For Mandy- Down's syndrome child from 2 normal parents

Ok this is how it works.... Meiosis occurs to produce sperm and egg cells. When sperm and egg cells are produced, they only contain half the chromosome number. A DS child is produced when the mother's 21st chromosome doesn't split, so the child gets 2 chromosome 21 from the mom and 1 chromosome 21 from the dad, resulting in 3 chromosome 21... The picture below( sorry, hand drawn on the iPad!) shows u what happens in the mother when the egg cells are produced

( I ONLY DREW CHROMOSOME 21)

From the picture above, you can see 4 egg cells, one of them has TWO chromosome 21. If that egg is fertilised, it grows into a DS child. If the second egg is fertilised, It will usually become non-viable (does not survive). The 3rd and4th egg, when fertilised, should result in normal children.

Therefore a DS child is caused by what we called, non- disjunction of the mother's chromosome21 during meiosis second stage... Btw, non- disjunction is an A-level term!

For you O level peeps, just say the child inherits 2 chromosome 21 from the mother instead of one. Reason? It's a mutation :)

Hope that helps... Sorry abt the retarded drawing :p

June 2010 Qn 7- Essay on Food Web and Conservation of species

JUNE 2010 QN 7

Clues to answering this question

1) The insects are HERBIVOROUS

2) These insects will COMPETE with OTHER HERBIVORES on that island, may even OUTCOMPETE island species

3) No natural predators so population of this insect may INCREASE RAPIDLY

4) Possible depletion of plant resources (because more herbivores now!)

5) Carnivores that eat the local species run out of food if the local species cannot compete with the new insects

ANSWER SCHEME

Possible answer: The new insects on the islands will eat the plants on the island and cause a decrease in plants, as their population increases (1^st and 2^nd point). This can lead to a decrease in population of other herbivores from the island as they compete for food (3^rd and 5^th point). The carnivores on the island may have less food as the local herbivores decrease (6^th point). The population of the new insects may not be easily controlled as there may not be any natural predators on the island (8^th point) Total: 6 marks

This is a general knowledge question. Conservation is important due to possible undiscovered medical cures, maintaining biodiversity (this is a big thing at the moment!), prevention of extinction of species as it can lead to effects on the food web/ecological balance

Possible answer: It is important to conserve species as it maintains the ecological balance and biodiversity in an ecosystem (1^st and 6^th point) and may affect the food web if the species decreases in number (2^nd point). Some species may also have medicinal values and hold clues to cure diseases (3^rd and 4^th point) Total: 5 marks (4 max)