3. Meiosis and Genetic Variation Flashcards
Which two processes promote genetic variation among the haploid cells produced by meiosis?
- random orientation of pairs of homologs chromosomes in metaphase I
- crossing over during prophase I
How does ‘random orientation of pairs of homologs chromosomes in metaphase I’ produce genetic variation?
- for each pair of chromosomes = two possible orientations
- orientations determine which chromosome moves to each of the two poles in the cell.
- orientation is random, not influenced by other chromosomes so different combinations of chromosomes can be produced and therefore different combinations of alleles
- possible combinations: 2^n (where n is 23) = 8mil+
How does ‘crossing over during prophase I’ produce genetic variation?
- very early stages: homologous chromosomes pair up and parts of non-sister chromatids can be exchanged between them (process called crossing over)
- produces chromatids with a new combination of alleles
- significant source of genetic variation because it is random where along the length of the chromosomes the exchange occurs
Name another source of genetic variation (apart from the 2 in meiosis). Explain why this is a source of genetic variation.
fusion of gametes during fertilisation
- alleles from two different parents are brought together in one new individual
- fertilisation is a random process - any gamete produced by the father could fuse with any produced by the mother
How do species that reproduce sexually generate genetic variation?
- by meiosis
- by random fusion of gametes
What can sometimes happen during meiosis to chromosomes that should separate and move to opposite poles? When could this happen? What is the result?
they do not (separate and move to opposite poles) - instead move to same pole:
- first division (anaphase I)
- second division (anaphase II)
gametes are produced with either one chromosome too many or too few
What is non-disjunction? What is the result?
non-separation of chromosomes (the type of genetic variation you don’t want to happen)
gametes are produced with either one chromosome too many or too few
What is likely if the gamete produced has one chromosome too few? What about with one chromosome too many?
- too few: the gamete will quickly die
- too many: sometimes will survive
What can you say about the zygote that is produced by the fusion of gametes where one of the gametes contains one chromosome too many? What is this called?
- the zygote is produced with three chromosomes of one type instead of two
- called: trisomy
Give an example of a condition brought about by trisomy. What causes this condition?
Down Syndrome - three chromosomes of type 21
= also called: trisomy 21
- non-disjunction during the formation of the sperm or egg
- many research studies have shown that the chance of Down Syndrome increases with the age of the parents
With mother’s age at: 25, 30, 35, 40, 45. What is the chance of having a baby with Down Syndrome?
25: 1/1250
30: 1/1000
35: 1/400
40: 1/100
45: 1/30
State two methods for obtaining the cells of an unborn child for chromosome testing.
- amniocentesis
2. chorionic villus sampling (CVS)
Outline how amniocentesis as a method for obtaining the cells of an unborn child for chromosome testing.
sample of amniotic fluid is removed from amniotic sac around fetus:
- hypodermic needle inserted through wall of mother’s abdomen and wall of uterus
- amniotic fluid is drawn out into a syringe
- contains cells from the fetus
Outline how chorionic villus sampling (CVS) as a method for obtaining the cells of an unborn child for chromosome testing.
cells are removed from fetal tissues in placenta called chorionic villi:
- hypodermic needle inserted through mother’s abdomen and uterus wall
- cells from chorion removed
Which of the two methods of chromosome testing has a higher chance of miscarriage?
CVS has a slightly higher chance of miscarriage:
- CVS: 2%
- amniocentesis: 1%
both methods have a very small risk of infections in maternal or fetal tissues
