Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Genes and Inheritance III Genes and sex • The examples we have seen so far were genes on autosomes, so it didn’t matter which parent was mother or father • Many organisms are “monoecious” - an individual can produce both male and female gametes • Others (including humans, birds, fruit-flies) are “dioecious” so individuals are either male or female • But the way in which chromosomes determine sex is different in different species Sex determination • Bees - unfertilised eggs (haploid) produce males (drones) and fertilised eggs (diploid) produce females (workers, queen); same in ants and many other invertebrates • Grasshoppers - all diploid, males have only one X chromosome, females have 2 X’s • Humans, fruit flies - sperms (haploid) contain an X or a Y chromosome, females and males are XX and XY • Birds, butterflies - males are ZZ, and females are WZ The sex life of bees The sex life of bees • Here is the story • To be female, bee has to be heterozygous for CSD gene • CSD has 19 different alleles so diploid bee is likely to be heterozygous • Rare homozygous diploid bees develop as sterile males and are killed by workers • Intensive selection by bee-keepers can reduce genetic diversity and increase chances of CSD homozygosity (which is bad for the bees) Functions of sex chromosomes • In humans (and other mammals) there is a gene SRY on the Y, and DAX1 on the X • Dax1 protein prevents the development of testis, allows development of ovary, whereas Sry protein overcomes Dax1 and allows testis to develop - primary sexual characteristics • XO individuals are female, XXY male • Secondary sexual characteristics (body type, voice, body hair) are controlled by hormones, not by X and Y directly Functions of sex chromosomes • • • • In fruit flies it’s a bit different XO is a sterile male, XXY a fertile female It is the ratio of X to Y that determines sex There is no gene on the Y corresponding to the mammal’s SRY Sex-linked inheritance • There are very few genes on the Y, but lots on the X, and their inheritance shows sex-linkage in pedigrees • White eye colour in fruit flies is sex linked (Dr Thomas Morgan again) • The outcome of a cross depends on which parental genotype is from the father and which from the mother • Figure 10.23 in textbook Sex-linkage in humans • There are thousands of genes on the human X, some cause diseases like muscular dystrophy and haemophilia that mostly affect males • This can be seen from inheritance in pedigrees - for example, colour blindness • Figure 10.24 Haemophilia in the Royal Family • Haemophilia is a hereditary disorder caused by defective gene on X-chromosome for a blood clotting factor • Queen Victoria was a carrier – her family was linked to other European royals • It did not get passed on to the present Royals……. Non-nuclear inheritance • Everything we have seen so far, concerns genes on chromosomes in the nucleus • Mitochondria and plastids are organelles that have their own little chromosome • They may have evolved from bacteria that once colonised a primitive eukaryotic cell • They are found in eggs (but not sperm) so they show maternal inheritance - passed from mothers to all children Mitochondrial (maternal) inheritance