Sex Determination and Sex -Linked Traits How

Sex Determination and Sex -Linked Traits

How do we determine sex? What is the role of biology, specifically genetics? • What are some other factors in sex determination? • What is the “Gender Revolution”?

The Gender Revolution http: //channel. nationalgeographic. com/gender-revolution-a-journey-withkatie-couric/videos/gender-revolution-extended-trailer/

Understanding Sex-linked Inheritance • How sex is determined • How characteristics encoded by genes on the sex chromosomes are inherited.

Sex is determined by a number of different mechanisms • The term sex refers to sexual phenotype. ex – a XXY person may have a female anatomy and is considered a “female” • Hermaphroditism – organisms that bear both male and female reproductive structures…. . they are “monoecious” …. one house • Organisms that are either male OR female are “ dioecious” meaning two houses. • Sex can be determined by: chromosomes, genes, and environment.

Chromosomal Sex-Determining Systems • Chromosome Theory of Inheritance: Genes are located on chromosomes. • Proved by discovery that sex of certain insects is determined by presence or absence of particular chromosomes • Ex – in grasshoppers female XX male XO

Types of chromosomes • Sex-chromosomes • Autosomes Even though we think sex is determined by the presence of the sex chromosomes, the individual genes located on the sex chromosomes are usually responsible for the sexual phenotypes.

• Heterogametic sex – if two types of gametes are produced in respect to the sex chromosomes (for us, males) • Homogametic sex – gametes all the same (for us, females)

Chromosomal Sex determination Systems • XX-XO • XX-XY • ZZ-ZW • Haplodiploidy

XX – XO systems • XX (females) are homogametic • XO (males) are heterogametic • Males have only one sex chromosome.

XX-XY Sex Determination • X and Y NOT generally homologous • They pair because their chromosomes are homologous in small regions called the pseudoautosomal regions in which they carry the same genes. • In humans at both tips of X and Y.

ZZ – ZW sex determination • Female ZW is heterogametic • Male ZZ Homogametic • Found in birds, snakes, butterflies, some amphibians, and some fishes.

Haplodiploidy • Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid.

What about sex determination in plants? • Most plants have both sexes on each plant. • Some plants have one sex per plant. Ex – Bryophytes (mosses) have separate sexes in their gametophyte stages.

Silene latiflora has X and Y chromosomes

Environment and Sex Determination • Sex can be determined by the environment • Limpets’ sex is determined by their order in a stack; the uppermost animals are always males. Slipper limpet

• In some reptiles, sex is determined by temperature during embryonic development. • In turtles, warm temps produce females.

Temperature-dependent sex determination in three reptile species: the American alligator (Alligator mississippiensis), the red-eared slider turtle (Trachemys scripta elegans), and the alligator snapping turtle (Macroclemys temminckii).

• So it seems there is no genetic predisposition for the embryo of a temperature-sensitive reptile to develop as either male or female, so the early embryo does not have a "sex" until it enters thermosensitive period of its development.

And then there are the spoon worms. • As a larvae, it is sexually undifferentiated, where it settles determines its sex. If the spoon worm larvae lands on the seafloor it becomes female. • Should the larvae come in contact with a female, it will be masculinized and sucked into the spoonworm’s body through her feeding proboscis, where it will spend the rest of its life in her uterus.

What about fruit flies? • Y does not determine maleness. Each fly’s sex is determined by a balance between the genes on their autosomes and genes on the X chromosome. • Sex is primarily determined by the X: A ratio, or the ratio of the number of X chromosomes to the number of sets of autosomes.

• Fruit flies have 8 chromosomes – 3 pairs of autosomes 1 pair of sex chromosomes

Drosophila: genic balance. Sex chromosome Autosome (A) complement X: A ratio Phenotype XX and AA 1. 0 female Xy and AA 0. 5 male XXy and AA 1. 0 female XX and AAA 0. 67 intersex X: A > 1 metafemale (developmental problems) X: A < 0. 5 metamale (sterile) X: A between 1. 0 and 0. 5 intersex (mix)

Am I a male or female? I have no idea. I can’t do math!

How number of sex chromosomes affect human development • Turner syndrome (XO) female • Klinefelters (XXY) male • sterile, phenotypic traits related to secondary sexual characteristics, and frequently developmental problems

Human Karyotypes Klinefelter Syndrome One or more Y and multiple X. Non-disjunction NORMAL Turner Syndrome XO

Klinefelter’s Turner’s

Maleness Determination of maleness is due to sry gene of Y chromosome

SRY gene • Encodes a protein transcription factor that binds to DNA and stimulates the transcription of other genes that promote differentiation of the testes. SRY becomes active about 6 wks after fertilization. • A XX male would have this gene. • A XY female would be missing this gene.

How does this work? • Upon SRY activation, the former neutral gonads develop into testes, which secrete testosterone and Mullerianinhibiting substance which causes the degeneration of the female reproductive ducts.

• SRY is the primary determinant of maleness in humans, other genes (some X-linked, others Y-linked, and others autosomal) also have roles in fertility and development of sex differences.

What about crossing-over involving the SRY gene? • When crossing over extends past the boundary of the pseudoautosomal region and includes the SRY gene, sexual development will most likely be adversely affected. • The rare occurrences of chromosomally XX males and XY females are due to such aberrant crossing over, in which the Y chromosome has lost—and the X chromosome has gained—this sexdetermining gene.

This could happen during meiosis to produce the sperm.

• Expression of gender is more complex than just sry gene. • Example: androgen insensitivity – XY genotype – Many female characteristics!

Androgen-insensitivity syndrome • In XY females • Vagina ends blindly, no uterus, oviducts, or ovaries • Pair of testes in abdominal area that produce testosterone • Cells have X and Y • Testosterone is produced (has SRY gene) but receptor is missing • Androgen-receptor gene on X chromosome

• So, the key to maleness and femaleness lies not in the genes, but in the control of their expression! OR NOT!

Sex Linked (X-linked) Traits – determined by Thomas Morgan • The mystery of the white-eyed Drosophila male

Human Color Blindness

Hemophilia X h. Y x X HX h Pedigree of hemophilia ½ XH ½ Xh ½ X h ¼ X HX h ¼ X h ½Y ¼ X HY ¼ X h. Y When working problems, indicate sex chromosomes.

Y-linked characteristics • 2/3 of Y consists of repeated short DNA sequences and contain no active genes • Only about 350 genes in human Y; only half code for proteins • Some genetic elements can influence expression of genes on autosomal and X chromosomes though.

• Mutations can form Y-linked markers and these can be traced to the male exclusively • Thomas Jefferson’s sons? ?

Dosage compensation for X chromosomes • Amt of gene product would be 2 X in females • How compensate: In fruit flies, X double activity of genes on the X • Lyon hypothesis: in females one of the two X’s is inactivated and forms a Barr body (observed initially in cats)

Barr bodies Lyon hypothesis The hypothesis that one X-chromosome is inactive during interphase in normal females and is represented in interphase cell nuclei as the sex chromatin body.

Number of Barr Bodies in Human Cells

X-Inactivation in cats Tortoiseshell Color

De-evolution of the Y chromosome • • When is testosterone the highest? Only males have testosterone? Only females have estrogen? SRY begins to be expressed at what age of development? • What is the NRY?

• What percent of functional genes of the genome on the Y? • Breaking that percent down, how many are used to produce sperm? • What is DAZ? • Where did the SRY gene come from? • Why has the Y chromosome become smaller over the years? • Where did DAZ come from? • What is the result if the DAZ gene is deleted in males?

The De-evolution of the Y

Back to the Gender Revolution https: //www. facebook. com/katiecouric/videos/10154790608436005/

So what does determine sex? • • • Chromosomes Hormones Gene expression Genitalia How you feel about your sex?

The End