©2000 Timothy G. Standish Genesis 1:27 27 So God created man in his own image, in the image of God created he him; male and female created he them.

©2000 Timothy G. Standish Sex Determination and Sex Chromosomes Timothy G. Standish, Ph. D.

©2000 Timothy G. Standish What Is Male and Female? What characteristics define being male or female? Male: –Produce sperm... –What are sperm? Female: –Produce eggs... –What are eggs? Organisms like Chlamydomonas are isogamous, producing morphologically indistinguishable gametes

©2000 Timothy G. Standish Many Plants and Animals are Hermaphrodites Plants like corn and peas have both male and female parts which produced sperm and eggs respectively Some animals, for example Caenorhapditis elegans, are also hermaphroditic In both cases there are not genetic differences producing dimorphic genders, instead the various parts develop as a result of developmental differences within the organism C. elegans produces males via non-disjunction

©2000 Timothy G. Standish Sex Determination Two ways in which sex can be determined: Environment: Turtles - Temperature of development Some fish - Social structure Chromosomes - Three methods: XO - Haploid/diploid, i.e., bees, haploid males diploid females ZW - Heterogametic (ZW) females, homogametic (ZZ) males, i.e., birds XY - Heterogametic (XY) males, homogametic (XX) females, i.e., humans and Drosophila

©2000 Timothy G. Standish X Chromosome Human and Drosophila Genes Are Easy To Find In humans and Drosophila, males are XY Thus males are haploid for the X chromosome Because of this, recessive genes on the X chromosome show up far more commonly in male than female phenotypes

©2000 Timothy G. Standish Human X-linked Recessive Genes Brown enamel - Tooth enamel appears brown rather than white Hemophilia - Two types: –A - Classic hemophilia, deficiency of blood clotting factor VIII –B - Christmas disease, deficiency of blood clotting factor IX

©2000 Timothy G. Standish X-linked Recessive Genes Related to sight Coloboma iridis - A fissure in the eye’s iris Color Blindness - Two types: –Deutan - Decreased sensitivity to green light –Protan - Decreased sensitivity to red light Congenital night blindness - Not due to a deficiency of vitimin A Mocrophthalmia - Eyes fail to develop Optic atrophy - Degeneration of the optic nerves

©2000 Timothy G. Standish Human Y Chromosome Centromere q arm p arm

©2000 Timothy G. Standish Human Y Chromosome Centromere q arm p arm Pseudoautosomal region (PAR)

©2000 Timothy G. Standish Human Y Chromosome Centromere Nonrecombining region of the Y (NRY) Pseudoautosomal region (PAR) Heterochromosin (essentially blank?) Sex-determining region Y (SRY): Testis-determining factor (TDF) location

©2000 Timothy G. Standish Variation In Chromosome Number - Polyploidy Polyploid individuals have more than two sets of chromosomes Many important commercial plants are polyploid: –Roses –Navel oranges –Seedless watermelons Polyploid individuals usually result from some sort of interruption during meiosis Pro or Metaphase I Interruption of meiosis Metaphase II 2n Gametes + 1n Gamete 3n Zygote

©2000 Timothy G. Standish Variation In Chromosome Number - Aneuploidy Polyploid humans are unknown, but individuals with extra individual chromosomes are known. Having extra chromosomes or lacking some chromosomes is called aneuploidy Aneuploid individuals result from nondisjunction during meiosis Metaphase IAnaphase I Zygote + +

©2000 Timothy G. Standish Aneuploidy In Humans Most human aneuploids spontaneously abort The most viable variations in chromosome number are those that deal with the sex chromosomes: XO - Turner’s Syndrome - Phenotypically females XXX…- “Super” females XYY… - “Super” Males - On average tend to be larger and less intelligent XXY - Klinefelter’s Syndrome - Phenotypically male Of the non-sex chromosome aneuploidys, Down’s Syndrome, extra chromosome 21, tends to be the most viable Down’s Syndrome is more common in children of mothers who gave birth after age 40

©2000 Timothy G. Standish Gene Dosage There seem to be elegant mechanisms for maintaining the correct dosage of genetic material in each cell When aneuploidy causes a change in the relative dose of one chromosome, problems result Another way in which dosage of genetic material can be changed is via macromutations

©2000 Timothy G. Standish The Lyon Hypothesis Having extra chromosomes causes problems (i.e., Downs Syndrome) Men have only one X chromosome and they are normal (at least they think so) Women have two X chromosomes and they are normal Mary Lyon proposed that the extra dosage of X chromosome that women have is compensated for by turning off one of the X chromosomes. This turned off chromosome can be observed as a “Barr Body” in metaphase female nuclei

©2000 Timothy G. Standish Consequences of X Chromosome Dosage Compensation During early development, X chromosomes are randomly turned off in female cells All daughter cells have the same X chromosome inactivated as their parental cell. Thus, females are a mosaic of patches of cells some patches expressing the genes on the paternal X chromosome, other patches expressing the maternal X chromome

©2000 Timothy G. Standish Consequences of X Chromosome Dosage Compensation XX At some point (probably later than the 4 cell stage) half the X chromosomes are turned off Daughter cells inherit the mother cell’s combination off and on X chromosomes Because of dosage compensation, females are thought to be a mosaic of patches of cells with each patch expressing the same X chromosome, but none expressing both chromosomes Different patches of cells inherit different act X chromosomes XX Cell division XX Zygote X X XX

©2000 Timothy G. Standish Why Calico Cats Are Usually Female Orange coat color is a sex-linked trait in cats (it is on the X chromosome) A female cat heterozygous for orange has skin patches expressing the orange X with the other X chromosome turned off. In other patches the opposite occurs.

©2000 Timothy G. Standish

Why Have More Than One Chromosome? If only one chromosome, no crossing over, thus all bad or good genes go to offspring...