid vegetative phase gametes formed under unfavorable nutrient conditions there are two types of isogametes (+ and -). the only types of matings involve opposite types of gametes: + and - or - and + Zea mays - corn; cultivated from a wild grass in Mexico monoecious (both anisogametes in a single plant) Caenorhabditis elegans - roundworm; popular in genetic studies because it is known that the hermaphroditic adult has exactly 959 cells; genome has been sequenced X and Y chromosomes were first linked to sex determination early in the 20th century XX/XO sex determination In 1906, Edmund B. Wilson demonstrated that females of the hemipteran insect Protenor contain 14 chromosomes, including 2 X chromosomes. Oogenesis produces gametes with 7 chromosomes. Males contain 13 chromosomes, including a single X. Spermatogenesis produces gametes with either 6 (without a X) or 7 (with a X) chromosomes. X and Y chromosomes were first linked to sex determination early in the 20th century XX/XY sex determination Wilson also experimented with Lygaeus turicus. The species has 12 autosomes. In addition, females have 2 X chromosomes, while the males have a single X and a smaller heterochromosome labeled the Y chromosome. Oogenesis produces (6A + X) gametes. Spermatogenesis produces (6A + X) and (6A + Y) gametes. Other sex chromosome configurations Homogametic sex: sex that produces uniform gametes with regard to chromosome numbers and types (female humans, female Protenor, female Lygaeus) Heterogametic sex: sex that produces unlike gametes with regard to chromosome numbers and types (male humans, male Protenor, male Lygaeus) In some organisms, the heterogametic sex is the female instead of the male. To designate this, we use the notation ZZ/ZW for the sex chromosomes instead of XX/XY. Examples where the female is the heterogametic sex include birds, butterflies, some reptiles, and at least one species of plant. How was it determined that the Y chromosome plays a role in sex determination? By studying the genetics of mutant phenotypes Klinefelter syndrome (47, XXY): 2 per 1000 male births; male genitalia and ducts, but the testes are rudimentary and fail to produce sperm. They are generally tall and have long arms and legs and large hands and feet. Also have some feminine characteristics (slight enlargement of breasts, rounded hips). Turner syndrome (45, X): 1 per 2000 female births; female genitalia and ducts, but the ovaries are rudimentary. They are generally short in stature (< 5 ft), have skin flaps on the back of their neck, and underdeveloped breasts. A broad, shieldlike chest is sometimes noted. Intelligence is usually normal. Other sex chromosome abnormalities 47, XXX syndrome: 1 per 1200 female births; results in a female; the expression of this chromosomal abnormality is highly variable; some women with it are perfectly normal. In other cases, underdeveloped sex characteristics, sterility, and mental retardation may occur 47, XXY syndrome: A 1965 study of a male prison population in Scotland found that 9 of 315 males had this karyotype. All 9 were above average in height and all had been incarcerated because of antisocial criminal acts. 7 of the 9 were of subnormal intelligence and all had personality disorders. Initiated a large study in 1974 to identify to identify XXY individuals at birth and track their behavioral patterns. Study abandoned in 1975. The Y chromosome pseudoautosomal regions (PAR) homologous with regions on X; synapsis and recombination occurs during meiosis. nonrecombining region (NRY) - everything else. euchromatin - region that contains functional genes heterochromatin - region that lacks genes sex-determining region Y (SRY) - gene that controls male sexual development; produces testisdetermining factor (TDF), a product that triggers the formation of testes from undifferentiated embryonic gonadal tissue Dosage compensation prevents excessive expression of X-linked genes in humans and other mammals Since females have two copies of the X chromosome and males only one, there is the potential for females to produce twice as much of each gene product for all X-linked genes. In mammals, one of the X chromosomes in females is inactivated in cells early in the embryonic cycle These inactivated chromosomes are called Barr bodies, and are observable as darkly staining bodies in a Feulgen reaction for DNA. Dosage compensation prevents excessive expression of X-linked genes in humans and other mammals In females, one X is from the father and one is from the mother. So, which one is inactivated? The Lyon Hypothesis postulates that inactivation occurs randomly in somatic cells at a point early in embryonic development. Thus females are mosaics for all heterozygous X-linked alleles. Depending on the embryonic origin of a cell, the adult may express one or the other allele. The Lyon Hypothesis postulates that inactivation occurs randomly in somatic cells at a point early in embryonic development. Thus females are mosaics for all heterozygous X-linked alleles. Depending on the embryonic origin of a cell, the adult may express one or the other allele. Black and yellow-orange patches of female tortoise-shell and calico cats due to Lyon Hypothesis. Neither occur in males because males are hemizygous (only have one X). Sex Determination in Drosophila Drosophila has a haploid number of 4, composed of 3 autosomes and 1 sex chromosome. They have XX/XY inheritance. In 1916, Calvin Bridges performed a series of experiments that elucidated sex determination in Drosophila Nondisjunction during meiosis produced abnormal gametes, some with n+1 chromosomes, some with n-1 chromosomes. Bridges crossed these with normal haploid gametes and found that the resulting XXY flies were normal females and the XO flies were sterile males. Concluded that Drosophila Y lacks male determining factors, but does contain genetic information related to male fertility.