Topic 7- Genetic variation

Question 1

Define phenotype

Answer 1

Observable characteristics

Question 2

Define phenotypic variation

Answer 2

the difference in phenotypes between organisms of the same species

Question 3

What is the phenotypic variation of individuals in a population is determined by?

Answer 3

genetic variation within the population and the interaction of the environment on the individuals:

Question 4

Define genetic variation

Answer 4

differences in DNA base sequences between individual organisms within a species population

Question 5

How does genetic variation create phenotypic variation?

Answer 5

Genetic variation is transferred from one generation to the next

Question 6

What is the primary source of genetic variation?

Answer 6

mutation

Question 7

Why do mutations cause genetic variation?

Answer 7

-results in the generation of new alleles

Question 8

Give some features of alleles

Answer 8

The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype
New alleles are not always seen in the individual that they first occur in
They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

Question 9

Name 3 things that result in a new combination of alleles

Answer 9

-Crossing over
-Independent assortment
-Random fertilisation

Question 10

What is crossing over?

Answer 10

non-sister chromatids exchange alleles

Question 11

What happens during crossing over?

Answer 11

-During meiosis I homologous chromosomes pair up and are in very close proximity to each other
-The non-sister chromatids can cross over and get entangled
-These crossing points are called chiasmata
-The entanglement places stress on the DNA molecules
-As a result of this a section of chromatid from one chromosome may break and re-join with the chromatid from the other chromosome

Question 12

What does crossing over result in?

Answer 12

a new combination of alleles on the two chromosomes

Question 13

Where is crossing over most likely to occur?

Answer 13

further down the chromosome away from the centromere

Question 14

What is independent assortment?

Answer 14

different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase

Question 15

What does independent assortment result in?

Answer 15

increased genetic variation between gametes

Question 16

What happens during independent assortment?

Answer 16

-In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
-Each pair can be arranged with either chromosome on top, this is completely random
-The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
-The homologous chromosomes are then separated and pulled apart to different poles
-The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up

Question 17

How do you work out the number of different possible chromosome combinations?

Answer 17

-2n (squared)
-n corresponds to the number of chromosomes in a haploid cell

Question 18

How does random fertilisation of gametes cause genetic variation?

Answer 18

creates genetic variation between zygotes as each will have a unique combination of alleles
(any male gamete can fuse with any female gamete to form a zygote)