The roles of genes in determining the phenotype

Cells known as melanocytes produce the black pigment melanin, which affects hair, skin and eye colour.

Feb/March 2018

Fig. 10.1 illustrates chemoreceptor cells inside a taste bud. Two of the chemoreceptor cells have formed synapses with dendrites of sensory neurones.

Feb/March 2025

Explain what the term heterozygous genotype means.

May/June 2012

Raspberry plant stems bear spines. Fig. 7.1 shows part of a raspberry plant. Two genes, A/a and B/b, determine spine colour. These genes are located on separate chromosome pairs. Allele A causes a pink anthocyanin pigment to form in the spines. Allele B does not act on its own, but it intensifies the colour produced by allele A so that the spines become red. Alleles a and b do not affect colour. When anthocyanin is absent, the spines are green.

May/June 2017

In Europe, cats that are black or white are common, while brown cats are less common. One gene, which codes for an enzyme needed for pigment production, has two alleles. • The dominant allele, $B$, produces black fur. • The recessive allele, $b$, produces brown fur. A second gene can influence fur colour. • The dominant allele, $A$, prevents pigment production, so the cat has white fur. • The recessive allele, $a$, has no effect on fur colour. The two genes lie on separate pairs of autosomes.

May/June 2017

Oculocutaneous albinism (OCA) is one category of albinism, and OCA occurs in several distinct forms. One variant of OCA is OCA1A, which is caused by a recessive mutation in the autosomal gene, TYR, that codes for the enzyme tyrosinase. Tyrosinase functions in the biosynthetic pathway that leads to melanin production, and melanin is the pigment that determines the colour of hair, skin and eyes. A person with OCA1A has white hair, very pale skin and pink eye colour.

May/June 2019

The patty pan squash plant, $Cucurbita\ pepo$, bears edible fruits that differ in colour.

May/June 2020

Haemoglobin consists of two $\alpha$-globin chains and two $\beta$-globin chains. A person may carry a mutation in the gene that codes for $\beta$-globin. This results from a base substitution and causes abnormal $\beta$-globin to be made and, therefore, abnormal haemoglobin. A person homozygous for the mutant allele has a condition known as sickle cell anaemia. Outline the phenotypic effects of having abnormal haemoglobin in a person with sickle cell anaemia.

May/June 2021

The fruit fly, Drosophila melanogaster, normally has red eyes. The gene controlling eye colour has four alleles: red, apricot, honey and white.

May/June 2022

The jaguar, Panthera onca, is a large cat found mainly in South America. Most jaguars have light brown fur with black spots, as shown in Fig. 1.1. Some jaguars have entirely black fur, as shown in Fig. 1.2. The pigments responsible for fur colour are made through biochemical pathways that occur in cells known as melanocytes. These pathways are similar to those found in human melanocytes. The melanocortin 1 receptor (MC1R) is found on the cell surface membrane of melanocytes and is coded for by the MC1R gene. Fig. 1.3 shows the processes that take place in jaguar melanocytes.

May/June 2022

Epistasis is when a gene at one locus influences how a gene at a different locus is expressed.

May/June 2022

For plants and humans alike, an organism’s phenotype is controlled by its genotype together with environmental influences.

May/June 2024

Rice, $\textit{Oryza sativa}$, is a major grain crop. A rice grain is a seed and may have a structure called an awn, which extends from the tip of the grain. Fig. 3.1 shows one rice grain with an awn present and another rice grain with no awn present.

May/June 2025

Inherited diseases result from genetic mutations.

May/June 2025

The disease-causing bacterium, Pseudomonas aeruginosa, can occur as a ‘biofilm’. A biofilm is a layer of bacteria growing on a surface and attached to one another. Antibiotics find such biofilms difficult to control. A mutant strain of P. aeruginosa has been discovered that forms biofilms indistinguishable from those made by the wild-type bacteria. However, the mutant strain differs from the wild-type in its resistance to antibiotic A.

Oct/Nov 2010

Section B Select one question to answer.

Oct/Nov 2010

A collection of plants called Rapid Cycling Brassicas (RCBs) has been created for use in schools and colleges in genetics experiments. As RCB seedlings grow, they may show either purple stems or non-purple stems. Their seed leaves may be green or yellow-green. Purple stems and green seed leaves are each controlled by dominant alleles. The genes for stem colour and seed-leaf colour are found on separate chromosomes.

Oct/Nov 2012

Fig. 4.1 illustrates the stages of spermatogenesis in a mammal.

Oct/Nov 2012

Huntington’s disease (HD) is an inherited disorder of the central nervous system. The symptoms of HD usually begin in adulthood and can include uncontrollable muscular movements, short-term memory loss and changes in mood. HD results from a dominant allele of the huntingtin gene on chromosome 4.

Oct/Nov 2013

Section B. Attempt only one question.

Oct/Nov 2013

Maize first arose in the Americas, and $55\%$ of global maize production comes from that region. Fig. 5.1 displays the mean maize yields in the USA from $1860$ to $2010$.

Oct/Nov 2013

Phenylketonuria (PKU) is an inherited disorder that causes the amino acid phenylalanine to build up in the blood at a higher concentration. If a newborn baby is not treated, brain damage may follow. Therefore, babies can be screened for PKU soon after birth.

Oct/Nov 2014

In shorthorn cattle, coat colour is determined by a codominant pair of alleles. The colour may be red, white, or roan, meaning a blend of red and white. Horn development is controlled by a separate pair of alleles. The allele for horns is recessive to the allele for hornless cattle.

Oct/Nov 2015

Describe the kinds of gene mutation that may happen and explain how certain mutations can produce a shorter polypeptide.

Oct/Nov 2017

Galactosaemia is a rare inherited disorder in which a build-up of the monosaccharide galactose can lead to an enlarged liver, kidney failure and brain damage. Galactose is made in the body when the sugar lactose in milk is digested. Galactosaemia results from a recessive mutation in the GALT gene. The usual dominant allele codes for an enzyme that converts galactose into glucose.

Oct/Nov 2017

Section B. Choose and answer one question.

Oct/Nov 2017

Describe the types of gene mutation that can occur and explain how some mutations can result in a shortened polypeptide.

Oct/Nov 2017

Galactosaemia is an uncommon inherited disorder in which galactose, a monosaccharide, builds up and may lead to an enlarged liver, kidney failure and brain damage. Galactose is formed in the body when lactose, the sugar in milk, is digested. Galactosaemia results from a recessive mutation in the $\textit{GALT}$ gene. The normal dominant allele codes for an enzyme that changes galactose into glucose.

Oct/Nov 2017

Tyrosinase is an enzyme found in mammals. It plays a part in making melanin pigment. Mutations in the tyrosinase gene alter a mammal’s hair colour. Table 5.1 sets out the DNA sequences for codons 974-985 of: • the normal tyrosinase gene in humans (human) • the normal tyrosinase gene in cats with pigmented hair (normal cat) • the tyrosinase gene in cats with an albino phenotype (albino cat) The amino acid sequences for each tyrosinase are given in the shaded rows.

Oct/Nov 2020

The Labrador is one type of domestic dog. Labradors can have brown, black or yellow fur. In Labradors, $TYRP1$ is one gene that determines fur colour. This gene has two alleles, $B$ and $b$. - The dominant allele, $B$, codes for the enzyme tyrosinase, which acts in the pathway that makes melanin and so produces black fur. - Melanin production in Labradors is very similar to melanin production in humans. - The recessive allele, $b$, codes for an enzyme that leads to a brown form of melanin being produced, so the fur is brown.

Oct/Nov 2021

The enzyme iduronate $2$-sulfatase breaks down particular complex molecules. Hunter syndrome is an uncommon inherited human condition caused either by iduronate $2$-sulfatase not working correctly or by iduronate $2$-sulfatase not being made. This leads to accumulation of the complex molecules and progressive harm to many organs. Explain how a gene mutation can lead to iduronate $2$-sulfatase not functioning properly or not being produced.

Oct/Nov 2021

In domestic turkeys, sex is determined differently from the way it is in humans. Their sex chromosomes are called Z and W. Male turkeys are ZZ and female turkeys are ZW. The gene for feather colour is found only on the Z chromosome. The dominant allele gives bronze feathers, whereas the recessive allele gives brown feathers.

Oct/Nov 2022

The tiger barb, Puntigrus tetrazona, is a fish native to South America that is widely kept as an aquarium fish around the world. Fig. 6.1 illustrates the appearance (phenotype) of a normal (wild-type) tiger barb. Tiger barbs with a wild-type phenotype are gold with black stripes. Tiger barbs with an albino phenotype are gold with white stripes. In 2012, a fish breeder found a tiger barb showing a new transparent phenotype. This fish had a transparent body and black stripes. The fish breeder crossed the tiger barb with the new transparent phenotype and a tiger barb with the albino phenotype. All the F1 offspring were wild-type. These F1 offspring were then crossed with each other. Table 6.1 gives the phenotypes produced in the F2 generation and the number of fish with each phenotype.

Oct/Nov 2023

The South American fish Puntigrus tetrazona is a popular aquarium species around the world. Fig. 6.1 shows the phenotype of a normal (wild-type) tiger barb. Tiger barbs with the wild-type phenotype are gold with black stripes. Tiger barbs with the albino phenotype are gold with white stripes. In 2012, a fish breeder found a tiger barb with a novel transparent phenotype. This fish had a transparent body and black stripes. The breeder crossed the tiger barb with the new transparent phenotype with a tiger barb with the albino phenotype. All the F1 offspring were wild-type. These F1 offspring were then crossed with each other. Table 6.1 gives the phenotypes in the F2 generation and the number of fish showing each phenotype.

Oct/Nov 2023

Guinea pigs, Cavia porcellus, differ in both fur length and fur colour. Fig. 1.1 shows a guinea pig with short black fur. The A/a locus and the B/b locus each carry a gene that controls fur length and colour. These two gene loci lie on different autosomal chromosomes. Allele A gives short fur. Allele a gives long fur. A is dominant over a. Allele B gives black fur. Allele b gives chocolate fur. B is dominant over b.

Oct/Nov 2025

Explain how genes, proteins and phenotype are linked, with reference to two examples of human genetic diseases.

Oct/Nov 2025

Define the terms gene, genotype and phenotype.

Oct/Nov 2025

Guinea pigs, Cavia porcellus, show variation in fur length and fur colour. Fig. 1.1 shows a guinea pig with short black fur. Two genes that control fur length and fur colour are located at the A/a locus and the B/b locus. These two gene loci are on different autosomal chromosomes. Allele A gives short fur. Allele a gives long fur. A is dominant to a. Allele B gives black fur. Allele b gives chocolate fur. B is dominant to b.

Oct/Nov 2025

Explain the relationship between genes, proteins and phenotype, using two examples of human genetic diseases.

Oct/Nov 2025

The wolf, Canis lupus, is found in North America. Wolves can have either a grey coat or a black coat. The coat colour of one wolf depends on the DNA it inherits at the CPD103 gene locus. Each wolf receives two copies of CPD103, one from each parent. A wolf that inherits one copy of the black version of the CPD103 gene has a black coat. Besides determining black coat colour, the protein coded for by the CPD103 gene also provides protection against infectious lung disease. Canine distemper virus (CDV) causes serious lung disease in wolves. Wolves that were infected by CDV in the past have antibodies against CDV (anti-CDV antibodies) in their blood.

Oct/Nov 2025

The fruit fly, Drosophila melanogaster, feeds on sugars present in rotten fruit. A fruit fly with typical characteristics is called wild type. It has a grey body and wings that are longer than its abdomen. The genes controlling body colour and wing length are found on separate chromosomes. A fruit fly carrying mutations in these two genes has a black body and short wings. Fig. 5.1 shows a wild type fruit fly and a mutant fruit fly.

Oct/Nov 2025