Inheritance (SL IB Biology)

Contrast continuous and discontinuous variation.

Describe sex linkage.

Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen). The ABO blood groups are controlled by a single gene with multiple alleles (A, B, O). The table below shows all the genotypes for all the possible blood groups.

A child has blood group AB and their father has blood group A. 

Identify the possible phenotypes of the mother.

Suggest which pattern of inheritance is exhibited in the AB blood group.

A man with normal colour vision wishes to start a family with his partner who is a carrier of the allele for colour-blindness. They wanted to work out the probability of having a child with colour-blindness.

Using the following symbols: 

X^B = an X chromosome carrying the normal allele for colour vision
X^b = an X chromosome carrying the allele for colour blindness

Hypophosphatemia is a sex-linked inherited condition which results in abnormally low levels of phosphate in the blood which can cause the disease rickets. It is caused by a dominant allele.

The diagram below shows the inheritance of hypophosphatemia in one family.

State the evidence that suggests that hypophosphatemia is a sex-linked, dominant inherited disease.

Using the following symbols,

         X^H = an X chromosome carrying the allele for hypophosphatemia
         X^h = an X chromosome carrying the normal allele
         Y = a Y chromosome 

Identify all the possible genotypes of each of the following persons from the diagram in part (a):

 1  :

 4  :

 5  :

13 :

Person 20, from the diagram in part (a), is pregnant for the fourth time. As the family has a history of hypophosphatemia, a test was carried out to discover the sex of the embryo. 

Describe what possible observations of the chromosomes would be expected when determining the sex of an embryo.

State the probability that the child Person 20 is pregnant with will be a male with hypophosphatemia.

Explain your answer by drawing a genetic diagram, using the following symbols:

 X^H = an X chromosome carrying the allele for hypophosphatemia

X^h = an X chromosome carrying the normal allele

Y = a Y chromosome

A horticulturist investigated the inheritance of flower colour in Camellia japonica, a widely cultivated ornamental plant commonly known as Japanese camellia.The horticulturist crossed a homozygous parent with red flowers and a homozygous parent with white flowers. All of the F1 generation had the same colour flowers. Using the following symbols: 

C^R  = Red flowers
C^W = White flowers 

Sketch a genetic diagram / Punnett square to deduce all the genotypes in this cross

Each of the F1 generation plants had flowers that were patterned red and white. The horticulturist undertook a self-cross with these F1 hybrids.

Describe, with a reason, what pattern of inheritance is exhibited in the horticulturist’s experiment.

FG syndrome is a recessive disorder that can cause a characteristic facial appearance, developmental delays and hyperactivity. FG syndrome is a rare X-linked genetic disorder that occurs almost exclusively in males, it is caused by a mutation in the MED12 gene on the X chromosome.

Suggest why FG syndrome occurs almost exclusively in males.

The diagram below shows the familial inheritance of an X-linked recessive disease.

Describe the patterns of inheritance that hold true for X-linked conditions.

Haemophilia is due to a sex-linked recessive gene X^h  whereas the normal gene is X^H. A haemophiliac man and a woman, who does not have haemophilia, have two children. Their first child is male and has haemophilia. 

Deduce what this tells us about the mother.

Their second child is female. 

Deduce, with a reason, the probability that their daughter will also have haemophilia.

Outline the inheritance of colour-blindness.

Human eggs and sperm cells are very different in size to each other.

Explain why, despite this size difference, both contribute equally to the genetic composition of a zygote. 

Explain why knowledge of blood groups is of critical importance when planning a blood transfusion. 

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.

Using your knowledge of the A, B, O blood grouping system, suggest why people of blood group O are sought-after as blood donors. 

As stated in part (c), group O blood is highly valued for transfusions into other patients. 

Explain the main disadvantage of a person having group O blood. 

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. 

Explain why the traits shown in part (a) are referred to as codominant.

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.

Use the information above to explain why the condition is regarded as codominant. 

Explain how mutations can have a positive effect on a species. 

Galactosaemia is a condition that prevents sufferers from fully metabolising the sugar galactose.

The pedigree chart below shows part of a family in which galactosaemia is an inherited condition.

Explain how this pedigree chart indicates whether galactosaemia is recessive, sex-linked or both. 

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]

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. 

[4]

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. 

Huntington's disease is described as an autosomal dominant disorder.

Explain the term autosomal dominant.

[2]

(ii)

The mutation in the Huntington's disease allele contains a higher-than-normal number of repeats of a nucleotide sequence coding for a protein called huntingtin.

Suggest the consequence to the protein huntingtin of the extra nucleotide repeats. 

[2]

When investigating variation scientists often study sets of twins, both identical and non-identical. 

Suggest the advantage of studying twins when investigating variation.

A scientific study carried out in 2013 investigated whether academic achievement was influenced primarily by genetics or the environment. 

11 117 pairs of identical twins participated in the study, which found that around 55% of the outcomes in core GCSE subjects (English, maths, and science) were explained by genetic influence, 25% by shared environmental influences, such as parental support, and the remaining 20% by environmental influences that were not shared between the two twins, such as teacher quality and class grouping. 

Using this information, evaluate the statement "intelligence is determined by genetics". 

Twin studies tend to be run using two main assumptions:

• That the identical twins in the study share 100% of their DNA
• That all twins are raised in exactly equal environments at home

Discuss whether it is correct to make these assumptions about twin studies.

Drosophila melanogaster is a useful organism to use in studies on inheritance patterns. Female fruit flies can lay up to 400 eggs, which can develop into adults in 7-14 days. They have simple nutrient requirements.

Explain why Drosophila melanogaster is a useful organism in studies of inheritance patterns.