Question 18N.2.HL.TZ0.7
| Date | November 2018 | Marks available | [Maximum mark: 15] | Reference code | 18N.2.HL.TZ0.7 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Describe, Explain, Outline | Question number | 7 | Adapted from | N/A |
Isolated communities in rural Finland, Hungary and some of the Scottish islands have a high incidence of red-green colour blindness. Describe the inheritance of red-green colour blindness.
[3]
a. sex linked/gene is on the X chromosome ✔
b. allele «for red-green colour blindness» is recessive/colour blindness is recessive trait/disorder ✔
c. heterozygous females are unaffected/carriers ✔
d. XB denotes normal allele and Xb denotes colour blindness allele ✔
e. more frequent in males because they only have one X chromosome ✔
f. 50% chance of colour blindness in sons whose mother is heterozygous/XBXb ✔
Accept any other letter for the alleles.
Award mpb, mpc, mpd and mpf if these points are clearly made on a Punnett grid.
Outline the causes of variation in one example of continuous variation in humans.
[5]
a. height/skin colour/other valid example ✔
b. with continuous variation any level of the variable/phenotype is possible/OWTTE ✔
c. polygenic inheritance/combined effect of more than one gene on the trait ✔
d. additive effects on the trait of alleles of multiple genes/OWTTE ✔
e. histogram showing effects of alleles of multiple genes ✔
f. environment «may» also affect the trait/sunlight affects skin colour/other example ✔
g. normal distribution curve drawn or described to show typical pattern with continuous variation ✔
Explain how evolution occurs and which factors can cause the process to be rapid.
[7]
a. evolution is a change in the heritable characteristics «of a species» ✔
b. natural selection «causes evolution» ✔
c. overpopulation/over-reproduction/more offspring «than the environment can support» ✔
d. competition «for resources/mates» ✔
e. variation in population/species ✔
f. mutation/meiosis/sexual reproduction contributes to variation ✔
g. adaptation increases chance of survival ✔
h. reproduction/offspring produced «by the better adapted/by those that survive» ✔
i. characteristics passed to offspring by reproduction/variation is heritable ✔
j. allele frequencies/number of organisms carrying a gene changes/gene pool changes ✔
k. environmental change stimulates/triggers/speeds up natural selection/evolution ✔
l. increase in rainfall/introduction of antibiotic/pollution on tree trunks/other valid example of environmental change/new selection pressure ✔
m. artificial selection/selective breeding can speed up evolution ✔
Mark points can be awarded if explained using an example.