3.4 Inheritance

3.4 Inheritance

Lesson one: Punnet squares.

Essential questions: What rules govern the inheritance of traits eg. hair colour?

Objectives: Practice solving punnet square inheritance predictions of increasing complexity. Introduce codominance and then sex-linkage. Link mutations to radiation


Key words:

  • Phenotype: the expression of the alleles which are present for a particular trait eg. having sickle cell anaemia
  • Genotype: the presence of alleles for a particular trait eg. HBS HBS gives you sickle cell anaemia
  • Co-dominance: when more than one allele is expressed in determining a trait eg. blood group AB resulting from IAIB
  • Sex-linkage: when a gene is located on a sex chromosome and therefore inheritance depends on sex eg. red-green colourblindness is carried on the X chromosome
  • Mutation: a change in DNA
  • Mutagen: a chemical or non-chemical agent that causes mutations eg. radiation or benzene.



Modern genetics is still based on the work of Gregor Mendel.

Q) State Mendel’s first and second law of Genetics?

Gametes and Alleles

Gametes have a haploid number of chromosomes, this means they have one of each homologous chromosome pair. This means that for each gene they will have only one allele. When fertilisation happens, two different alleles (from egg and sperm), are combined, making a new individual.

Phenotype and Genotype

The expression of the genes which are present, is called the phenotype. This is easy to discern. The actual genes which are present, is the genotype. It is more difficult to know the genotype, and these are often deduced based on ‘family trees’

 Dominant, recessive, and co-dominant

Some alleles are always expressed over others, these are called dominant alleles. Dominant alleles mask the effects of recessive alleles. Mendel found that the gene for Tall pea plants (T), is dominant over the gene for short pea plants (t).

Tt – is the genotype. Q) What would be the phenotype?

Recessive alleles are only expressed if there are two of them. The short gene for pea plants (t) is recessive.

Describing genotypes

  • Both alleles are the same – Homozygous
  • Both alleles are different – Heterozygous
  • Tt – Heterozygous
  • TT – Homozygous dominant
  • tt – Homozygous recessive

Basic Punnett squares:

Punnett squares are used to predict the results of genetic crosses between different organisms. Punnet squares show all the possible combinations of alleles to produce the next generation. Typically inheritance questions ask for the phenotype ratio of the next generation

At this SL, we will focus on the effects of one gene: monohybrid inheritance: or the study of the inheritance of one characteristic eg. height of pea plants

Setting out a punnet square inheritance question with full workings involves the following:

  1. Parent’s genotypes and phenotypes
  2. Alleles in gametes
  3. Punnet square combinations
  4. Phenotype ratio (genotype ratio is rarely asked for but it is possible)



CO-DOMINANCE – a complication in patterns of inheritance

Co-dominant alleles both have an effect on the phenotype. It has been discovered that the allele for red flowers flowers Cr, and the allele for white flowers Cw, produce a pink flower if both alleles are present in the genotype in the flowering perennial Mirabilis Jalapa (Marvel of Peru).

image credit: visuals unlimited.com

image credit: visuals unlimited.com

The same phenomenon has been observed in Snapdragon flowers (Antirrhinum spp)

image credit: buzzle.com

image credit: buzzle.com

Task Q) What would be the phenotype, and the genotype for a pink snapdragon or marvel of peru flower?


Sex-Linkage – a further complication

Sex-linkage refers to the phenomenum where a specific allele is carried on a sex-chromosome. As the X-chromosome is much larger, many more characteristics are sex-linked on the X-chromosome, than on the Y.




Red-green colourblindness is sex-linked. This means the allele causing this condition is carried on the X chromosome.

Because a male must inherit his X chromosome from his mother, it is common for males to inherit this condition from their mothers. Females are less likely to be colour-blind, as they inherit two chromosomes, and the allele is recessive. It is more likely that they become carriers of colourblindness. Carriers will have a copy of an allele causing a condition, but will not have the phenotype.

Q) Males cannot be carriers of red-green colourblindness.Why?

Haemophilia is also a sex-linked disease

  • Also carried on the X chromosome
  • Characterised by a lack of blood clotting factors in the blood, with the result that blood clots too slowly. This leads to the risk of bleeding to death.
  • Haemophilia has a similar pattern of inheritance to red-green colourblindness.
  • XH  is used for the normal allele, and Xh for the haemophiliac allele
  • Q) What is the phenotype of a heterozygous male and female for haemophilia?

Quite a few Famous people are rumoured to have been haemophiliacs:

image credit: missingfactor.weebly.com


image credit: missingfactor.weebly.comimage credit: missingfactor.weebly.com


3 Well-studied genetic disorders (which may be mentioned in IB exams)

Unlike diseases, which are caused by pathogens, genetic diseases are caused by faulty genes. They are inherited.

Experts state that a typical human carries between 75 and 200 faulty alleles among the 25,000 genes in their genome. The emergence of a genetic disease occurs only if two individuals with the same faulty alleles have children.

Here are the patterns of inheritance of 3:

1) Cystic fibrosis:

  • Genetic cause: Recessive mutated allele of the CFTR gene carried on chromosome 7.
  • Main effect: Causes a Chloride ion channel in the respiratory linings to become faulty. Insufficient sodium chloride is found in mucus, reducing the amount of water absorbed by osmosis. This causes the mucus to become dry and sticky.
  • Other effects: Causes secretions of pancreatic juice to become blocked
  • Incidence: 1 in 400 people (European statistics)

2) Huntingdon’s disease:

  • Genetic cause: Dominant allele of the HTT gene on chromosome 4.
  • Main effect: Slow Degeneration of brain neural pathways, leading to eventual full nursing care, heart failure, pneumonia or other infectious disease
  • Other effects: Changes in behaviour, thinking, emotions.
  • Incidence: One in 30,000 (US statistics)

3) Sickle-celled anaemia:

  • Genetic cause: Co-dominant allele of the HBB gene on chromosome 11 (remember 11p15.5)                                       Cause by a SNP, this leads to the swapping of the sixth amino acid glutamic acid for valine. The haemoglobin is deformed, causing the structure of the red blood cell to alter.
image credit: HBS

image credit: mcGrawhill.edu


  •  Main effect: Deformation of red blood cells, and reducing saturation with oxygen. This blocks capillaries, and leads to lower supply of oxygen to the tissues (anaemia: low levels of oxygen in blood)
  • Other effects: Inability to do strenuous exercise, dizziness, shortness of breath.Resistance to the malarial parasite (plasmodium)
  • Incidence: 1 in 5,000 (US). In parts of Africa with malaria can be as high as 1 in 30.

 CAUSES OF MUTATION – what causes DNA changes?

The faulty alleles that cause genetic diseases are thought to have been caused by gene mutations (random change in the base sequence of a gene DNA).

There is a natural background rate of mutations, and many mutant genes are not harmful and can even be beneficial (eg. the allele for lactose tolerance).

Some things increase the rate at which mutations occur naturally, these are called mutagens.

List of mutagens:

  • Radiation
  • Chemical agents:                          -benzo(a)pyrene (tobacco smoke)                         -nitrosamines (tobacco smoke)                         -mustard gas

Different chemical agents have different levels of effects on DNA. An increasing number of chemicals are reported to be mutagens (including burnt toast!), but it is important to bear in mind that the strength of the mutagen varies.

The test for mutagenicity is called the Ames test and was invented by Dr. Bruce Ames (US, Berkely).


Reminder: Cancer is caused by mutations of the genes that code for cyclins (the proteins that control the stages of mitosis).

Two historical examples of mass exposure to radiation:

1) Hiroshima and Nagasaki (1945): Detonation of two atomic bombs over these Japanese cities killing 250,000 people, leading to the surrender of Japan in the second world war (1986). Survivors of the atomic blasts have not succumbed to a large number of disorders or cancers as predicted, but have felt stigmatised socially as people were reluctant to marry them for fear that their children might carry genetic diseases.

2) Chernobyl: The fire and subsequent explosion of a nuclear power plant released tonnes of radioactive material into the atmosphere in the Ukraine. The effects have included the death of forests, domestic cattle, bioaccumulation in food chains or caesium and iodine to unacceptable levels, 6,000 cases of thyroid cancer and deaths due to leukaemia.



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