Passage of information from parents to offspring

Gamete formation in male and female vertebrates, humans included, happens by meiosis.

Feb/March 2016

In one snail species, shell colour is determined by a gene with three alleles: allele $C^B$ gives a brown shell; allele $C^P$ gives a pink shell; allele $C^Y$ gives a yellow shell. Allele $C^B$ is dominant over both $C^P$ and $C^Y$, whereas $C^P$ is dominant over $C^Y$. Snail shells may also be banded, meaning they have dark stripes, or non-banded. The allele for non-banded, $N$, is dominant over the allele for banded, $n$.

Feb/March 2016

The foxglove, \textit{Digitalis purpurea}, is a plant commonly found in many regions of the world. Fig. 5.1 shows a foxglove. In foxgloves, flower colour is determined by two genes that interact with one another. Dominant allele $M$ codes for an enzyme involved in the production of a purple pigment. Recessive allele $m$ codes for a non-functioning enzyme, so no purple pigment is produced, giving a white colour. Dominant allele $D$ interacts with allele $M$ to produce dark purple flowers. Recessive allele $d$ does not interact with allele $M$. Neither allele $D$ nor allele $d$ interact with allele $m$. A double homozygous foxglove with dark purple flowers was crossed with a double homozygous recessive foxglove with white flowers. All of the offspring had dark purple flowers.

Feb/March 2017

MELAS syndrome is an inherited disorder caused by a mutation in a gene within mitochondrial DNA (mtDNA). Every mtDNA molecule is inherited from the mother. Fig. 6.1 depicts four generations of a family in which several members have MELAS syndrome.

Feb/March 2019

Fig. 7.1 illustrates two chromosomes at one phase of meiosis. The letters G to M indicate the dominant alleles of seven genes, while g to m indicate the recessive alleles of those same seven genes.

Feb/March 2021

Meiosis is a key process that adds to genetic variation within a population. Fig. 4.1 shows one pair of homologous chromosomes in prophase I of meiosis.

Feb/March 2022

Meiosis is termed a reduction division because the chromosome number in the daughter cells is reduced by half.

Feb/March 2023

Gregor Mendel, the scientist, examined how stem length varied in the pea plant, Pisum sativum. He published the findings of this study of the trait (characteristic) in 1866. Fig. 2.1 contains a diagram of a pea plant. Mendel noted that the pea plants he cultivated had either tall stems or dwarf (short) stems. In his investigation, he used crosses between pea plants showing these two phenotypes.

Feb/March 2024

Cystic fibrosis is an autosomal recessive genetic disease. Individuals with cystic fibrosis have a homozygous recessive genotype.

Feb/March 2024

The $HFE$ gene specifies the $HFE$ protein, which helps regulate how much iron the body absorbs. Iron is an essential mineral that can be obtained only from food in the diet. A mutation of the $HFE$ gene called C282Y causes hereditary haemochromatosis, which is an autosomal recessive disease. The mutant allele codes for a non-functioning $HFE$ protein. Individuals who are homozygous for the mutant allele make no functioning $HFE$ protein, so an excess of iron is absorbed by the body. The build-up of iron in body organs over many years can lead to organ damage. People who are heterozygous for the $HFE$ gene do not have hereditary haemochromatosis. They absorb more iron from their diet than people without the mutation, but this usually has no health effects.

Feb/March 2025

Fig. 5.1 shows a photomicrograph of one plant cell at a stage in meiosis.

Feb/March 2025

Fig. 3.1 illustrates the arrangement of a section of a seminiferous tubule in the human testis.

May/June 2010

Explain what the term gene mutation means.

May/June 2010

In humans, a rare recessive allele that is sex-linked causes the iris in the eye to change shape. This disorder is called cleft iris (CI).

May/June 2010

In humans, a rare recessive allele that is sex-linked causes the iris in the eye to have a different shape. This condition is called cleft iris (CI).

May/June 2010

Outline the procedure used in in-vitro fertilisation (IVF).

May/June 2011

Canavan disease is an inherited condition that is not sex-linked and causes progressive damage to neurones in the brain. Symptoms of the condition include loss of motor skills and mental retardation. These symptoms appear in early infancy, and many children with this condition die by the age of four years. People with Canavan disease do not have an enzyme called aspartoacylase, which breaks down N-acetyl aspartate. The accumulation of N-acetyl aspartate can interfere with the formation of the myelin sheath, especially in neurones of the brain.

May/June 2011

Meiosis is a kind of nuclear division that forms gametes for sexual reproduction.

May/June 2011

Huntington’s Disease (HD) is a serious neurological disorder whose symptoms usually start only after a person has reached sexual maturity. These symptoms can include memory loss and changes in personality and mood. HD is caused by a gene mutation on chromosome 4 in which the triplet code CAG is repeated many times. The allele formed is dominant.

May/June 2011

Meiosis is a form of nuclear division that produces gametes for sexual reproduction.

May/June 2011

Huntington’s Disease (HD) is a serious neurological disorder whose symptoms normally begin only after sexual maturity has been reached. These symptoms can include memory loss together with changes in personality and mood. HD results from a gene mutation on chromosome 4, where the triplet code $\text{CAG}$ is repeated numerous times. The allele produced is dominant.

May/June 2011

The fruit fly, Drosophila melanogaster, displays a wide range of phenotypic variation and has been used in experiments to illustrate inheritance principles. Most fruit flies have red eyes, although a white-eyed variant also occurs. Fig. 7.1 shows the red-eyed and white-eyed variants of the fruit fly. The eye-colour gene is found on the X chromosome.

May/June 2012

The fruit fly, Drosophila melanogaster, shows many phenotypic variations and has been used in experiments to illustrate the principles of inheritance.

May/June 2012

In in-vitro fertilisation (IVF), several oocytes are taken from a woman receiving treatment. Each oocyte is examined under a microscope. Explain why oocytes with a first polar body are selected for the fertilisation process.

May/June 2013

In cats, coat colour is controlled by a sex-linked gene that has two alleles for black or orange. If black cats are mated with orange cats: • all female offspring are tortoiseshell (black and orange patches) • all male offspring have the same colour as their mother.

May/June 2013

Resistance to the poison warfarin is now very widespread among rats. Warfarin blocks an enzyme in the liver, vitamin K epoxide reductase, which is needed to recycle vitamin K. This vitamin is used in making the substances needed for blood clotting. Rats that are vulnerable to warfarin die from internal bleeding. Rats that are homozygous for resistance to warfarin do not suffer internal bleeding when their diet supplies more than $70\,\mu\text{g}$ of vitamin K per kg body mass per day. Heterozygous rats are resistant to warfarin when their diet supplies about $10\,\mu\text{g}$ of vitamin K per kg body mass per day.

May/June 2013

During in-vitro fertilisation (IVF), several oocytes are collected from a woman receiving treatment, and each oocyte is examined under a microscope.

May/June 2013

In cats, coat colour is controlled by a sex-linked gene with two alleles that determine black and orange. If black cats are crossed with orange cats, every female offspring is tortoiseshell (black and orange patches), while every male offspring has the same colour as the mother.

May/June 2013

Russian scientists uncovered the fruits of the flowering plant, Silene stenophylla, preserved in a ground squirrel burrow within frozen sediments in Siberia. Dating methods indicate that the ground squirrel stored the fruits about $32\ 000$ years ago, just before the ground became permanently frozen. Tissue samples were removed from the fruits and cultured on a nutrient medium. After treatment with plant hormones to promote root and shoot development, $36$ whole plants were obtained. These regenerated plants, which were identical in appearance to each other, flowered and, after cross-pollination, formed seeds that could germinate.

May/June 2014

Bread wheat, $Triticum\ aestivum$, came about through hybridisation between three grass species, A, B and C. Each of those species had seven pairs of chromosomes $(2n = 14)$. The hybridisation process is illustrated in Fig. 3.1.

May/June 2014

(a) Spermatogenesis occurs in the seminiferous tubules of the testis. Fig. 4.1 shows a diagram of some of the cells in a small section of a seminiferous tubule.

May/June 2014

Deer mice, *Peromyscus maniculatus*, are petite rodents native to North America. Since they are mammals, their blood has haemoglobin, which binds oxygen in the lungs and releases it to respiring tissues. Deer mice vary in their genotypes for the genes coding for the $\alpha$-polypeptide chain of haemoglobin. In most deer-mouse populations, the majority of individuals are $A^1A^1$, whereas a smaller proportion are $A^0A^0$.

May/June 2014

At times during meiosis, homologous chromosomes do not separate at anaphase; this is called non-disjunction. Turner’s syndrome is the most common chromosome mutation in females. It may arise because non-disjunction occurs in meiosis during gametogenesis. Some of the gametes formed will therefore lack an X chromosome. In some cases of Turner’s syndrome, one of the pair of X chromosomes is not absent but has been damaged. The damaged X chromosome may have broken and then rejoined so that part of its structure is lost. Fig. 7.1 shows a normal X chromosome and two types of ‘damaged’ X chromosomes, $X_1$ and $X_2$: • In $X_1$, part of the ‘p’ arm of the chromosome is absent. This deletion causes reduced height in the female and defects such as narrowing of the aorta. • In $X_2$, part of the ‘q’ arm of the chromosome is absent. This deletion causes little or no development of the ovaries.

May/June 2014

Neurofibromatosis (NF) is a condition inherited genetically in humans in which tumours form on nervous tissue. One symptom, which may appear at about 20 years of age, is loss of sight caused by tumours on the optic nerve.

May/June 2014

Spermatogenesis occurs within the seminiferous tubules of the testis. Fig. 4.1 is a diagram of some of the cells in a small region of a seminiferous tubule.

May/June 2014

Deer mice, $Peromyscus\ maniculatus$, are small rodents native to North America. As with all mammals, their blood contains haemoglobin, which binds with oxygen in the lungs and releases oxygen to respiring tissues. Deer mice show variation in the genotypes of the genes that encode the $\alpha$-polypeptide chain of haemoglobin. In most deer mouse populations, most individuals have genotype $A^1A^1$, whereas fewer individuals have genotype $A^0A^0$.

May/June 2014

During meiosis, homologous chromosomes can sometimes fail to separate at anaphase. This is called non-disjunction. Turner’s syndrome is the most common chromosome mutation in human females. It may arise because of non-disjunction in meiosis during gametogenesis. As a result, some gametes lack an X chromosome. In other cases of Turner’s syndrome, one chromosome from the X pair is not missing, but it is damaged instead. The damaged X chromosome may have broken and then rejoined, so that part of its structure has been lost. Fig. 7.1 shows a normal X chromosome together with two ‘damaged’ X chromosomes, $X_1$ and $X_2$. For $X_1$, part of the ‘p’ arm of the chromosome is absent. This deletion causes the female to have reduced height and abnormalities such as narrowing of the aorta. For $X_2$, part of the ‘q’ arm of the chromosome is absent. This deletion causes little or no development of the ovaries.

May/June 2014

In mice, the degree of fur pigmentation is determined by multiple alleles of one gene. The alleles are arranged below in descending dominance, with $C$ being the most dominant: $C$ = full colour $C^{ch}$ = chinchilla $C^{h}$ = himalayan $C^{p}$ = platinum $C^{a}$ = albino

May/June 2015

A single autosomal gene that determines fur colour in cats has three alleles: • allele $C^B$ specifies black fur • allele $C^{CH}$ specifies chocolate fur • allele $C^{CM}$ specifies cinnamon (orange-brown) fur. Allele $C^B$ is dominant over both $C^{CH}$ and $C^{CM}$. Allele $C^{CH}$ is dominant over $C^{CM}$.

May/June 2015

In mice, the degree of fur pigmentation is determined by several alleles of one gene. These alleles are arranged in dominance order, with $C$ the most dominant: $C$ = full colour, $C^{ch}$ = chinchilla, $C^{h}$ = himalayan, $C^{p}$ = platinum, $C^{a}$ = albino.

May/June 2015

Describe how chromosomes behave during meiosis.

May/June 2016

The fruit fly, Drosophila melanogaster, has eyes, a striped abdomen and wings that are longer than its abdomen; this phenotype is known as a wild-type fly. Mutation has produced many different forms of these features. Table 6.1 presents diagrams of a wild-type fly and three other flies, each displaying one recessive mutation.

May/June 2016

In domestic chickens, the sex chromosomes are called W and Z. A male has two Z chromosomes, whereas a female has one W chromosome and one Z chromosome. The gene controlling feather colour lies on an autosome. This gene (C) has two alleles: black ($C^{B}$) and splashed-white ($C^{S}$). If a black-feathered chicken is crossed with a splashed-white chicken, every offspring shows blue feathers. The gene controlling feather pattern is found on the Z chromosome, not on the W chromosome. This gene has two alleles: barred (striped) feathers (A) and non-barred feathers (a).

May/June 2016

Describe how chromosomes behave throughout meiosis.

May/June 2016

The fruit fly, Drosophila melanogaster, has eyes, a striped abdomen, and wings that are longer than its abdomen. A fly with these characteristics is called a ‘wild-type’ fly. Mutation has produced many different forms of these characteristics. Table 6.1 presents diagrams of a wild-type fly and three other flies, each showing one recessive mutation.

May/June 2016

Raspberry plant stems carry spines. Fig. 7.1 illustrates part of a raspberry plant. Two genes, $A/a$ and $B/b$, determine the colour of the spines. These genes are located on separate pairs of chromosomes. Allele $A$ causes a pink anthocyanin pigment to be produced in the spines. Allele $B$ has no effect on its own, but it enhances the colour produced by allele $A$ so that the spines are red. Alleles $a$ and $b$ do not affect colour. If anthocyanin is absent, the spines are green.

May/June 2017

Domestic goats, Capra hircus, display a very broad variety of coat patterns and colours. A single gene affecting coat colour and pattern has multiple alleles. Four of these alleles are: A, the allele for white, which is dominant over all the others; $A^{b}$, the allele for badgerface (stripes on face) and $A^{g}$, the allele for grey, which are codominant; and a, the allele for black, which is recessive to all the others.

May/June 2018

In the sweet pea plant, Lathyrus odoratus, one gene determines flower colour and a different gene determines pollen grain shape. Flowers may be purple or red. Pollen grains may be long or round. The inheritance of these genes is an example of autosomal linkage. The allele F for purple flowers is dominant over the allele f for red flowers. The allele G for long pollen grains is dominant over allele g for round pollen grains.

May/June 2018

Differences in a single characteristic across a population are generally due to genetic influences and environmental influences.

May/June 2018

In the fruit fly Drosophila melanogaster, genes determine eye colour and wing shape. E/e are alleles of a gene controlling eye colour: E gives red eyes, while e gives purple eyes. N/n are alleles of a gene controlling wing shape: N gives normal (functional) wings, while n gives vestigial (short, non-functional) wings. Both of these genes are autosomal. A dihybrid cross was performed between one fly with red eyes and normal wings and another with purple eyes and vestigial wings. Each parent was homozygous for both genes. The F1 offspring were crossed to produce the F2 generation. The results are shown in Table 2.1.

May/June 2018

Meiosis is referred to as reduction division. Explain why meiosis is needed in the life cycle of a sexually reproducing organism.

May/June 2018

In the brain, some neurones make the neurotransmitter dopamine. Parkinson’s disease develops when the neurones that make dopamine die. This disease usually appears in people older than 55 years. People younger than this who show these symptoms are described as having early onset Parkinson’s disease (EOPD). Recessive mutations in a gene called $\textit{PINK1}$, found on chromosome 1, an autosome, are thought to be one cause of EOPD. A person with this type of EOPD has a homozygous recessive genotype.

May/June 2019

Albinism develops when the biosynthetic pathway involving the enzyme tyrosinase is disrupted. It is estimated that 1 in 17 000 people in the world has albinism.

May/June 2019

Mitochondrial complex I is a very large enzyme complex and forms one component of the electron transport chain. It contains many different polypeptides. The genes that code for these polypeptides are found either in mitochondrial DNA (mtDNA) or in nuclear DNA. If mutations occur in these genes, the enzyme produced may work less effectively. The resulting condition is called mitochondrial complex I deficiency. When it is severe, it may cause death in early childhood.

May/June 2020

The summer squash plant, $\u001a\textit{Cucurbita pepo}$, bears edible fruits that occur in different shapes. Fig. 2.1 shows the fruits of three squash varieties. In squash plants, fruit shape is controlled by two genes, $A/a$ and $B/b$, which are found on different chromosomes. • A disc-shaped fruit is formed when both dominant alleles, $A$ and $B$, are present. • A spherical fruit is formed when either allele $A$ or allele $B$ is present, but not when both $A$ and $B$ are present. • A long fruit is formed when both allele $A$ and allele $B$ are absent.

May/June 2020

The courgette plant, *Cucurbita pepo*, bears edible fruits that differ in colour and form. Fruit colour in courgettes is determined by gene $A/a$. Fruit shape in courgettes is determined by gene $B/b$. A yellow fruit appears when the dominant allele $A$ is present. A round fruit appears when the dominant allele $B$ is present. Genes $A/a$ and $B/b$ are located on separate chromosomes. Table 1.1 gives the genotypes and phenotypes of four courgette varieties with regard to fruit colour and shape.

May/June 2020

Individuals differ in the molecules found on the surface of their red blood cells. For instance, in the ABO blood group system, people can be classified as one of four blood groups: A, B, AB and O. A single gene, $I$, with three alleles, controls these blood groups. Gene $I$ is an example of a gene with multiple alleles. Alleles $I^A$ and $I^B$ are codominant. Allele $I^o$ is recessive to alleles $I^A$ and $I^B$.

May/June 2021

The fruit fly, Drosophila melanogaster, feeds on sugars present in damaged fruit. A fruit fly showing normal characteristics is called wild type. It has red eyes and wings that are longer than its abdomen. Mutant forms include flies with purple eyes or short (vestigial) wings. Fig. 7.1 shows a wild type fruit fly and a mutant fruit fly with purple eyes and vestigial wings. The genes coding for eye colour and wing length are located on the same chromosome. Allele R for red eyes is dominant to allele r for purple eyes. Allele N for long wings is dominant to allele n for vestigial wings.

May/June 2021

Domestic rabbits show variation in both fur length and fur colour. Fig. 6.1 illustrates a domestic rabbit with short fur and the Himalayan fur-colour pattern. The two genes controlling the length and colour of this rabbit’s fur are located at the A / a locus and the B / b$^h$ / b locus. These two gene loci are found on different chromosomes. • The allele A gives short fur. • The allele a gives long fur. • A is dominant over a. • The allele B gives black fur over the whole body. • The allele b$^h$ gives black fur on the nose, ears, paws and tail of the rabbit, with white fur on the rest of the body (Himalayan pattern). • The allele b gives white fur over the whole body (albino). • B is dominant over b$^h$ and b$^h$ is dominant over b.

May/June 2023

Fig. 8.1 shows a cell from the testis of a locust at the late prophase I stage of meiosis.

May/June 2023

Several different factors influence the rate of cellular respiration.

May/June 2023

In sexual reproduction, meiosis takes place to form haploid gametes from a diploid cell.

May/June 2023

In the freshwater fish species $Oryzias\ latipes$, individuals show four body colour patterns, as set out in Table 5.1. Two unlinked genes determine the body colour patterns shown in Table 5.1. One gene governs whether the body colour is red or white: • dominant allele $R$ = red • recessive allele $r$ = white. The other gene governs whether black spots are present or absent: • dominant allele $B$ = with black spots • recessive allele $b$ = without black spots. A fish that is homozygous recessive at both loci is white. Genetic crosses were used to investigate how the four body colour patterns are inherited. Males that were red with black spots, and homozygous at both loci, were crossed with females that were white. The F1 offspring were all red with black spots. These F1 offspring were then crossed to produce the F2 generation.

May/June 2023

A wide range of genes contributes to pigment production in mammals. One example is the TYR gene.

May/June 2024

The tortoise beetle, Chelymorpha alternans, is an insect found in Panama that displays several distinct colour patterns. Fig. 3.1 shows a tortoise beetle. Researchers have identified a gene, $L$, that determines colour pattern in the pronotum and elytra. Gene $L$ has four different alleles: $L^{V}$, $L^{T}$, $L^{R}$ and $L^{r}$. Table 3.1 presents five different colour pattern phenotypes of tortoise beetles together with their genotypes.

May/June 2024

Porphyria is a rare set of inherited diseases in which porphyrins, molecules called porphyrins, build up in the body. Three examples of porphyria are: X-linked protoporphyria; variegate porphyria; congenital erythropoietic porphyria (CEP).

May/June 2025

In plants and animals, sexual reproduction includes: - gamete formation through meiosis - fertilisation.

May/June 2025

A recent advance in fertility treatment is known as in-vitro maturation (IVM). It is less expensive and safer than the usual procedure used in in-vitro fertilisation (IVF), particularly for women with polycystic ovaries. Hormone treatment can be hazardous for women with this condition, in which several ovarian follicles mature at the same time. IVF and IVM are compared in Fig. 3.1.

Oct/Nov 2010

Section B Answer one question.

Oct/Nov 2010

Section B. Answer only one question.

Oct/Nov 2010

Describe how a spermatid matures into a spermatozoon (sperm).

Oct/Nov 2010

Tuberous Sclerosis Complex (TSC) is a genetic disorder caused by a dominant allele in the TSC gene, which results in abnormal tissue growth in organs such as the heart, lungs and kidneys. With treatment, children who have TSC can lead fairly normal lives. Around 33% of people with TSC have at least one parent who also has the condition.

Oct/Nov 2011

Tuberous Sclerosis Complex (TSC) is a genetic disorder caused by a dominant allele of a TSC gene, leading to abnormal tissue growth in organs such as the heart, lungs and kidneys. With treatment, children with TSC can live reasonably normal lives. Roughly 33% of people with TSC have at least one parent who has the condition.

Oct/Nov 2011

Explain how alterations in the nucleotide sequence of DNA can change the amino acid sequence in a protein.

Oct/Nov 2011

In the small intestine, the enzyme lactase hydrolyses the disaccharide lactose into the monosaccharides glucose and galactose. If lactase is lacking, a condition called lactose intolerance can result. The lactose then moves undigested into the large intestine, causing diarrhoea. Some babies are born with congenital lactase deficiency, which is an inherited disorder, and they need lactose-free milk from birth. Suggest how two parents who can digest lactose may have a child with congenital lactase deficiency.

Oct/Nov 2011

Spermatogenesis, which is the formation of male gametes, takes place in the testes of a male starting at puberty. Fig. 3.1 presents the order of events during spermatogenesis.

Oct/Nov 2011

Flowers are the structures in which sexual reproduction occurs in plants. Before fertilisation and development can happen, pollination has to take place. This may be self-pollination or cross-pollination, and it can be carried out by insects or by wind.

Oct/Nov 2012

Flowers are the structures used for sexual reproduction in plants. Pollination has to happen before fertilisation and development can occur. This may be self-pollination or cross-pollination, and it may be carried out by insects or wind.

Oct/Nov 2012

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

Oct/Nov 2012

Describe the initial division of meiosis (meiosis I) in animal cells.

Oct/Nov 2012

In mice, fur colour is determined by a gene with several alleles. These alleles are given here in no special order: black and tan = $C^{bt}$, agouti = $C^{a}$, yellow = $C^{y}$, black = $C^{b}$.

Oct/Nov 2012

A gene mutation in the fruit fly, $\textit{Drosophila melanogaster}$, produces white-eyed flies rather than the usual red-eyed flies. The allele for red eyes ($R$) is dominant over the allele for white eyes ($r$). A student mated a red-eyed fly with a white-eyed fly. The outcomes are shown in Table 1.1.

Oct/Nov 2013

Phenylketonuria (PKU) is an inherited disorder that causes the concentration of the amino acid phenylalanine in the blood to increase. If a newborn baby is not treated, it may result in brain damage. Because of this, babies may be screened for PKU shortly after birth. In normal conditions, any surplus phenylalanine is changed into the amino acid tyrosine by the enzyme phenylalanine hydroxylase (PAH). PKU may arise from a recessive mutation in the gene that codes for PAH.

Oct/Nov 2014

Phenylketonuria (PKU) is a genetic disorder that causes the amino acid phenylalanine to build up at a higher concentration in the blood. If a newborn baby is not treated, this may result in brain damage. Because of this, babies may be screened for PKU shortly after birth. Normally, any excess phenylalanine is changed into the amino acid tyrosine by the enzyme phenylalanine hydroxylase (PAH). PKU may arise from a recessive mutation in the gene that codes for PAH.

Oct/Nov 2014

In humans, the ABO blood grouping system is controlled by one gene with $I^A$, $I^B$ and $I^O$.

Oct/Nov 2014

The fruit fly, Drosophila melanogaster, is used extensively in genetic research. It shows many phenotypic variants in traits such as body colour, wing shape and eye colour. Two alternatives to the normal-winged, grey-bodied phenotype are: • vestigial (very short) wings, controlled by the recessive allele of the gene N/n • ebony (black) body colour, controlled by the recessive allele of the gene G/g.

Oct/Nov 2015

Diabetes insipidus (DI) is a disorder that causes extreme thirst and the passing of very large volumes of dilute urine. One major cause of DI is a lack of ADH. ADH normally attaches to receptor proteins on the cell surface membranes of cells in the collecting ducts of nephrons.

Oct/Nov 2016

Diabetes insipidus (DI) is a disorder that causes intense thirst and the passing of very large volumes of dilute urine. One important cause of DI is a lack of ADH. ADH normally attaches to receptor proteins in the cell surface membranes of cells in the collecting ducts of nephrons.

Oct/Nov 2016

The Labrador retriever is a dog breed developed in modern times and may show yellow, black or brown fur, together with pale, black or brown noses. Fur and nose colour are inherited because genes at two separate loci, the B locus and the E locus, interact with one another. Fig. 1.1 shows a Labrador retriever. Table 1.1 shows how gene interaction produces different phenotypes. A male Labrador retriever, heterozygous at the B locus and homozygous recessive at the E locus, was mated with a female Labrador retriever heterozygous at both loci.

Oct/Nov 2016

Huntington’s disease results from a dominant allele of the gene responsible for producing the huntingtin protein. This protein influences the development of a wide range of tissues, including brain tissue. The Huntington allele contains a number of repeats of the base sequence CAG, which codes for glutamine. As a result, the protein made includes a polyglutamine region. A gene with more than $39$ CAG repeats makes a protein that does not fold correctly and does not work properly. Symptoms of Huntington’s disease usually begin first between the ages of $30$ and $45$ years. There is no treatment for the disease, which gets worse over time and is always fatal. Some individuals with between $27$ and $35$ CAG repeats do not develop the disease, yet they may still pass the Huntington allele to their children, and these children may develop the disease because the number of repeats often increases when gametes are formed.

Oct/Nov 2017

The sweet pea, $\textit{Lathyrus odoratus}$, is a flowering plant found in many regions of Europe. The inheritance of flower colour and pollen grain shape is controlled by genes showing autosomal linkage.

Oct/Nov 2017

In Drosophila melanogaster, the fruit fly, body colour and eye colour are determined by two separate genes. G/g are alleles of the body colour gene: G gives a grey body, whereas g gives a black body. R/r are alleles of the eye colour gene: R gives red eyes, whereas r gives brown eyes. Both genes are autosomal. A dihybrid cross was set up between a fly with grey body and red eyes and a fly with black body and brown eyes. Each parent was homozygous for both genes. The F1 offspring were then crossed to produce the F2 offspring.

Oct/Nov 2018

Mammals like sheep, $Ovis\ aries$, and goats, $Capra\ hircus$, are significant farm animals and are sometimes reared together in mixed flocks. Very rarely, live young are produced when a male sheep mates with a female goat. Sheep have $2n = 54$ and goats have $2n = 60$.

Oct/Nov 2018

In the fruit fly, Drosophila melanogaster, body colour and eye colour are each governed by a different gene. G/g are alleles of the body colour gene, with G producing a grey body and g producing a black body. R/r are alleles of the eye colour gene, with R producing red eyes and r producing brown eyes. Both genes are autosomal. A dihybrid cross was done using a fly with a grey body and red eyes and a fly with a black body and brown eyes. Both parents were homozygous for both genes. The F1 offspring were crossed to produce the F2 offspring.

Oct/Nov 2018

Meiosis plays a key part in sexual reproduction. In humans it takes place during gametogenesis, and in plants it happens when pollen grains and embryo sacs are formed.

Oct/Nov 2019

Meiosis plays a key part in sexual reproduction. In humans, meiosis takes place during gametogenesis, and in plants it occurs when pollen grains and embryo sacs are formed.

Oct/Nov 2019

Mus musculus, the house mouse, has a diploid chromosome number of 40. Fig. 2.1 displays 6 of these chromosomes.

Oct/Nov 2019

Fig. 5.1 shows a cat with the recessive phenotype for two unlinked genes. • $H/h$ determines hair length. The allele for short hair is dominant over the allele for long hair. • $R/r$ determines coat pattern. The allele for 'full colour', in which pigment is present throughout the body, is dominant over the allele for 'pointed', where pigment is confined to the ears, face, paws and tail.

Oct/Nov 2020

Domestic goats are small herbivores that supply milk for human consumption. In rural China, goat milk is a major food source. Xinong Saanen and Guanzhong are the names of two goat breeds that are common in China. Within these breeds, there is genetic variation at nucleotide position 5752 of a gene that codes for a growth factor. At this site, the nucleotide may be either cytosine (C) or guanine (G). Some goats are homozygous for the allele with C at this position (CC), some are homozygous for the allele with G at this position (GG), and others are heterozygous (CG). Table 2.1 compares the mean milk yield from the first milk-producing period (first lactation) and the following milk-producing period (second lactation) for Xinong Saanen goats of each genotype.

Oct/Nov 2020

Fig. $5.1$ depicts a cat displaying the recessive phenotype for two unlinked genes. $H/h$ determines hair length. The allele for short hair is dominant to the allele for long hair. $R/r$ determines coat pattern. The allele for 'full colour', with pigment on all parts of the body, is dominant to the allele for 'pointed', where the pigment is confined to the ears, face, paws and tail.

Oct/Nov 2020