Monohybrid inheritance

Some people keep rabbits as pets.

Figure 1 shows two types of rabbit.

The fur colour pattern in the Himalayan rabbit is caused by a dominant allele.

What is a dominant allele?

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In Questions 01.2 and 01.3, use the following symbols: H = dominant allele for Himalayan fur pattern h = recessive allele for white fur.

What is the genotype of a white rabbit?

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A male Himalayan rabbit was crossed with a female Himalayan rabbit.

The male rabbit and the female rabbit each had the heterozygous genotype Hh.

Some of the offspring were Himalayan rabbits and some of the offspring were white rabbits.

Complete the Punnett square diagram in Figure 2.

You should show:

• the male gametes

• the offspring genotypes.

Some of the offspring in Figure 2 are homozygous and some are heterozygous.

Draw a ring around the genotype of one homozygous offspring in Figure 2.

Four offspring were born.

How many of the four offspring would you expect to be Himalayan rabbits?

Use Figure 2.

Tick (✓) one box.

Inheritance of sex in rabbits is the same as inheritance of sex in humans.

A female rabbit has two X chromosomes in each body cell.

How many X chromosomes does a male rabbit have in each body cell?

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Some of the offspring in Figure 2 would be male and some of the offspring would be female.

How many of the four offspring in Question 01.5 would you predict to be female?

Some people keep rabbits as pets.

Figure 8 shows two types of rabbit.

The Himalayan fur pattern is controlled by a dominant allele.

A heterozygous male Himalayan rabbit was crossed many times with each of two female Himalayan rabbits, A and B.

The results of the two types of cross were different.

• Crosses with female A: all of the 210 offspring were Himalayan rabbits.

• Crosses with female B: some of the 240 offspring were Himalayan rabbits and some were white rabbits.

In Questions 08.1 and 08.2, use the following symbols to represent alleles: H = dominant allele for Himalayan fur h = recessive allele for white fur.

What is the genotype of female Himalayan rabbit A ?

Give the reason for your answer.

Determine the probability that one of the offspring from a cross with female Himalayan rabbit B will be white.

You should:

• draw a Punnett square diagram

• use the symbols H and h

• identify the phenotype of each offspring genotype.

Probability of offspring being white =

Adult Himalayan rabbits have:

• white fur over most of the body

• black fur on the nose, ears and feet.

The black colour is caused by production of a substance called melanin.

Figure 9 shows how melanin is made from another substance called tyrosine.

Figure 9

White rabbits have one base change in the gene that codes for the enzyme tyrosinase.

Explain how a change of one base in the DNA of the gene coding for tyrosinase could stop the tyrosinase working.

For the first four weeks of life, Himalayan rabbits produce white fur over the whole body.

The black areas of fur start to develop after four weeks.

In the first four weeks, young rabbits stay in the nest with their mother.

A rabbit’s body temperature is 39 °C.

The tyrosinase in Himalayan rabbits is:

• most active between 15 °C and 25 °C

• not active above 35 °C.

The fur on the nose, ears and feet of Himalayan rabbits is white for the first four weeks of life.

Explain what causes the fur to be white in the first four weeks of life.

From the age of four weeks, young rabbits move from the nest to the outside environment.

Explain why a Himalayan rabbit develops black nose, ears and feet from the age of four weeks.

Melanin is also produced in human skin.

Melanin absorbs ultraviolet (UV) radiation from sunlight and prevents the radiation reaching deep into the skin.

UV radiation is a type of ionising radiation.

Explain the advantage of producing melanin in human skin when exposed to bright sunlight.

In 1866, a monk called Gregor Mendel published the results of his investigations into inheritance in pea plants.

Pea plants produce seeds in a pod.

Figure 9 shows a pea pod.

Pea seeds can be round or wrinkled in shape.

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

Figure 10 shows the results.

In questions 08.1 to 08.3 use the following symbols to represent the alleles: R = dominant allele for round seeds r = recessive allele for wrinkled seeds.

In Figure 10, the genotype of plant 1 is RR.

Give the genotype of plant 2.

Mendel collected the seeds from plants 3 and 4 and grew new plants from the seeds.

Mendel crossed the new plants.

Complete the Punnett square diagram in Figure 11.

You should show:

• the male gametes

• the offspring genotypes.

Give the ratio of round seeds to wrinkled seeds in the offspring in Figure 11.

Some of the offspring in Figure 11 are homozygous and some are heterozygous.

What does ‘heterozygous’ mean?

Mendel published his work in 1866.

Suggest two reasons why the importance of Mendel’s work was not recognised until the early 1900s.

Some human disorders are inherited.

Polydactyly is an inherited disorder.

• A person with polydactyly has extra fingers or toes.

• Polydactyly is caused by a dominant allele.

What is a dominant allele?

Figure 10 shows the inheritance of polydactyly in one family.

In questions 06.2 and 06.3, use the following symbols: D = allele for having polydactyly d = allele for not having polydactyly.

Person 1 is heterozygous.

Explain how Figure 10 shows that person 1 is heterozygous.

Persons 6 and 7 are expecting a fourth child.

A doctor states that the probability of having a child with polydactyly is 0.5

Explain how the doctor determined this probability.

You should:

• draw a Punnett square diagram

• give the genotype of person 6 and the genotype of person 7

• identify all the offspring that will have polydactyly.

Cystic fibrosis (CF) is another inherited disorder caused by a mutation.

The mutation occurs in a gene called CFTR.

For the CFTR gene, one allele in every 50 in the UK population is the cystic fibrosis allele.

Explain why only one person in 2500 in the UK population has cystic fibrosis.

Hormones are important for regulating the menstrual cycle. During the menstrual cycle, eggs mature inside follicles in the ovaries. A 27-year-old woman was infertile. A doctor tested a sample of the woman’s blood. The test did not detect any follicle stimulating hormone (FSH) in the woman’s blood. The doctor gave the woman daily injections of FSH for 7 days. The doctor measured:

• the concentration of FSH in the woman’s blood

• the concentration of oestrogen in the woman’s blood

• the volumes of developing follicles in the ovaries.

Figure 12 shows the results.

Give evidence from Figure 12 that the follicles in the ovaries release oestrogen.

Injection of FSH caused the development of a number of follicles. The mean diameter of the follicles on day 11 was 22 millimetres. Calculate the number of follicles in the woman’s ovaries on day 11. Assume each follicle is a sphere.

r = radius

Give your answer to the nearest whole number.

Before treatment with FSH, the woman had underdeveloped breasts. Explain why the lack of FSH in the woman’s blood caused underdeveloped breasts.

Usually males and females both produce FSH. The woman had inherited a faulty gene for FSH production from each of her parents. The woman’s parents both produce FSH. Show how the woman’s parents could have a child that does not produce FSH. You should:

• draw a Punnett square diagram

• identify the phenotype of each offspring genotype

• use the symbols below: H = allele for making FSH h = allele for not making FSH

The woman continues to have injections of FSH. The woman has a child with a man who is heterozygous for the FSH gene. Explain why the probability that the child will be able to produce FSH is 0.5.

Maple syrup urine disease (MSUD) is a rare inherited human condition. The allele for MSUD is recessive.

What is a recessive allele? Tick (✓) one box.

Figure 1 shows the inheritance of MSUD in one family.

The symbol is not in the key for Figure 1. What would this symbol represent?

Persons 1 and 2 in Figure 1 have a child with MSUD and some children

without MSUD.

Complete Figure 2 to show the possible genotypes of the children. Use the following symbols: N = allele for not having MSUD n = allele for MSUD

What is the phenotype of a person with the genotype Nn ?

What percentage of the offspring in Figure 2 will have MSUD? Tick (✓) one box.

Which scientific term describes the allele N ? Tick (✓) one box.

Alleles are found in the nucleus of a cell. What chemical substance are alleles made from?

People with MSUD must eat a special diet to reduce their intake of some types of amino acid. Which component of the diet is made of amino acids? Tick (✓) one box.

Maple syrup urine disease (MSUD) is a rare inherited human condition. MSUD is usually diagnosed early in childhood and can be controlled by having a low-protein diet. Figure 4 shows the inheritance of MSUD in one family.

The allele for MSUD is recessive.

Give one piece of evidence from Figure 4 which shows that MSUD is a recessive condition.

Persons 7 and 8 in Figure 4 are expecting a fourth child. Determine the probability that the child will have MSUD. You should:

• draw a Punnett square diagram

• identify the phenotype of each offspring genotype

• use the symbols:

N = allele for not having MSUD n = allele for MSUD.

Figure 5 shows chemical reactions involved in the normal breakdown of some types

of amino acid inside body cells.

A person with MSUD cannot make Enzyme 2.

One of the final products shown in Figure 5 is urea. Where in the human body are the reactions shown in Figure 5 most likely to occur? Tick (✓) one box.

Scientists can analyse blood samples or urine samples to see if a person has MSUD. The test identifies high concentrations of toxic substance P, shown in Figure 5.

Explain why the blood of a person with MSUD will have a high concentration of toxic substance P. Use information from Figure 5.

Explain why the urine of a person with MSUD will have a high concentration of toxic substance P.

Explain why a person with MSUD must have a low-protein diet.

Potato blight is a disease of potato plants. Potato blight is caused by the fungus Phytophthora infestans.

What is the genus of the fungus that causes potato blight?

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The fungus grows near the surface of the potato. How does growing near the surface help the fungus to respire?

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A farmer sprays his potato plants with a pesticide.

The pesticide kills the fungus that causes potato blight. Spraying the crop with a pesticide could decrease biodiversity in a river flowing through his farm.

What does ‘biodiversity in a river’ mean?

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The farmer sprayed pesticide on his potato plants. The next day it rained heavily. Explain why the biodiversity in the river decreased.

Another method of preventing potato blight is to breed potatoes that are resistant to blight. Resistance to potato blight is controlled by two alleles: R = a dominant allele for having resistance to blight. r = a recessive allele for not having resistance to blight. A scientist crosses two potato plants. Each plant has the genotype Rr.

Complete Figure 9 to show the possible genotypes of the offspring produced.

Draw a ring around one of the homozygous genotypes in Figure 9.

What percentage of the offspring in Figure 9 will be resistant to potato blight?

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Potatoes can also reproduce asexually. Potatoes from one plant can be planted in the ground to produce new potato plants. All the new plants from a parent plant that is resistant to blight will also be resistant to blight. Explain why.

Sickle cell anaemia is an inherited condition that affects red blood cells. Sickle cell anaemia is caused by a mutation in the gene for haemoglobin. Haemoglobin is the red pigment found in red blood cells. A person who is homozygous for the normal haemoglobin allele (H^A) produces normal red blood cells. A person who is homozygous for the mutated allele (H^S):

• produces red blood cells with abnormal haemoglobin

• has red blood cells that can form an altered shape

• has sickle cell anaemia and becomes ill.

A person who is heterozygous:

• has both normal and abnormal haemoglobin in the red blood cells

• has sickle cell trait

• is generally healthy but can become ill in certain circumstances.

Give the reason why a mutation in the gene coding for haemoglobin could be harmful.

Figure 13 shows some red blood cells from the blood of a person with sickle cell trait.

Calculate the proportion of cells in Figure 13 that have an altered shape.

Figure 14 shows the inheritance of sickle cell anaemia in one family.

Persons 8 and 9 in Figure 14 are expecting a second child. Determine the probability that the child will be a girl with sickle cell trait. You should:

• draw a Punnett square diagram

• identify the phenotype of each offspring genotype

• use the symbols: H^A = normal haemoglobin allele H^S = mutated haemoglobin allele.

Without medical treatment, people with sickle cell anaemia are frequently ill and have a reduced life expectancy. The malarial parasite cannot live in the red blood cells of a person who has

A scientist stated: ‘It is an advantage for people to have the H^S allele in countries where malaria occurs.’ Evaluate the scientist’s statement.

Sperm cells and egg cells are formed by meiosis.

During meiosis a cell divides twice.

How many sperm cells are formed when a cell divides by meiosis?

Human body cells contain 46 chromosomes.

How many chromosomes are in each human egg cell?

Dupuytren’s is a disorder that affects the hands.

One form of Dupuytren’s is caused by a dominant allele (D).

The allele for not having Dupuytren’s is recessive (d).

What is an allele?

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A man with Dupuytren’s has the genotype Dd.

Which word describes the man’s genotype?

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The man with Dupuytren’s (Dd) and a woman who does not have Dupuytren’s (dd) plan to have a child.

Complete the genetic diagram in Figure 4 to show the possible genotypes of the child.

Draw a ring around the genotype of a child in Figure 4 who will have Dupuytren’s.

What is the chance of the child having Dupuytren’s?

Tick (✓) one box.

A genetic disorder develops as a result of a change in a gene.

What scientific term describes a change in a gene?

People with a family history of some genetic disorders are offered embryo screening.

Suggest one way embryo screening can help people with a family history of a genetic disorder.

There are two types of cell division: mitosis and meiosis.

Describe three differences between the processes of mitosis and meiosis.

Describe one similarity between the processes of mitosis and meiosis.

Dupuytren’s is a disorder that affects the hands.

Figure 6 shows the inheritance of Dupuytren’s in one family.

Dupuytren’s is caused by a dominant allele in this family. D = dominant allele d = recessive allele

Give the genotype of person 1. Explain your answer.

Person 7 and person 8 in Figure 6 are expecting a fourth child. What is the probability of the child having Dupuytren’s? You should:

• draw a Punnett square diagram

• identify which offspring have Dupuytren’s

Explain how Figure 6 shows the allele for Dupuytren’s is not on the Y chromosome.

The shape of a person’s earlobes is controlled by a gene.

Figure 3 shows two types of earlobe.

A dominant allele codes for free earlobes.

What is a dominant allele?

Tick (✓) one box.

A man with free earlobes and a woman with attached earlobes have children together.

Complete Figure 4 to show the possible genotypes of the children.

Use the symbols:

E = allele for free earlobes e = allele for attached earlobes

What is the probability that one of the children would have attached earlobes?

Use Figure 4.

Tick (✓) one box.

Figure 5 shows the inheritance of the sex chromosomes, X and Y.

Complete Figure 5 to show the sex chromosomes in the gametes of the man and the woman.

Calculate the probability that the man and the woman’s next child will be a girl with attached earlobes.

Use the equation:

probability of a girl with attached earlobes

= probability of attached earlobes probability of being a girl

Table 2 gives the classification of four plant species.

Species 1 and 3 are the most closely related.

What information in Table 2 gives evidence for this?

Figure 6 shows the inheritance of flower colour in two species of plant.

• In pea plants and in snapdragon plants, flower colour is controlled by one pair of alleles.

• In Figure 6 the parental generation plants are homozygous for flower colour.

• In heterozygous pea plants, the allele for red flower colour is dominant.

• In heterozygous snapdragon plants, the alleles for flower colour are both expressed.

Use the following symbols for alleles in your answers to Questions 05.2 to 05.4:

Pea plants
Snapdragon plants

R = allele for red flowers C^R = allele for red flowers r = allele for white flowers
C^W = allele for white flowers

What is the genotype of the red-flowered pea plants in the F₁ generation?

What is the genotype of a white-flowered snapdragon plant?

A gardener crossed two pink-flowered snapdragon plants.

Draw a Punnett square diagram to show why only some of the next generation plants had pink flowers.

Identify the phenotypes of all the offspring plants.

What percentage of the offspring would you expect to have pink flowers?

Commercially, hundreds of pink-flowered snapdragon plants can be produced from one pink-flowered plant.

Figure 7 shows a tissue culture technique used for producing many plants from one plant.

Give a reason for each of the following steps shown in Figure 7.

Several groups of cells are scraped off the leaf:

Nutrients are added to the agar jelly:

Hormones are added to the agar jelly:

The plant cells are kept in sterile conditions:

The plant cells are kept at 20^oC:

Explain why the method shown in Figure 7 produces only pink-flowered plants.

In the mid-19th century, a scientist studied inheritance in pea plants.

The scientist’s work was the beginning of our modern understanding of genetics.

What is the name of this scientist?

Tick one box.

In the mid-20th century, other scientists identified the chemical substance that makes up genetic material.

What is the name of the chemical substance that makes up genetic material?

Tick one box.

A gene often has two alleles.

One allele is dominant and the other allele is recessive.

When is a recessive allele expressed as a characteristic?

Tick one box.

A scientist investigated the inheritance of height in pea plants.

The scientist crossed tall pea plants with short pea plants.

Figure 4 shows the scientist’s results.

In Questions 04.4 and 04.5, use the following symbols to represent alleles:

T = the dominant allele for tall. t = the recessive allele for short.

In Figure 4, the genotype of plant 1 is TT.

Give the genotype of plant 2.

The scientist crossed plant 3 with plant 4.

Complete Figure 5 to show the offspring produced from this cross.

Draw a circle around one of the homozygous offspring in Figure 5.

What is the ratio of tall plants: short plants in the offspring in Figure 5 ?

Scientists want to breed cows that produce milk with a low concentration of fat.

Figure 10 shows information about the milk in one group of cows.

The cows were all the same type.

In Figure 10 the mean percentage of fat in the milk is equal to the modal value.

Give the mean percentage of fat in the milk of these cows.

A student suggested:

‘The percentage of fat in milk is controlled by one dominant allele and one recessive allele.’

How many different phenotypes would this produce?

Tick one box.

Give the evidence from Figure 10 which shows the percentage of fat in the milk is controlled by several genes.

One of the genes codes for an enzyme used in fat metabolism.

A mutation in this gene causes a reduction in milk fat.

The mutation changes one amino acid in the enzyme molecule.

Explain how a change in one amino acid in an enzyme molecule could stop the enzyme working.

The scientists found one cow with a mutation.

The cow’s milk contained only 2.9% fat.

Figure 11 shows the percentage of fat in the milk of cattle related to the cow with the mutation.

The values for male cattle are the mean values of their female offspring.

Animal 8 is homozygous.

The mutation in animal 7 produced a dominant allele for making low-fat milk.

Give evidence from Figure 11 that animal 7 is heterozygous.

Animals 7 and 8 produced 11 offspring. These offspring were produced by in vitro fertilisation (IVF).

The embryos from IVF were transferred into 11 other cows.

Suggest why IVF and embryo transfer were used rather than allowing animals 7 and 8 to mate naturally.

Draw a Punnett square diagram to show a cross between animals 7 and 8 .

Identify which offspring produce low-fat milk and which offspring produce high-fat milk.

Use the following symbols: D = dominant allele for making low-fat milk d = recessive allele for making high-fat milk

low-fat milk.

The scientists will selectively breed some of the animals shown in Figure 11.

Describe how the scientists would do this.