Inheritance

Describe how chromosomal mutation introduces genetic variation in a population.

Chromosomal mutation can be defined as: • a change in the structure of a chromosome (involving several gene loci) or • a change in the number of chromosomes.

What is the classical ratio for the F2 generation for monohybrid inheritance?

3:1

What is the phenotypic ratio of heterozygote x heterozygote cross?

3:1

What is the definition of a gene?

A unit of inheritance located at a particular locus of a chromosome. A specific DNA nucleotide sequence which codes for RNA (mRNA, tRNA or rRNA) or a polypeptide.

What is the modified F2 phenotypic ratio for multiple alleles?

No fixed ratio

What is the definition of recessive alleles?

Produce their effects only in homozygous condition, e.g. allele t causes the plant to be short only in the tt condition. An organism homozygous for a recessive allele is known as homozygous recessive.

Describe the sex-linked inheritance of duchenne muscular dystrophy.

This X-linked, recessive condition affects muscle development. Progressive weakness develops as the muscles of the body are replaced by fibrous tissues. It is believed that the allele for DMD codes for an enzyme that induces the replacement of muscle by fibre. A person with this disease rarely survives beyond the age of 20. The gene for DMD is sex-linked, and found on the X chromosome.

What is the effect of diet on late-onset diabetes in humans?

This develops in genetically diabetic individuals who over-eat but not in genetically diabetic individuals whose diet is low in sugars and lipids. The hormone insulin is secreted when blood glucose level increases. Over-eating of sugary foods for a long period of time causes repeated stimulation of the pancreas, which responds by secreting high levels of insulin. However, repeated exposure of target cells to large amounts of insulin desensitises the cells' responsiveness to insulin. This may result in the target cells failing to dispose of glucose in the usual way, resulting in diabetes.

What is heredity?

Transmission of genetic characteristics from one generation to the next and the effects of this transmission.

What is the coupling arrangement of linked genes?

Two dominant alleles are on one chromosome and the two recessives are on the homologous partner.

What are non-epistatic gene interactions?

Two independently assorting genes (i.e. unlinked) may interact to influence a single character.

Describe the no. Of genes controlling phenotypic variation in discontinuous variation.

Variation controlled by a single or a few gene(s). This gene may have two or more alleles.

Describe the no. Of genes controlling phenotypic variation in continuous variation.

Variations controlled by the combined effect of multiple additive genes and are thus known as polygenic inheritance. Genes act on phenotype in an additive manner. Effects of single genes are too slight to be detected. The combined effect of these genes can produce individuals of infinite phenotypic varieties.

How is the chi-squared value calculated?

Σ(d^2)/(E) = Σ(O-E)^2/E Where Σ is the sum of D is the difference between observed (O) and expected (E) results

What is the phenotypic ratio of Double heterozygote (e.g. AaBb) x Double heterozygote (e.g. AaBb)?

9:3:3:1

What are the steps of chi-squared test?

1. State the hypotheses 2. Calculate the expected number of individuals for each phenotypic class 3. Calculate the χ2 value 4. Compare the χ2calc value against a χ2 probability table (a) degree of freedom (b) level of significance (c) critical chi-squared value 5. Write a conclusion

What were the advantages of Mendel's choice of garden peas that led to his success?

1. The plant had varieties of distinct characteristics (discontinuous variation). 2. The peas were easy to cultivate. 3. They had a relatively short life cycle of one year. 4. The peas produced a large number of seeds. 5. Pollination of the pea plant was easy to control. Foreign pollen could be kept out and cross fertilization could be accomplished artificially. 6. Pure breeds available.

What is the phenotypic ratio of heterozygote x homozygous recessive (test cross)

1:1

What is the phenotypic ratio of Double heterozygote x Double homozygous recessive (Test cross)?

1:1:1:1

What is the modified F2 phenotypic ratio for codominance?

1:2:1

What is the modified F2 phenotypic ratio for incomplete dominance?

1:2:1

What is the modified F2 phenotypic ratio for lethal genes?

2:1

What is the effect of temperature on coat colour in Himalayan rabbits?

A Himalayan rabbit has a white body with black ears, nose, feet and tail. If the fur on the back of the rabbit is shaved and an ice-pack is fixed on the shaved region, left in position for weeks and kept cold, new black fur instead of the original white fur) begins to grow below the ice-pack This is because all these Himalayan rabbits are homozygous for the ch allele of the tyrosinase gene, which codes for a heat-sensitive form of the enzyme tyrosinase, which is needed for melanin production resulting in black fur. Heat sensitive tyrosinase is active only when the air temperature is below 33ºC. Hence, only in parts of the body that are cool enough, e.g. extremities or underneath the ice-pack does black fur grow.

What is the effect of diet on honey bees?

A bee colony comprises three types of bees, mainly the queen, drones and workers. Drones are males and are developed from unfertilized haploid eggs. On the other hand, the queen and the workers are females and they develop from fertilised diploid eggs. Although the queen and worker bees have the same amount of genetic material, they are phenotypically different. Not only the worker bees are sterile, they are smaller and have larger mouthparts and modified legs as compared to the queen bee. The phenotypic differences between the queen and the worker bees are due to the diet of the bee larvae. After hatching, all the larvae are initially fed with royal jelly. Larvae destined to be worker bees are switched to a diet containing honey and pollen. The larvae destined to be the queen are fed with royal jelly. The high content of protein in royal jelly stimulates the formation and maturation of the female reproductive system.

Describe what occurs in the cross when a trait exhibits incomplete dominance.

A cross between two heterozygotes in F1 generation produces a 1:2:1 phenotypic ratio in the F2 progeny. The 1:2:1 genotypic ratio for the F2 generation is identical to that of Mendel's monohybrid crosses. Main difference from Mendel's classical monohybrid crosses - Phenotypic ratio is identical to the genotypic ratio.

What is a dihybrid cross?

A dihybrid cross is a genetic cross involving two characters in which the parents possess different forms of each character. The two characters are controlled by two genes (2 pairs of alleles) located on two gene loci on two different chromosomes.

What is the definition of carriers?

A person or other organism that has inherited a recessive allele for a genetic trait or mutation but does not display that trait or show symptoms of the disease.

What is the purpose of a reciprocal cross and its outcomes?

A reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome (X-linked) or on an autosomal chromosome. A reciprocal cross is a pair of crosses in which the traits of the two parents are reversed. To determine if the locus for eye colour in Drosophila is on the X chromosome, the following crosses are performed: (a) Crossing a red-eyed male Drosophila with a white-eyed female Drosophila, (b) Crossing a red-eyed female Drosophila with a white-eyed male Drosophila. The allele for red eye is dominant to that for white eye. reciprocal crosses involving X-linked traits yield non-identical results in contrast to reciprocal crosses involving autosomal traits which yield identical results.

What is autosomal dominant inheritance?

A single copy of the dominant allele is sufficient to allow the mutant phenotype to be expressed, i.e. even if the individual is heterozygous. Example: Huntington disease

Describe the how the type of linkage can be detected.

A test cross between a double heterozygous individual with a double homozygous recessive individual can be performed to detect if genes are unlinked, completely linked or incompletely linked. If the genes are present on different chromosomes: • 4 different phenotypes (gametes) are produced. • Offspring should consist of four phenotypes with a ratio of 1:1:1:1. If the genes are completely linked: • Only 2 phenotypes (gametes) are produced. • Offspring should consist of two parental phenotypes with a ratio of 1:1. • There are no recombinant phenotypes. If the genes are incompletely linked, i.e. crossing over occurs between the linked genes: • 4 different phenotypes are produced. • Two of them are parental and two are recombinant. • Larger percentage of parental phenotypes and smaller percentage of recombinant phenotypes, i.e. no fixed ratio.

What is the phenotypic ratio of Double homozygous dominant (e.g. AABB) x Double homozygous recessive (e.g. aabb)?

All dominant (double heterozygous)

What is the phenotypic ratio of homozygous dominant x homozygous recessive cross?

All dominant (heterozygous)

What is the link between genotype and phenotype?

Alleles being alternative or different forms of a gene are transcribed to form mRNA and translated into different polypeptide chains and proteins which may affect different metabolic pathways resulting in formation of different phenotypes. Each allele thus specifies or codes for the specific 3D conformation of a particular protein which directly determines a trait

What are some principles regarding sex-linked inheritance?

An affected father will transmit the recessive X-linked allele to all daughters but not to any sons because his son will inherit his Y chromosome only and the X chromosome from mother. If the mother is homozygous dominant, the daughters will have the normal phenotype but will be carriers of the mutation i.e. they are heterozygous and have the ability to pass the recessive X- linked allele to their offspring even though they do not express the recessive phenotype. Females who are heterozygous are described as carriers of the trait. If the carrier female marries with a normal male, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. It can thus be inferred that if the carrier female marries with an affected male, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not display the recessive phenotype will be carriers, whereas males without the disorder will be completely free of the recessive allele.

What is the definition of an allele?

An alternative form of a gene at a particular gene locus. Responsible for determining contrasting traits of the same character. All alleles of a gene determine the same character, but each has a unique DNA nucleotide sequence, which may result in different phenotypes. Alleles occur in pairs in a diploid cell although only one of the pair is

What is an epistatic gene and hypostatic gene?

An epistatic gene is one which is able to suppress / inhibit the effect of a gene at a different locus. The epistatic gene can be (i) recessive or (ii) dominant in their effects. The suppressed gene is termed the hypostatic gene.

Describe an example of codominance.

An example of codominance is the coat colour of short horn cattle. A cross between homozygous red shorthorn cattle and homozygous white shorthorn cattle results in heterozygous offspring with a roan coat. The roan coat consists of a mixture of all red hairs and all white hairs. As each hair is either all red or all white, the condition is codominance. A subsequent cross between the roan offspring results in offspring of all three coat colour.

What is the definition of true-breeding?

An organism that gives rise to all offspring of the same phenotype i.e. organism is homozygous.

Why do human x-linked disorders mainly affect males?

As a male has only one sex-linked gene locus (1 copy of X chromosome instead of 2 as seen in females), he is neither homozygous nor heterozygous for his sex-linked loci; instead he is always hemizygous for every sex-linked locus (hemi means 'half'). As males possess only a single X chromosome, whatever allele present on the X chromosome of males will be directly expressed in the phenotype, whether or not if the allele is dominant or recessive. Thus, X linked disorders mainly affect males as males are hemizygous.

What are the 4 main modes of inheritance?

Autosomal recessive Autosomal dominant Sex-linked recessive Sex-linked dominant

Describe the biochemical basis of codominance in coat colour of short horn cattle.

Both alleles of the hair-colour gene code for functional products. Both products appear in the phenotype of the heterozygote. In this example, a heterozygote with roan coat consists of a mixture of red hairs and white hairs

Describe the gene interaction of chicken comb shape.

Chicken comb shape is determined by interactions between 2 gene loci. Looking at one gene locus at a time When both dominant alleles are present, as in PPRR, PpRR, PPRr and PpRr genotypes, a walnut comb is observed. The shape of the chicken comb is determined by two independently assorting genes, P and R, each with two alleles, P/p and R/r, respectively. The F1 hybrids between a pure-breeding rose-comb cock (ppRR) and a pure-breeding pea-comb hen (PPrr) are of double heterozygous genotype (PpRr) - they will have the walnut comb phenotype. If these F1 hybrids are crossed with each other, all four types of combs will appear in the F2 progeny ratio of 9:3:3:1

What is the definition of heterozygous?

Condition in which the alleles of a gene pair in diploid condition are different, e.g. Tt. An organism with this condition is known as a heterozygote. 50% of its gametes will carry one allele while the other 50% will carry the other allele.

What is the definition of homozygous?

Condition in which the alleles of a gene pair in diploid condition are identical, e.g. TT or tt, where T is the dominant allele for tall stem and t is the recessive allele for short stem. An organism with this condition is known as a homozygote and referred to as true or pure breeding. All gametes produced by this organism will carry the same allele.

What are the principles of chromosome mapping?

Crossing over can occur at any point along the chromosome. The chance of crossing over occurring between two linked genes on the chromosome is proportional to the distance between them. If the two linked genes are far apart on a chromosome, the greater the statistical chance that crossing over will separate them than if they were closer and therefore the greater the proportion of recombinants that will be formed. The distance between genes can therefore be determined by the proportion of recombinants. For any particular pair of genes, the proportion of recombinants will always be about the same when the testcross is repeated because the genes occupy fixed positions in the chromosomes and there is a certain probability of a crossover occurring between them. Therefore, the percentage of crossing over between the two linked genes can be taken as the arbitrary units to represent the relative distance (not actual) between the genes concerned.

Describe the biochemical basis of comb shape in chickens.

Different combinations of alleles from the two genes result in different phenotypes of a single character, presumably due to the interaction of their gene products, each of which contributes to the comb shape at the biochemical or cellular level.

What is the repulsion arrangement of linked genes?

Dominant allele is linked with a recessive on one chromosome.

What are the characteristics of an autosomal recessive inheritance?

Dominant because: 1. Unaffected parents should not have affected children. 2. Trait should not skip generations, i.e. affected individuals must have affected parent(s). 3. When an affected individual mates with an unaffected individual, 50% of their offspring should be affected (indicating that the affected individual is heterozygous). Autosomal because: 1. Both males and females are affected with equal probability.

Describe dominant epistasis in fruit colour in summer squash.

Dominant epistasis is seen in the interaction of two gene loci that determine fruit colour in summer squash, which is commonly found in one of three colours: yellow, white, or green. • 12/16 or 3/4 of the plants produce white squash. • 3/16+1/16=4/16=1/4ofthe plants produce coloured squash. • This suggests that a dominant allele (W) at one locus encodes an inhibitor for the production of pigment (YY, Yy, yy), resulting in white progeny. • Among the F2 plants with pigmented fruit, the ratio of yellow to green fruit is 3:1. • The coloured squash hence shows monohybrid 3:1 ratio produced by a cross between two heterozygous individuals. • The second locus determines the type of pigment produced in the squash, with yellow (Y_) dominant over green (yy). Dominant epistasis shows 12: 3: 1 phenotypic ratio with a double heterozygous cross.

Describe dominant lethal genes including examples.

Dominant lethal genes are expressed in both homozygotes and heterozygotes. They are rarely detected due to their rapid elimination from populations. For example, Huntington's disease, a neurological disorder in humans, which reduce life expectancy. Since the onset of Huntington's disease is slow, individuals carrying the allele can pass it on their offspring.

What is Mendel's first law of segregation?

During the formation of gametes, the paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood. Genes occur in pairs, with each being located on one of the pair of homologous chromosomes in a diploid organism. When the pair of homologous chromosomes segregate from each other during anaphase I of meiosis, they take their genes with them, resulting in each gamete receiving only one of each type of chromosome (either the paternal or maternal chromosome), and thus one allele instead of the normal two

What is epistasis?

Epistasis (Greek for "stoppage") is inferred when the expression of an allele of one gene suppress / inhibit the expression of alleles of a different gene. When two or more genes influence one characteristic, an allele at one gene locus may have an overriding effect on the genotype at a different locus. • Result: different phenotypic expression • Epistasis is different from complete dominance, i.e. complete dominance entails the masking of alleles at the same gene locus.

What are examples of sex-linked inheritance?

Examples of sex-linked inheritance include haemophilia, red-green colour blindness and Duchenne muscular dystrophy in humans, and white eye colour in Drosophila.

What is dominant epistasis?

For dominant epistasis, one dominant allele at the epistatic gene locus will suppress / inhibit the effect of both alleles of the hypostatic gene at a different locus.

What is duplicate recessive epistasis?

For duplicate recessive epistasis, two recessive alleles at either of the two gene loci will suppress / inhibit the effect of the dominant allele at the other locus.

How to distinguish codominance from incomplete dominance?

For incomplete dominance, the heterozygote phenotype is intermediate between the two homozygote phenotypes (e.g. pink flower versus red / white flowers). For codominance, the heterozygote phenotype is NOT intermediate e.g. we do not get pink hair cattle from breeding red and white cattle between the homozygote phenotypes. Instead, the heterozygote possesses both parental traits in equal expression i.e. coat with a mixture of hairs that are either completely red or completely white.

What is recessive epistasis?

For recessive epistasis, two recessive alleles at the epistatic gene locus will suppress / inhibit the effect of either allele of the hypostatic gene at a different locus.

Describe how gene mutation introduces genetic variation in a population.

Gene mutation is defined as an inheritable change in the nucleotide sequence of DNA, which occurs at a single gene locus on a chromosome, resulting in formation of new alleles. It can be brought about by the following mechanisms: • Deletion This occurs when one or several nucleotides are removed from a sequence of nucleotides. • Insertion This occurs when one or several nucleotides are added to a sequence of nucleotides. • Substitution This occurs when a nucleotide is replaced by another.

What are lethal genes?

Mutations which lead to a non- functional gene product can sometimes be tolerated in the heterozygous state. If a mutation results in lethality, this is indicative that the affected gene has a fundamental function in the growth, development and survival of an organism. Lethal genes can be recessive or dominant.

Explain why Mendel's Second Law of Independent Assortment does not apply to linked genes on the same chromosome.

Genes located close together on the same chromosome (linked genes) do not assort independently as they are linked / joined together on the same chromosome. Note that independent assortment involves the arrangement and separation of whole chromosomes and therefore all the alleles that are on them will be moved together as one entity.

What are linked genes?

Genes that control different characters and situated on the same chromosome at different loci Linked genes located on the same chromosome tend to be inherited together in genetic crosses because the chromosome is passed together as a unit. Linked genes hence do not show independent assortment and therefore fail to produce the expected 9:3:3:1 or 1:1:1:1 ratio in breeding situation involving the inheritance of two pairs of contrasted characteristics. The linkage of linked genes can be either complete or incomplete, depending on the relative distance between each gene.

What is incomplete linkage?

Genes which are located some distance apart on the same chromosome generally show incomplete linkage and thus can be separated when crossing over occurs during meiosis: • Prophase I of meiosis: chiasmata may form between non-sister chromatids of homologous chromosomes. • At a chiasma, the non-sister chromatids break at corresponding points, exchange genetic material and rejoin to form recombinant chromatids: crossing over. • If the crossing over occurs between the 2 gene loci, there will be genetic recombination leading to a new combination of alleles in gametes. • Eventually, the separated chromatids end up in separate gametes, and after fertilisation, give rise to new combinations of alleles in the offspring - such offspring are called recombinants. As crossing over is a random process, offspring produced show a majority of parental allele combinations and hence parental phenotypes and a minority of recombinant allele combinations and hence recombinant phenotypes.

Describe how alleles come in pairs.

Genetic characters are controlled by alleles existing in pairs in individual organisms. • In the monohybrid cross involving tall and dwarf plants, a specific allele, T and t exists for each trait respectively. • Each diploid individual receives one allele from each parent. • As the alleles occur in pair, 3 combinations are possible: - 2 factors for tallness, TT (homozygous) - 2 factors for dwarfness, tt (homozygous) - 1 of each factor, Tt (heterozygous). • Every individual possesses one of these three combinations, which determine stem height.

What is the effect of elevation on yarrow plants?

Genetically identical cuttings from yarrow plants grown at three elevations grew differently at different altitudes. Cuttings from one plant grew tall at the lowest and the highest elevation, but a third cutting remained short at mid-elevation. Even though these plants were genetically identical, their phenotypes differed in different environments.

Describe the sex-linked inheritance of haemophilia.

Haemophilia refers to the reduced ability of blood to clot, due to deficiency of one of the blood clotting factors. It is caused by the X-linked recessive allele represented by h, while the normal dominant allele is represented by H. In the female, the presence of a recessive allele for haemophilia (h) can be masked by a dominant allele for normal blood clotting (H). This type of individual is described as a carrier (XHXh). A carrier does not suffer from haemophilia, i.e. shows normal blood clotting, but can transmit the recessive allele for haemophilia to offspring. In the male, the presence of a recessive allele for haemophilia is sufficient to result in the disease because the Y chromosome does not carry a dominant allele for normal blood clotting to mask the recessive allele. Haemophilia is more common in males than in females. This is because males need only one copy of the defective recessive allele to suffer from haemophilia whereas females require two copies of the defective recessive allele to be a sufferer.

What is the effect of soil acidity on hydrangea?

Hydrangea, a common garden plant, may have different floral colours despite carrying the same alleles. This colour variation is due to the differences in soil acidity in which the plants grow, which affects the availability of aluminium to the plants. In acidic soils (pH 5.5 or lower): Aluminum assumes a form that is easily absorbed by plant roots, and thus flowers are predominately blue. In soils where pH is 6.5 or higher: Aluminum is unavailable and flower color is pink. A single plant can have both blue and pink flowers because of varying soil conditions around the plant.

What are the observed differences between complete linkage and dihybrid cross?

If these two pairs of genes (L, l and G, g) are located on different (non-homologous) chromosomes, according to Mendel's Second Law of Independent Assortment, the F2 generation should consist of four phenotypic groups at ratio 9:3:3:1 (for Example 1) and offspring generation should consist of four phenotypic groups at ratio of 1:1:1:1 (for Example 2). The results of the 2 crosses did not show the expected dihybrid ratio (for Example 1) and expected dihybrid testcross ratio (for Example 2). The two pairs of genes must be located very close together on the same chromosome and are closely linked together. No crossing over occur between the 2 gene loci in the F1 flies and thus they are transmitted together as a unit (as though they are one gene instead of two) to produce only two phenotypes with a monohybrid ratio 3:1 (in Example 1) and monohybrid test cross ratio of 1:1 (in Example 2). The two phenotypes are both parental phenotypes (absence of recombinant phenotypes)

What is the effect of temperature on vestigial wings in Drosophila melanoma stem?

In Drosophila, vestigial wings are recessive to that of long wings. The allele for vestigial wings is expressed at low temperatures. Drosophilas which are homozygous recessive for vestigial wings will develop vestigial wings at 21ºC, intermediate wings at 26ºC and long wings at 31ºC.

What is the causes of genetic variation in a population?

In asexually reproducing organisms: DNA replication is so highly accurate and almost error-free that there is little possibility of variation in the genotype. Any apparent variation between asexually reproducing organisms is almost always the result of environmental influences. In sexually reproducing organisms: Ample opportunities for genetic variation arise via: 1. Crossing-over between non-sister chromatids of homologous chromosomes during prophase I of meiosis. 2. Independent assortment of bivalents at the metaphase plate during metaphase I of meiosis. 3. Random fertilisation Above three accounts for new combination of alleles, which is the basis of continuous variation 4. A mutation is a change in the structure of a gene i.e. nucleotide sequence (gene mutation) or in the structure or number of chromosomes (chromosomal mutation). Most mutations occur in somatic cells, i.e. somatic mutations, and are not passed from one generation to the next. Only mutations occurring during the formation of gametes (i.e. germline mutations) can be inherited. These mutations produce distinct differences between individuals, and frequently form the basis of discontinuous variation.

What is complete linkage?

In complete linkage, no crossing over occurs between the linked genes. All the linked genes will pass into the same gamete, resulting in only parental gametes.

Describe the production of gametes and sex determination in humans.

In humans, the female produces one type of gamete. All the egg cells carry an X chromosome. Hence, the female is described as homogametic. In contrast, the male produces two types of gametes. Half of the sperm cells carry an X chromosome while the other half carries a Y chromosome. The male is thus heterogametic. The sex of an offspring is determined by the sperm that fertilises the egg cell • The egg cell may be fertilised by a sperm carrying an X chromosome: - zygote produced will have two X chromosomes and will develop into a female. • The egg cell is equally likely to be fertilised by a sperm carrying a Y chromosome: - zygote produced will have one X chromosome and one Y chromosome, and will develop into a male.

Describe the link between genotype and phenotype in pea plants.

In the pea plant with tall stem, when the genotype of the organism consists of either one or two copies of the dominant allele of the gene for stem height, the plant displays a phenotype of tall stem, as a result of the presence of a protein which contributes to growth of stem. If the genotype lacks a dominant allele or consists of two copies of recessive alleles of the gene for stem height, the plant display a phenotype of dwarf stem as a result of the lack of the protein which contributes to growth of stem.

Describe the biochemical basis of incomplete dominance in snap dragon flowers.

Incomplete dominance in the heterozygote of snapdragons can be explained by postulating that the allele CR allows the production of a functional enzyme required for the synthesis of red pigment. Heterozygotes possess only one copy of the allele per cell and hence produces insufficient enzyme to synthesise enough red pigment as compared to a homozygote CRCR that makes twice as much of the red pigment. Consequently, the heterozygotes are pink. The allele CW codes for a non-functional enzyme.

Describe how meiosis and sexual reproduction introduces genetic variation in a population.

Meiosis (Recall Cell and Nuclear Division) generates genetic variation in the following ways: • Crossing over of non-sister chromatids of homologous chromosomes during prophase I results in new combinations of paternal and maternal alleles in each chromatids. • Independent assortment of homologous chromosomes during metaphase I results in random distribution of paternal and maternal chromosome in each gamete. In sexual reproduction, the fusion of two haploid gametes (fertilisation) to form a diploid zygote restores the diploid number. It also results in genetic variation due to random fusion of gametes from each individual, as well as random mating between individuals in a population.

What is mono hybrid inheritance?

Mendel's earliest experiments involved the study of only one character. The inheritance of a single character of contrasting traits, e.g. tall stem vs short stem, is known as monohybrid inheritance. The character is controlled by a single gene (a pair of alleles). It is studied by a cross between two pure breeding organisms, which differ in one character. Such a cross is called a monohybrid cross.

What is the level of significance?

P=0.05

Describe the observable phenotype in discontinuous variation.

Phenotypes are definite and clear cut. They can be divided into distinct groups - discrete phenotypic classes observed. • Intermediates are not observed

Describe the effect of environment on phenotype in continuous variation.

Phenotypes can be modified by the cumulative effect of varying environmental factors acting on the different genotype. Although the genotype of an organism is determined at the time of fertilisation, the degree of expression allowed to this genetic potential hinges on environmental factors during the development of the organism

Describe the observable phenotype in continuous variation.

Phenotypes not clear cut and cannot be divided into distinct contrasting groups - range of phenotypes observed. • Intermediates are observed

How are phenotypic measurements of discontinuous variation normally represented?

Phenotypic measurements form a normal distribution curve that has a continuous range of intermediate forms between two extremes. These are normally represented on histograms. Most organisms in the population fall in the middle of the range with approximately equal numbers showing the extreme forms of the characteristic. Quantitative: statistical analyses give estimates of population parameters such as the mean and standard deviation.

What is phenotypic variance?

Phenotypic variance (VP) of a population has two components, genotypic variance (VG) and environmental variance (VE): VP =VG +VE VE is most easily determined when homozygous organisms are studied, i.e. VG = 0. VG is due to genotype, and includes the effects of additive genes, dominant genes and epistasis (KIV: Inheritance Part II). The contributions of each to the phenotype can be estimated from crosses involving homozygous varieties and their F1, F2 and backcross progeny.

What is the definition of dominant alleles?

Produce their effects in both homozygous and heterozygous condition i.e. one copy of the allele is sufficient to cause the organism to express the phenotype it encodes, e.g. allele T causes the plant to be tall in either TT or Tt condition. A dominant allele masks the influence of the recessive allele. An organism homozygous for a dominant allele is known as homozygous dominant.

What is the purpose of a test cross and the 2 possible outcomes?

Purpose: to find out genotype of organism with dominant phenotype This involves crossing the organism of the dominant phenotype but unknown genotype with a homozygous recessive individual. The phenotype of the offspring indicates the genotype of the organism tested. Possible outcomes: If the organism displaying a dominant phenotype is homozygous, all the offspring will express the dominant phenotype. However, if it is heterozygous, about half of the offspring will express the dominant phenotype while the other half expresses the recessive phenotype in a 1: 1 ratio.

What are the characteristics of an autosomal recessive inheritance?

Recessive because: 1. If both parents are affected, all children should be affected. 2. In most cases, when unaffected people mate with affected individuals, all children are unaffected. When at least 1 child is affected (indicating that the unaffected parent is heterozygous), approximately half the children should be affected. 3. Unaffected parents (heterozygous at gene loci) can produce affected individuals. 4. Trait often skips generations. Autosomal because: 1. Both males and females are affected with equal probability.

Describe recessive epistasis in the coat colour in Labrador retrievers.

Recessive epistasis is seen in the genes that determine coat colour in Labrador retrievers, which may be black, brown, or yellow; their different coat colours are determined by interactions between genes at two loci. A cross of true-breeding black (EEBB) and yellow (eebb) dogs produces all black dogs (EeBb). A cross between the F1 dogs produces a genetic ratio of 9 black : 3 brown : 4 yellow dogs in the F2 generation. • Yellow dogs can carry alleles B or b, for either black or brown pigment deposition respectively, but these alleles are not expressed in their coat colour. • Genotype ee is epistatic to B and b, because ee suppresses the expression of the alleles for black and brown pigments in the phenotype. • Alleles B and b are hypostatic to e as their expression is suppressed by the presence of 2 copies of e. • In this case, e is a recessive epistatic allele because: two copies of the recessive allele ee must be present to suppress the expression of the alleles encoding the black and brown pigments at the second locus.

Describe the sex-linked inheritance of red-green colour blindness.

Red-green colour blindness is the inability to distinguish between red and green. It is caused by the X-linked recessive allele represented by b, while the normal dominant allele is represented by B. Note: The recessive allele for colour blindness is passed from one sex to the other at each generation. The father passes it to his daughters (but not his sons), who thus become carriers. The daughters in turn may pass it to their sons, who are thus colour-blind.

What is the effect of light on the freckling of humans?

Several genes are responsible for freckling in humans but the extent of freckling is determined by exposure to light.

What is sex linkage?

Sex linkage refers to the carrying of genes on the sex chromosomes.

Describe how a genetic diagram is drawn?

Step 1: Define alleles Note: Not needed if alleles are already defined in question. Step 2: List down the parental phenotype and genotype Each true-breeding plant of the parental generation has matching alleles, TT or tt. Step 3: List down the parental gametes *Gametes should always be circled Each gamete contains only one allele for the stem-length gene. In this case, every gamete produced by one parent has the same allele. Step 4: List the F1 phenotype and genotype using a Punnett square (if necessary) Step 5: List the F1 gametes When the F1 hybrid plants produce gametes, the two alleles segregate, with half the gametes receiving the T allele and the other half receiving the t allele. Step 6: List the F2 genotypes using a Punnett square The Punnett square shows all possible combinations of alleles in offspring. Each square represents an equally probable product of fertilisation. Step 7: List the F2 genotypic and phenotypic ratio Note: It is important to show correlation between genotype & phenotype.

Describe duplicate recessive epistasis in flower colour in sweat peas.

Sweet pea flowers (Lathyrus odoratus) are either purple (containing anthocyanin pigment) or white (without pigment). A cross between two white flowers produced F1 heterozygotes which all had purple flowers. When the F1 heterozygotes were crossed, a ratio of 9 purple flowers: 7 white flowers were obtained in the F2 generation. Each dominant allele encodes an enzyme that controls a step in the synthesis of anthocyanin from a biochemical precursor. If a dominant allele is absent, its step in the biosynthetic pathway is blocked and anthocyanin will not be produced • There are two independently assorting gene loci, C and P, involved in anthocyanin synthesis. The gene at each locus has a recessive allele that abolishes pigment production. • F1 offspring are all CcPp double heterozygotes with purple flowers. • A dominant allele at each gene locus (i.e. C_P_) is necessary for the synthesis of anthocyanin. • In the F2 generation, 9/16 of the plants are C_P_ and have purple flowers while the remaining 7/16 are homozygous for at least one of the recessive alleles and have white flowers. • The double recessive homozygotes, ccpp, are not phenotypically different from the either of the single recessive homozygotes, C_pp and ccP_. • Each of the recessive alleles is epistatic over the dominant allele of the other gene. • cc is epistatic to the P locus because cc prevents the phenotypic expression of the alleles at the P locus. Similarly, pp is epistatic to the C locus.

Describe multiple alleles using the example of ABO blood groups in humans.

The ABO system, like the MN blood types, is characterised by the presence of antigens on surface of red blood cells. However, the A and B antigens are distinct from the MN antigens and are under the control of a different gene, located on chromosome 9. The three alleles responsible for the production of antigens are designated IA, IB and i, where I / i stand for isoagglutinogen, another term for antigen. The IA and IB alleles are codominant to each other, while the i allele is recessive to both. Only two of the three alleles can be present in an individual. Four possible blood groups but 6 possible genotypes can result from various combinations of these three different alleles

What is the alternative hypothesis?

The ALTERNATIVE HYPOTHESIS (HA), states that there is significant difference between the observed and expected results. It assumes that differences are not due to chance.

What is the null hypothesis?

The NULL HYPOTHESIS (H0) states that there is no significant difference between the observed and expected results. It assumes that any differences are due to chance.

What is the definition of genotype?

The complete genetic makeup / allelic composition of an organism. The term is also commonly used in reference to the paired alleles carried by an organism that give rise to a phenotype.

What is autosomal recessive inheritance?

The gene of interest for the trait is carried on an autosome. A recessive trait only becomes phenotypically apparent when two similar alleles of a gene are present. In other words, the subject must be homozygous for the trait. Example: albinism

What is x-linked recessive inheritance?

The gene of interest for the trait is carried on the sex (usually X) chromosome. Males have only one X chromosome i.e. they are hemizygous for the trait, thus a single recessive allele on the X chromosome will cause the male to display the disease phenotype. Hence, males display disease phenotype more often than females. • Females will require two recessive alleles, one on each of their X chromosomes, to display the disease phenotype. Examples: haemophilia, colour-blindness and Duchenne Muscular Dystrophy

What is X-linked dominant inheritance?

The gene of interest for the trait is carried on the sex (usually X) chromosome. A single abnormal allele on the X chromosome can cause a sex-linked dominant disease.

What are the observations in incompletely linkage?

The genes are incompletely linked, i.e. crossing over occurs between the linked genes. In a large sample of cells undergoing meiosis, crossing over (a rare event) will occur between the two linked genes in only some of the cells, depending on how closely linked they are. Thus, 4 types of gametes are produced. Two of them are parental (larger proportion) and two are recombinant (smaller proportion; only 2 of 4 chromatids cross over in some cells only). Percentage of recombinant gametes and thus recombinant offspring produced is comparatively small.

What is dihybrid inheritance.

The inheritance of two pairs of contrasting characters in each cross is known as dihybrid inheritance.

What is hybridisation?

The mating or crossing of two true-breeding varieties

What is the definition of wild-type?

The most common allele / phenotype in nature.

How is the percentage of crossing over calculated?

The percentage of crossing over is also known as the Crossover Value (COV) or the Recombination Frequency (RF): COV: Number of individuals showing recombination/Total number of offspring x 100% Note: By convention a COV of 1% represents a relative distance of 1 centimorgan (cM) on the chromosome.

What is the definition of phenotype?

The physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment.

What is variation?

The recognisable differences between individuals of the same species and between parents and offsprings.

What is Mendel's second law of independent assortment?

The segregation of one pair of alleles is independent of the segregation of other pairs. As a result of segregation, each gamete receives one member of each pair of alleles. For one pair, whichever allele is received does not influence the outcome of segregation of any other pair. In other words, each F1 gamete receives either a R or r allele and a Y or y allele. • Independent assortment stipulates that all four combinations (RY, Ry, rY and ry) will be formed with equal probabilities It is important to note that this pattern of segregation would not occur if the genes were located on the same chromosome, i.e. if they are linked genes.

Describe the examples of recessive lethal genes.

The snapdragon plant Antirrhinum with characterised condition aurea, has golden instead of green leaves. When two aurea snapdragon plants were crossed, a 2:1 ratio of yellow seedlings to green seedlings was observed. Homozygous aurea plants lack normal chlorophyll development and died either during embryonic stage or when the plant seedlings were two to three days old. Another example is a mutation that causes yellow coat colour in mice. Coat colour in mice is linked to a lethal gene. With regards to coat colour, the mutant yellow allele Y is dominant to the wild-type agouti allele y, hence heterozygous mice will have yellow coats. Cross breeding yellow mice (genotype Yy) produces offspring in the ratio 2 yellow fur: 1 agouti fur. The atypical Mendelian ratio is due to the fetal death of homozygous yellow (YY) coat mice, so the YY genotype is missing from the progeny.

What is the definition of a locus (plural loci)?

The specific location of a gene on a chromosome.

What is the effect of light on chlorophyll synthesis in plants?

The synthesis of chlorophyll in plants is genetically determined but light is also required.

What is gene interaction?

The term "gene interaction" is often used to describe the idea that two or more genes influence one particular character.

What is the P generation (parental)?

The true-breeding parents

Describe human sex chromosomes.

There are 23 pairs of chromosomes in humans: • First 22 pairs appear identical in both male and female. These chromosomes are known as autosomes and are not involved in sex determination. • The 23rd pair appears identical in the female (X chromosomes) but differs in the male (one X chromosome and one Y chromosome). These are the sex chromosomes and are involved in sex determination.

Describe the effect of environment on phenotype in discontinuous variation.

There is little or no environmental effect on the phenotypic expression of the gene(s).

Describe dominance or recessiveness.

While two unlike alleles responsible for a single character are present in a single individual, one allele is dominant to the other, which is said to be recessive. • In the monohybrid cross, the trait tall stem expressed in the heterozygous (Tt) F1 generation results from the presence of the dominant allele, T. • The trait dwarf stem not expressed in the F1 generation, but which reappears in the F2 generation, is under the influence of the recessive allele, t. • In the F1 generation, the recessive allele, t for dwarf stem is masked by the dominant allele, T for tall stem. • The recessive allele, t is only able to express itself in the outward appearance of the plant in the absence of the dominant allele, T as evident in the homozygous recessive (tt) F2 dwarf plants. • The trait tall stem is dominant to the recessive trait, dwarf stem.

What are the characteristics of X-linked dominant inheritance?

X-linked because: 1. Affected mothers produce affected sons (only 50% chance). 2. Affected females are the offspring of affected mothers or fathers. 3. All daughters, but none of the sons, of an affected father are affected, assuming mother is normal. Dominant because: 1. The trait does not skip generations. 2. Approximately half of the children of the affected heterozygote females are affected.

What are the characteristics of x-linked inheritance?

X-linked because: 1. Most affected individuals are males. 2. Affected mothers produce affected sons with 100% chance. 3. Affected females are the result of a mating between affected fathers and affected or carrier mothers. 4. Approximately half of the sons of carrier females should be affected. Recessive because: 1. If fathers are not affected, daughters will not be affected but may be carriers.

What is incomplete dominance?

a condition where neither of the two alleles is completely dominant to the other, so that the heterozygote has a phenotype which is intermediate • e.g. pink flowers between the two homozygous forms (red and white).

What is codominance?

a phenomenon in which both alleles are equally expressed in the phenotype of the heterozygote The heterozygote simultaneously expresses the phenotypes of both types of homozygotes.

How are phenotypic measurements of discontinuous variation normally represented?

bar graphs It is impossible to obtain a normal distribution curve for such variation because the phenotypic classes are distinct. Qualitative: analysed by making counts and ratios.

How is the number of degrees of freedom calculated?

df = n - 1 where n is the number of phenotypic classes First, determine how many phenotypic classes there are in the investigation being carried out. If there are 2 phenotypic classes, the degree of freedom (df) is 1. • The degree of freedom measures the number of classes of data (in this case, phenotypic classes) that are independent of each other. The degree of freedom measures the spread of the data and is always one less than the number of classes of data. • In the case of 2 phenotypic classes where the total number of offspring is known, when the number in any one phenotypic class has been determined, the number in the second class is fixed. Hence, one is subtracted from the number of phenotypic classes.

What is variation?

recognisable differences in characteristics (e.g. height, weight, skin colour) between organisms of the same natural population or species. In any large population, two forms of variations occur: discontinuous and continuous. Studies of variation in a certain character involve measuring the expression of that characteristic in a large number of organisms within the population.

What is the chi-squared (χ2) test?

statistical test for the significance of data that consists of discontinuous / discrete variables. Statistical tests are done to see how well the observed experimental results fit the expected results, i.e. 'the goodness of fit' between the observed and expected results. When observed ratios of phenotypic classes deviate from expected ratios, there is a need to test whether this deviation or difference is due to CHANCE (meaning: the expected ratio has been correctly predicted) or due to an incorrect prediction of expected ratios. O The larger the difference between observed and expected results, the more likely that the hypothesis is incorrect and hence, is not due to chance alone. O The smaller the difference between observed and expected results, the more likely that the hypothesis is correct, and hence is due to chance alone. The calculated When observed ratios of phenotypic classes deviate from expected ratios, there is a need to test whether this deviation or difference is due to CHANCE (meaning: the expected ratio has been correctly predicted) or due to an incorrect prediction of expected ratios. O The larger the difference between observed and expected results, the more likely that the hypothesis is incorrect and hence, is not due to chance alone. O The smaller the difference between observed and expected results, the more likely that the hypothesis is correct, and hence is due to chance alone. The calculated χ2 value is compared against a chi-squared distribution table to assess if the difference between observed and expected results is statistically significant.

What does it mean If χ2calc > χ2crit?

the probability that chance alone is the reason for the difference between observed and expected results / ratio is less than 5%. The deviation is significant. Hence, reject H0 in favour of HA.

What does it mean If χ2calc < χ2crit?

the probability that chance alone is the reason for the difference between observed and expected results / ratio is more than 5%. The deviation is not significant. Hence, do not reject H0.