Biology 7.4 and 7.5 Sex Determination The X & Y chromosomesare involved in producingthe sex of an individualthrough a process calledsex determination. In humans, sex isdetermined by the malesince males producesperm that carry eitheran X or a Y chromosome. The X and Y chromosomes The Y chromosome issmaller than the Xchromosome andcarries fewer genes. Sex Determination Birds: The male is ZZ The female is ZW
The female determinesthe sex of the offspring. Sex Determination Sex can be determinedby the incubationtemperature of the egg. For example, in turtles, females are born if theeggs are very hot, andmales are born at acooler temperature. Sex Determination Sex depends on fertilization.
If the eggs are fertilized, females are born. If the eggs are notfertilized, males areborn. Sex Determination Some species of bonyfish change their sexafter maturation. All individuals becomefemale unless they aredeflected from thatpathway by social signalsuch as dominanceinteractions. Sex Determination The nematode C.elegans can either bemale or have both maleand femalereproductive organs. Individuals with maleand femalereproductive organs arehermaphrodites. Questions How is sex determined in humans?
How is sex determined in birds? How is sex determined in turtles? How is sex determined in wasps, ants, and bees? How is sex determined in bony fish? What are the possible sexes of C. elegans? X-linked genes versus Y-linked genes
Genes on the X or Ychromosomes are called sexlinked genes because biologicalsex is inherited along withgenes on the X or Ychromosome. Sex linked genes on the Xchromosomes are X linkedgenes. Sex linked genes on the Ychromosome are Y linked genes. The Y chromosome carries verylittle genetic informationbecause it is smaller than the Xchromosome. X linked genes X linked genes are located on the X chromosome.
Males inherit X linked genes only from their mothers. Males are more likely to suffer from diseases caused byrecessive alleles on the X chromosome. A carrier of a recessively inherited trait has one copy ofthe recessive allele and one copy ofthe normal alleleand will not exhibit symptoms of the disease. Only females can be carriers of X linked recessive traits. Both males and females can be carriers of non sex- linked, autosomal traits. Example: Hemophilia A recessive trait that is carried on the Xchromosome. Hemophilia is a genetic disorder which causes poorblood clotting and coagulation. Because it is X linked it is more often found inmales. Females can carry one affected allele (carrier) butnot show any symptoms. Hemophilia Hemophilia is on the X chromosome, therefore wewill use the letter X. XH is considered normal and Xh is afflicted A normal female can be XHXH or XHXh. A female with the disease is XhXh A normal male will be XHY A male with the disease will be XhY Hemophilia Genetics Cross 1 Hemophilia Genetics Cross 2 Can you see it? 4% of males affected Cannot see red or green.
This is red-green colorblindness. Sex Linked Trait: Color Blindness
Color Blind: Male: XrY Female: XrXr Normal: Male: XRY Female: XRXR Female Carrier: XRXr Sex Linked Traits Worksheet Muscular Dystrophy Progressive, fatal Xlinked to disease ofmuscle wasting theaffects approximately 1in 3500 males. Muscle wasting startsbetween 1 and 12 yearsof age, and by age 12,affected boys are inwheelchairs. X inactivation X inactivation guarantees that allfemales actually receive only onedose of the proteins produced bygenes on the X chromosomes. This happens in the embryo when itimplants in the uterus. One chromosome is inactivatedwhen a string of RNA is wrappedaround it. Inactivation is irreversible and, assuch, is inherited during cellreplication. Once an X chromosome isinactivated in a cell, all descendantsof that cell continue inactivating thesame chromosome. X inactivation and patchy gene expression Tortoise Shell Almost always happens in females.
Can happen in males via nondisjunction if he is XXY. Y linked genes Y linked genes are passed from fathers to sons.
The SRY (sex determiningregion of the Y chromosome)gene is on the Ychromosome. The SRY gene develops thetestes and male sexualcharacteristics. Genes other than SRY, thatare on the Y chromosome,code for proteins that areunique to males but are notexpressed unless testesdevelop. Pedigrees A pedigree is a family tree thatfollows the inheritance of agenetic trait for manygenerations of relatives. Pedigrees are used in studyinghuman genetics. Scientists can use pedigrees todetermine whether a trait isinherited as autosomaldominant or recessive or assex linked recessive. Dominant trait: Polydactyly Recessive trait: Attached Earlobe Sex-linked trait: Muscular Dystrophy Origin and Inheritance of the Hemophilia Allele Pedigrees Worksheet