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Sex Determination XY Sex Determination WZ Sex Sex Determination XY Sex Determination • In most cases, the determination • In the XY type, sex determination is based on the presence or of the sex of an organism is controlled by the sex absence of the Y chromosome; without it, an individual will develop chromosomes provided by each into a female. Female Male parent. • XY sex determination occurs in: • These have evolved to regulate • Mammals (including humans) the ratios of males and females • Fruit fly Drosophila produced and preserve the • Some dioecious (separate genetic differences between the sexes. male and female) plants Y • In humans and other mammals, males such as kiwifruit. are the heterogametic sex because • Females are homogametic with each somatic cell has one X and one Parents XX XY Y chromosome (i.e. the two sex two similar sex chromosomes X chromosomes are different). • Males are not always the (XX). The male has two unlike heterogametic sex.In birds and chromosomes (XY) and is X X X Y butterflies, the female is the Gametes heterogametic sex, and in some heterogametic. insects the male is simply X whereas Possible the female is XX. • Primary sex characteristics are initiated by genes on the X. fertilizations X ‘Maleness’ is determined by the Offspring XX XY XX XY Y. Sex: Female Male Female Male WZ Sex Determination XO Sex Determination • In the WZ type,the • In some insect orders, the Female Male female determines female has two similar sex the sex of the chromosomes (XX) while the male only has one (XO). offspring. • In the sperm produced by • The male is the males, there is a 50% chance homogametic sex that it will have a sex Female Male chromosome and create a (ZZ ), while the female XX Parents ZW X ZZ Parents X X has two unlike female offspring when it fertilizes an egg. chromosomes ( ZW ). Z W Z Z X X X Gametes • XO sex determination occurs Gametes • WZ sex determination in: Possible • Grasshoppers Possible occurs in: fertilizations fertilizations • Birds • Aphids • Honey bees X X • Butterflies and moths Offspring ZZ ZZ ZW ZW Offspring XX XX • Hemiptera (bugs) • Some fish Male Sex: Male Male Female Female Sex: Female Male Female Sex Determination in Honey Bees Aneuploidy • In honey bees sex is determined by the number of sex The normal condition for a body cell is for chromosomes, but environment chromosomes to be present as homologous pairs in the hive has an influence too. • Males or drones (a condition known as disomy ). • Result from unfertilized Aneuploidy is a condition where one or more eggs and have one sex chromosomes are missing from or added to the Queen Drone chromosome ( monoploid ). normal somatic cell chromosome number. • Females XX X X Examples : • Females have two sex Nullisomy 0 homologues chromosomes but diet Monosomy 1 homologue X X Egg X determines if they are fertilized Trisomy 3 homologues Tetrasomy 4 homologues reproductively active. Not given Egg not XX Given • Queens are supplied with a fertilized royal jelly royal jelly Aneuploidies usually result from diet of royal jelly. non-disjunction during meiosis. • Sterile workers are not X XX XX given royal jelly. Drone Worker Queen (Fertile male) (Sterile female) (Fertile female) 1 Non-disjunction in Meiosis Non-Disjunction: Meiosis I WaikatoHospital Dept. Cytogenetics Photos: Non-disjunction Meiosis I The meiotic spindle normally A non-disjunction distributes chromosomes to daughter in meiosis I cells without error. However, occurs when mistakes can occur during meiosis in homologous which the chromosomes, or sister Trisomy 13 chromosomes Meiosis II chromatids, fail to separate properly fail to separate at anaphase . properly during This mishap is called non-disjunction . It can anaphase I. occur during meiosis I or meiosis II. One gamete Non-disjunction results in abnormal numbers receives two of the of chromosomes passing to the gametes. Trisomy 18 same sort of If either of these aberrant gametes unites with chromosome and a normal one at fertilization, the offspring will the other gamete have an abnormal chromosome number. receives no copy. Gametes This is known as an aneuploidy , e.g. n+1 n+1 n–1 n–1 trisomies. Trisomy 21 Non-Disjunction: Meiosis II Meiosis I A non-disjunction in meiosis II occurs when sister chromatids fail to separate Meiosis II properly during anaphase II. One gamete receives Non- two of the same sort disjunction of chromosome and the other gamete receives no copy. Some gametes are unaffected. Gametes n+1 n–1 n n 2.
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