Inheritance (Edexcel GCSE Biology)

Gregor Mendel carried out hundreds of breeding experiments with pea plants.

State what Gregor Mendel was able to show.

In one of Mendel's experiments using pea plants, he crossed a tall plant with a small plant.

The offspring plants in this first generation were all tall.

Mendel then crossed two tall plants from the first generation to produce new offspring plants (second generation).

Explain how the second generation plants led Mendel to conclude that there were two factors (alleles) that controlled height in pea plants with one being dominant over the other.

Explain why Mendel's work was not recognised until after his death.

Figure 1 shows a list of key terms and definitions relating to inheritance but they are in the incorrect order. 

Figure 1

In humans, ear wax type has two distinct types, sticky and dry. Ear wax is an inherited trait which is controlled by a single gene. The allele for sticky ear wax (E) is dominant over the allele for dry earwax (e).

Explain how inheritance of biological sex is determined in humans.

Figure 2 shows a genetic diagram.

Complete the genetic diagram to show the genotypes of the parents and any offspring

(3)

Figure 2

Human red blood cells can be categorised into different blood groups ABO.

The gene controlling human ABO blood groups has three alleles (I^A, I^B and I^o), not just two.

Figure 1 shows all the genotypes for all the possible blood groups.

Figure 1

Explain why the AB blood group in humans shows codominance.

State the genetic term that describes the genotype for blood group O.

Higher Tier Only

Red-green colour blindness is a sex-linked disorder as the allele for colour blindness is found on the X chromosome. 

Approximately 8% of men suffer from red-green colour blindness as opposed to only 0.5% of women.

Suggest why red-green colour blindness is less prevalent in women.

Higher Tier Only

The following symbols can be used to represent the alleles for red-green colour blindness:

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

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

Y = the Y chromosome does not have an allele for this gene

The genotypes for red-green colour blindness of a mother and father are shown in Figure 2.

Figure 2

Describe what the term 'variation' means in the context of a population.

State why the following phrase is incorrect.

"Variation between individuals within populations is only caused by a variation in their genes" 

Variation in organisms can be caused by genetic and environment factors or a combination of both.

Figure 1 shows a list of characteristics in humans.

Complete Figure 1, placing a tick (✓)  in the appropriate column for each example characteristic.

Give an explanation for each of your answers in Figure 1.

Describe the differences between continuous variation and discontinuous variation.

The Human Genome Project was an international scientific research project to determine the sequence of all the bases that make up human DNA and to identify and map all of the genes.

State two potential applications of the Human Genome Project within medicine.

There is usually extensive genetic variation within a population of a species due to variations in alleles.

State how this variation arises.

State two factors that can lead to changes in the base sequence of DNA.

Most mutations don't significantly affect the phenotype.

Explain why this is.

Mendel’s research on pea plants showed that genetic traits are inherited.

Which term is used to describe the expression of traits in an organism?

(1)

Mendel crossed pea plants that produced round seeds with pea plants that produced wrinkled seeds.

All the offspring produced round seeds.

He then crossed these offspring with each other.

Some pea plants in the next generation produced round seeds and the others produced wrinkled seeds.

Explain how this showed that some inherited traits are not expressed in an organism.

(3)

Higher tier content only

Duchenne muscular dystrophy is a recessive sex-linked genetic disorder.

This disorder causes muscle weakness.

Figure 1 shows the inheritance of Duchenne muscular dystrophy in a family.

State and explain the phenotype of person Z.

Gregor Mendel investigated genetic inheritance using pea plants.

Figure 1 shows some of the equipment used in this investigation.

Figure 1

Gregor Mendel crossed red-flowered pea plants with white-flowered pea plants.

Describe how this equipment could be used to do this cross and discover the flower colour of the new pea plants produced.

(3)

(ii)

Describe how to make sure that the results obtained from this investigation are not anomalous.

(2)

In a different investigation, Mendel crossed pea plants that produced yellow seeds (AA) with pea plants that produced green seeds (aa).

The dominant allele is shown as A.

The Punnett square shows the genotypes of the offspring from this cross.

Explain a conclusion that can be made from the results of this cross.

Mendel did his investigations in the 19th century.

State one reason why Mendel could not fully explain the results of his investigations.

Haemochromatosis is a disease that occurs when iron accumulates in the liver.

A person with haemochromatosis is treated by having 0.5 dm³ of their blood removed each week.

This lowers the level of iron in their blood.

(i)

Give two safety precautions needed when blood is removed from this person.

(2)

Haemochromatosis can be inherited.

Haemochromatosis occurs when a person inherits two copies of a recessive allele.

Figure 1 shows the inheritance of haemochromatosis in a family.

Figure 1

State and explain the genotype of female Z.

(3)

Sickle cell disease is a recessive genetic disorder in humans.

(i)

Two parents are heterozygous for sickle cell disease.

Complete the Punnett square to show the possible genotypes of their children.

(1)

State the percentage probability that their children could have sickle cell disease.

(1)

A father with the genotype dd and a mother with the genotype DD plan to have several children.

Explain why none of their children will have sickle cell disease.

(2)

A karyogram is a picture of the chromosomes found in the nucleus of a single cell.

Figure 1 shows a human karyogram.

State two reasons why this karyogram cannot be from a gamete (sex cell).

(2)

(ii)

State the gender shown by this karyogram.

(1)

(iii)

Complete the Punnett square to show how gender is inherited.

(2)

A gene coding for a protein has two alleles.

Figure 1 shows the first 5 codons of an mRNA strand for these alleles.

Allele 1 – AUG CCA CAG GAG UUC

Allele 2 – AUG CCA GAG GAG UUC

Figure 1

Allele 2 has a mutation.

Figure 2 shows the key needed to predict the translated amino acid sequence of the protein.

Figure 2

Explain how the mutation in allele 2 could affect the functioning of this protein.

The inheritance of different alleles affects the phenotype of an individual.

A child is blood group O.

The child’s mother is blood group A and the child’s father is blood group B.

Explain how this child is blood group O.

Use the Punnett square and probability in your answer.

Higher Tier Only

One gene can have several different alleles.

Explain why these alleles can produce proteins with different structures.

Higher Tier only

Monoclonal antibodies are proteins.

Figure 1 shows the process of monoclonal antibody production.

Figure 1

Which type of cell is added to the B lymphocytes to produce the hybridoma?

(1)

A person with blood group A has the A antigen on the surface of their red blood cells.

Monoclonal antibodies can be used to detect the A antigen on red blood cells to determine the blood group of a person.

Explain why monoclonal antibodies, used to detect blood group A, would not react with the blood of a person with blood group B.

(3)

The allele for blood group A and the allele for blood group B are codominant.

Gregor Mendel investigated the inheritance of alleles using flowering plants.

He showed that the allele for red flowers (I^R) is codominant with the allele for white flowers (I^W).

A heterozygous plant produces pink flowers.

(i)

Give the genotype for a plant producing white flowers.

(1)

(ii)

Explain the outcome if two plants that produce pink flowers are crossed.

You should complete the Punnett square as part of your answer.

(4)

Baldness is a sex-linked inherited disorder that affects around 70% of the male population.

However, only around 40% of women are affected by baldness.

Explain why more men than women are affected by baldness.

Baldness is caused by the dominant allele, B. Non-baldness is caused by the recessive allele, b.

Complete the Punnett square to show the outcome of a cross between a heterozygous female and an unaffected male.

Show the percentage probability of a male offspring being affected by baldness in your answer.

Percentage probability of male offspring affected by baldness ……………………… %

The pedigree diagram in Figure 1 shows the baldness pattern across four generations of one family.

Figure 1

(1)

(1)

The alleles for fur colour in cats are codominant. 

It has been found that the allele for ginger fur (CG)  is codominant with the allele for black fur (CB). A heterozygous cat has tortoiseshell-coloured fur. 

(1)

(4)

Sickle cell anaemia is another example of an inherited disorder whereby the alleles that cause it are codominant.

Sickle cell anaemia affects the shape of red blood cells, causing them to be shaped like sickles or crescent moons rather than their regular round shape. This means they can become rigid or sticky, which can slow blood flow. These red blood cells die much quicker than regular-shaped red blood cells.

Suggest why someone with sickle cell anaemia feel fatigued much more quickly than someone without sickle cell anaemia.

Figure 1 shows the inheritance of sickle cell anaemia in a family.

Figure 1

(1)

(1)

(1)

Tay-Sachs is a rare inherited disorder that stops nerves from working properly and often leads to progressive damage to and the death of cells due to a build up.of a lipid called GM2 ganglioside

In unaffected individuals, an enzyme called β-hexosaminidase A helps to break down GM2 ganglioside.

Explain how a change in one amino acid in β-hexosaminidase A could stop the enzyme working properly. 

Tay-Sachs is caused by a recessive allele.

The diagram in Figure 1 shows the inheritance pattern for Tay-Sachs in a family.

Figure 1

(2)

(3)

Another genetic disorder, called Duchenne Muscular Dystrophy (DMD), is a progressive condition that gradually causes muscles to weaken. It is caused by a mutation in a gene.

About 1 in 3600 men suffer with DMD, but only 1 in 50,000 women have the genetic condition. 

Explain why more men than women suffer with DMD.

There are few known cures for DMD, but one type of treatment currently being researched is the use of stem cell therapy. This involves using stem cells from the patient to make healthy muscle cells, which could then be used to replace the damaged ones that cause DMD. The new muscle cells have a low risk of being rejected by the patient however the gene mutation may still affect some of these stem cells.

The graph in Figure 2 shows how patients categorised their DMD symptoms following treatment by stem cell therapy.

Figure 2

(2)

(4)