Chromosomal Basis of Inheritance & Sex-Linked Genes Chapter 15 Section 1-2 Pgs. 286-292 Objective: I can describe how scientists proved genes are physically on chromosomes and use those ideas to solve sex-linked genetics problems.

Chromosome Theory of Inheritance Genes exist on chromosomes Behavior of genes (determined by Mendel in 1850’s) similar to behavior of chromosomes during mitosis/meiosis (seen in 1900’s), specifically…(2 concepts) Segregation (of alleles & homologs: disjunction) Independent assortment (of characters & non-homologous chromosomes) + crossing over (if on same chromosome) Theory was supported by Sutton, et al (read book!) But there was no definitive proof!

Proving Genes on Chromosomes Thomas Hunt Morgan & Fruit Flies Determined wild types for many characters Wild type = most prevalent (abundant) trait Is it AUTOMATICALLY dominant? Is it likely dominant? Others are “mutant” types A different “standard” to symbolize allele Wild-type = red = w+ Mutant = white = w

Proving Genes on Chromosomes Morgan’s Experiment This proves that wild-type red is 3:1 ratio still proved red was dominant, but only MALES were white-eyed! WHY?!? dominant

Proving Genes on Chromosomes Since sex of fruit fly involved, had to look at sex chromosomes (XX = female, XY = male) The only way for F2 sons to get white eyes is if gene for white eyes on X chromosome… This proves gene linkage What is “this”? Sex-linked traits (type of) Gene linkage !!! Wild-type = red = w+ Mutant = white = w

Sex Chromosomes A set of chromosomes that determines (genetic) sex of the offspring Other chrmsmes. = autosomes (determine “body”) Different organisms have different “systems” X-O system Z-W system X-Y system Diploid/Haploid system

Human Sex Chromosomes Women = XX Men = XY Which is more important for survival? X – Why? Who determines sex? Men with Y Punnett Squares can be for just 1 gene on chromosome or entire chromosome itself! A B C D E F a b c d e f A B C d E F a b c d e f X Y X X X Y D d D d d d

Inheritance of Sex Chromosomes Can treat sex as a totally different character! 2 traits: male / female Gene = entire chromosome (“alleles” = X or Y) Genetics Problem: The gene for eye color (found on autosome #10 in rabbits) is controlled by simple dominance with red eyes dominant over white. What are the chances that a heterozygous red-eye male and a white-eye female will have red-eye kids? 50% What are the chances that a heterozygous red-eye male and white-eye female will have a red-eye boy?

1 1 1 1 Looking @ 2 characters (each with own alleles) Eye Color: Red = White = R r Sex: Female-ness = Male-ness = X Y Because looking at 2 characters at same time… heterozygous red-eye male x white-eye female rrXX RrXY …must do… Genotype Count RrXX = RrXY = rrXX = rrXY = 1 1 1 1 Phenotype Count Red-eye male = Red-eye female= White-eye male = White-eye female= RX RY rX rY RrXX RrXY rrXX rrXY 1 1 1 Chance of having red-eye boy? 1 25% Chance that a son will have red eyes? 50%

Sexual Development Everyone is conceived as a “female” offspring when examine fetus under 4 months, genital area looks identical between males and females At 2 months (embryo), IF a Y chromosome is present, SRY gene activates Sex Determining Region on Y chromosome (sorry) Trigger that causes testes to develop Testes start producing testosterone which changes body chemistry and body parts Other genes involved too! At 4 months (fetus), can see physical difference http://www.baby2see.com/gender/external_genitals.html

Sex-Linked Genes X chromosome carries genes that are NOT involved in sex determination (still vital to survival) Will be inherited WITH the X chromosome (linked!) Traits determined by these genes are sex-linked! Y chrmsme mostly determines if male  why? Size difference! X bigger than Y  X carries more genes! Types of Sex-Linked X-linked genes Y-linked genes (most of problems are X-linked, because…)

Sex-Linked Traits – Geno./Pheno. Eye color is a sex-linked trait for fruit flies. Red eyes are dominant over white. Key: Red = White = Add as superscripts to X chromosome only! R r Females (♀) Males (♂) Red Eyes (Homozygous) X X X Y Red Eyes (Heterozygous) White eyes Only X chromosome can hold alleles XRY Red eye male No homozygous or heterozygous (hemizygous) R R R R r R r r r

Sex-Linked Traits – Punnett Sq. Cross a female fly that has homozygous red eyes with a white-eyed male fly CROSS: _______ x _______ ♀ ♂ What are the chances of having a red-eyed female? What % of daughters have red eyes? What % of sons will XR XR Xr Y Xr Y XR XR Xr XR Y 50% 100% 100%

Sex-Linked Traits – Punnett Sq. Cross a white-eyed female fly with a red-eyed male fly CROSS: _______ x _______ ♀ ♂ What are the chances of having a red-eyed male? What % of daughters have red eyes? What % of sons will have white eyes? Xr Xr XR Y XR Y Xr XR Xr Xr Y 0% 100% 100%

Sex-Linked Traits Conclusions Explains why sons tend to take after mothers Sons get only Y chromosome from dad Any traits on X can only come from mother Explains why daughters tend to take after fathers Fathers can contribute genes on X chromosome Daughters still get an X from mom, too Daughters/fathers seems similar when compared to sons/mothers (more relative feel) Sex-Linked truly proved that chromosomes hold genes (X NEVER crosses over with Y)

Solving Pedigree Charts

Human Sex-Linked Disorders Red-Green color-blindness X-linked recessive Duchenne muscular dystrophy (dies @ 20) Hemophilia (blood cannot clot) Hairy Ears Y-linked recessive Incontinentia Pigmenti (skin, eyes, nerves  lethal) X-linked dominant

X Inactivation in Female Mammals X-Inactive Specific Transcript Men have only 1 X chromosome Implies need only 1 X to survive normally Women have 2 X chromosomes (don’t need both) One of them inactivates (DNA methylation / XIST) Condenses into a compact object: Barr body If a woman is heterozygous, then which X will be inactivated? it’s random! X inactivation is random in EACH cell Thus, a female body is composed of some cells with XA and some cells with Xa

X Inactivation in Female Mammals Implications If each cell has XA and Xa Then XA expression dominates of Xa But if some cells have only 1 XA and other cells have 1 Xa (each cell sort of becomes hemizygous) Then some cells express dominant phenotype and others express recessive phenotype So, body becomes a mosaic! Human female sweat glands http://www.youtube.com/watch?v=BD6h-wDj7bw