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 Sex Determination Determination

and Sex and Sex ChromosomesChromosomes

Cpt. 7Cpt. 7

Asexual ReproductionAsexual Reproduction Parthenogenesis: development of a new Parthenogenesis: development of a new

individual from an unfertilized egg individual from an unfertilized egg ((Cnemidophorus uniparens Cnemidophorus uniparens whiptail whiptail lizard)lizard)

Adventitious embryony: development of Adventitious embryony: development of a new individual from a single somatic a new individual from a single somatic cell (e.g. millet cell (e.g. millet Pennisetum Pennisetum squamulatumsquamulatum))

Vegetative reproduction: development of Vegetative reproduction: development of a new individual from a group of somatic a new individual from a group of somatic cells (e.g. cells (e.g. Iris Iris bulbs)bulbs)

Chlamydomonas - the “cockroach” of the algae world

most of life spent in haploidvegetative phase

gametes formed underunfavorable nutrientconditions

there are two types ofisogametes (+ and -). the onlytypes of matings involveopposite types of gametes:+ and - or - and +

Chlamydomonas - the “cockroach” of the algae world

most of life spent in haploidvegetative phase

gametes formed underunfavorable nutrientconditions

there are two types ofisogametes (+ and -). the onlytypes of matings involveopposite types of gametes:+ and - or - and +

Zea mays - corn; cultivated from a wild grass in Mexicomonoecious (both anisogametes in a single plant)

Caenorhabditis elegans - roundworm; popular in geneticstudies because it is known that the hermaphroditic adulthas exactly 959 cells; genome has been sequenced

X and Y chromosomes were first linked to sexdetermination early in the 20th century

XX/XO sex determinationIn 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 sexdetermination early in the 20th century

XX/XY sex determinationWilson also experimented withLygaeus turicus. The species has12 autosomes. In addition, femaleshave 2 X chromosomes, while themales have a single X and asmaller heterochromosome labeledthe Y chromosome. Oogenesisproduces (6A + X) gametes.Spermatogenesis produces(6A + X) and (6A + Y) gametes.

Other sex chromosome configurations

Homogametic sex: sex that produces uniform gametes with regardto chromosome numbers and types (female humans, female Protenor, female Lygaeus)

Heterogametic sex: sex that produces unlike gametes with regardto chromosome numbers and types (male humans, male Protenor, male Lygaeus)

In some organisms, the heterogametic sex is the female instead of themale. To designate this, we use the notation ZZ/ZW for thesex 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 wereabove 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 testis-determining factor (TDF), a product that triggers the formation of testes from undifferentiated embryonic gonadal tissue

Dosage compensation prevents excessive expression ofX-linked genes in humans and other mammals

Since females have two copies ofthe X chromosome and males onlyone, there is the potential for femalesto produce twice as much of eachgene product for all X-linked genes.

In mammals, one of the X chromosomes in females is inactivatedin cells early in the embryonic cycle

These inactivated chromosomes arecalled Barr bodies, and are observableas darkly staining bodies in a Feulgenreaction for DNA.

Dosage compensation prevents excessive expression ofX-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 insomatic 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 insomatic 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-shelland calico cats due to Lyon Hypothesis. Neither occur inmales because males are hemizygous (only have one X).

Sex Determination in Sex Determination in DrosophilaDrosophila

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 thatelucidated sex determination in Drosophila

Nondisjunction during meiosis produced abnormal gametes, somewith n+1 chromosomes, some with n-1 chromosomes. Bridgescrossed these with normal haploid gametes and found that theresulting XXY flies were normal females and the XO flies weresterile males. Concluded that Drosophila Y lacks male determiningfactors, but does contain genetic information related to male fertility.