3. Meiosis and Genetic Variation Flashcards Preview

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Flashcards in 3. Meiosis and Genetic Variation Deck (15)
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1
Q

Which two processes promote genetic variation among the haploid cells produced by meiosis?

A
  1. random orientation of pairs of homologs chromosomes in metaphase I
  2. crossing over during prophase I
2
Q

How does ‘random orientation of pairs of homologs chromosomes in metaphase I’ produce genetic variation?

A
  • 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+
3
Q

How does ‘crossing over during prophase I’ produce genetic variation?

A
  • 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
4
Q

Name another source of genetic variation (apart from the 2 in meiosis). Explain why this is a source of genetic variation.

A

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
5
Q

How do species that reproduce sexually generate genetic variation?

A
  • by meiosis

- by random fusion of gametes

6
Q

What can sometimes happen during meiosis to chromosomes that should separate and move to opposite poles? When could this happen? What is the result?

A

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

7
Q

What is non-disjunction? What is the result?

A

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

8
Q

What is likely if the gamete produced has one chromosome too few? What about with one chromosome too many?

A
  • too few: the gamete will quickly die

- too many: sometimes will survive

9
Q

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?

A
  • the zygote is produced with three chromosomes of one type instead of two
  • called: trisomy
10
Q

Give an example of a condition brought about by trisomy. What causes this condition?

A

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
11
Q

With mother’s age at: 25, 30, 35, 40, 45. What is the chance of having a baby with Down Syndrome?

A

25: 1/1250
30: 1/1000
35: 1/400
40: 1/100
45: 1/30

12
Q

State two methods for obtaining the cells of an unborn child for chromosome testing.

A
  1. amniocentesis

2. chorionic villus sampling (CVS)

13
Q

Outline how amniocentesis as a method for obtaining the cells of an unborn child for chromosome testing.

A

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
14
Q

Outline how chorionic villus sampling (CVS) as a method for obtaining the cells of an unborn child for chromosome testing.

A

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
15
Q

Which of the two methods of chromosome testing has a higher chance of miscarriage?

A

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

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