chapter 18 - variation and selection

Question 1

*define:

variation

Answer 1

the differences between individuals of the same species.

Question 2

what does ‘variation’ refer to?

Answer 2

it refers to observable differences within a species.

Question 3

*Distinguish between phenotypic variation and

genetic variation:

Answer 3

genetic variations can be inherited, since they are determined by genes.
phenotypic variations is caused by environmental factors, genetic factors or both.

Question 4

what are acquired characteristics?

Answer 4

features which you acquire from your environment.

ex) brown skin from tanning

Question 5

what are inherited characteristics?

Answer 5

features which you inherit according to your genes.
ex) you cannot inherit a tan, but you can inherit the genes for producing melanin pigment, which allows you to tan.
black skin is inherited.

Question 6

state the 2 types of phenotypic variation:

Answer 6

1- continuous variation

2- discontinuous variation

Question 7

*what is discontinuous variation?

Answer 7

• variations which take the form of distinct, alternative phenotypes with no intermediates.
• result in a limited number of phenotypes.
• mostly caused by genes only
• cannot usually be altered by the environment.
• under the control of a single pair of alleles or a small number of genes.
  ex) human blood groups, sex, tongue rolling.

Question 8

*what is continuous variation?

Answer 8

• variation where there is no distinct categories and there is possible intermediates.
• results in a range of phenotypes between two extremes.
• influenced by a combination of both genetic and environmental factors.
  ex) height

Question 9

*Define:

mutation

Answer 9

a spontaneous genetic change.

mutation is the way new alleles form.

Question 10

*define:

gene mutation

Answer 10

a change in the base sequence of DNA.

Question 11

• what would increase the rate of mutation?

Answer 11

mutagens, which are:
1- ionizing radiation from X-rays and radioactive compounds and ultraviolet radiation.
2- some chemicals (ex: substances in tobacco smoke such as tar).

Question 12

how can a mutation occur?

Answer 12

in a gene:
- one or more genes are not replicated correctly.

in a chromosome:

• damage/ gain/ loss of part of a chromosome during mitosis and meiosis.
• gain of an extra chromosome causes down syndrome.

Question 13

what is the effect of mutations?

Answer 13

• causes an abrupt change in a gene or chromosome
• -> result in defective enzymes –> disrupt the complex reactions in my body cells.
• so usually mutations are harmful.
• rarely ever produces a beneficial effect.

Question 14

explain why in some cases changes in genes and chromosomes do not cause mutations:

Answer 14

• only about 3% of human DNA consist of genes.
• the rest is “junk DNA’ which consists of repeated sequences of nucleotides.
• junk DNA don’t code for DNA.
• if a mutation occurs in them they are unlikely to have any effect –> described as ‘neutral’.

Question 15

difference between mutation in a gamete and a somatic cell:

Answer 15

in a gamete:
- affects all the cells of the individual that develops from the zygote.
- causes a genetic disorder (ex: hemophilia, cystic fibrosis)
in a somatic cell:
- affects only the cells produced by meiosis from the affected cell.
- promotes uncontrolled cell division in affected tissue (cancer).

Question 16

How do individuals get sickle-cell anemia?

Answer 16

• it is inherited.

- a person with sickle-cell anemia has inherited both recessive alleles (Hb^s Hb^s).