Aside from his ground-breaking experiments in genetics, Mendel is also credited with establishing one of the most fundamental principles of good experimental practice.
Explain how his experiments allowed him to accomplish that.
A man of blood group AB and a woman of blood group B have four children together.
One child is blood group AB
One child is blood group A
Two children are blood group B
Assuming that the genotypes of the four children are representative of the expected genotype ratios, deduce the mother's and father's genotypes under the A, B, O blood grouping system.
Chickens can produce pigmentation in their feathers to make them white, black or speckled, as shown in the diagram below.
Homozygous white-feathered chickens can be crossed with homozygous black-feathered chickens to produce speckled offspring. This occurs via codominance.
Construct a Punnett grid to show the results of two of the speckled offspring being crossed. Use your Punnett grid to deduce the ratios of the various phenotypes that would come out of the cross.
The genetic disease sickle cell anaemia is caused by a faulty allele of the beta-globin gene, needed for the production of functional haemoglobin in red blood cells.
The faulty allele causes red blood cells to adopt a sickle shape when oxygen availability is low, as opposed to the conventional biconcave disc structure, as shown below. Sickle cells cause considerable suffering including severe cramping pains in fingers and toes, and general fatigue.
The following table gives information about the condition in its various forms.
Genotype
Disease manifestation
Homozygous dominant
No disease
All red blood cells are biconcave discs
Heterozygous
Mild symptoms at worst
Mixture of biconcave discs and sickle-shaped cells at low oxygen levels
Homozygous recessive
Severe disease
All red blood cells sickle-shaped at low oxygen levels, considerable suffering
Use the information above to explain why the condition is regarded as codominant.
Outline the cause of the genetic disease cystic fibrosis, along with its main symptoms.
[3 marks]
ii)
Two individuals, who do not exhibit symptoms of cystic fibrosis, want to have children together and have asked for guidance as to their potential risk of having a child with cystic fibrosis. The parents are not aware of their own genotypes.
What should they be told about their potential risks?
One mark is available for clarity of communication throughout this question.
a)
i)
A certain species of flower grows with either red or white petals. The allele for red flowers, R, is dominant to the allele for white flowers, r.
Construct a genetic diagram to predict the outcome of crossing pure-bred red flowers with pure-bred white flowers. State the genotype and phenotype ratios that would be expected in the F1 generation.
[3 marks]
ii)
Plants from the F1 generation were crossed.
Construct a second genetic diagram to predict the outcomes.
State the genotype and phenotype ratios that would be expected in the F2 generation.
Huntington's disease is a genetic condition that affects the brain progressively. Problems with coordination worsen over time and can ultimately cause death by, for example, an inability to swallow or by injuries associated with falling.
i)
Huntington's disease is described as an autosomal dominant disorder. Explain the meaning of this description.
[2 marks]
ii)
The mutation in the Huntington's disease allele contains a higher-than-normal number of repeats of a certain nucleotide sequence coding for a protein called huntingtin.
Suggest the consequence to the protein huntingtin of the extra nucleotide repeats.
