inheritance

Question 1

what is the definition of a gene?

Answer 1

1. a gene is a unit of inheritance located at a particular locus of a chromosome
2. a gene is a specific DNA nucleotide sequence which codes for RNA or a polypeptide

Question 2

what is the definition of a locus?

Answer 2

a locus is the specific location of a gene on a chromosome

Question 3

what is the definition of an allele?

Answer 3

1. an allele is an alternative form of a gene at a particular gene locus, and is reponsible for determining contrasting phenotypes of the same character by coding for the specific 3D conformation of a particular protein which directly determines a trait
2. alleles occur in pairs in a diploid cell although only one of the pair is represented in a gamete

Question 4

what is the definition of a genotype?

Answer 4

1. a genotype is the complete genetic makeup/allelic composition of an organism
2. a genotype is used in reference to the paired alleles carried by an organism that gives rise to a phenotype

Question 5

what is the definition of a phenotype?

Answer 5

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

Question 6

what is the definition of a wild-type?

Answer 6

a wild-type is the most common allele/phenotype in nature

Question 7

what is the definition of heterozygous?

Answer 7

1. heterozygous is the condition in which the alleles of a gene pair in diploid condition are different
2. 50% of its gametes will carry one allele while the other 50% will carry the other allele

Question 8

what is the definition of homozygous?

Answer 8

1. homozygous is the condition in which the alleles of a gene pair in diploid condition are identical
2. all gametes produced by this organism will carry the same allele

Question 9

what is the definition of dominant allele?

Answer 9

1. a dominant allele produces its effects in both homozygous and heterozygous conditions, and masks the influence of the recessive allele
2. an organism homozygous for a dominant allele is known as homozygous dominant

Question 10

what is the definition of a recessive allele?

Answer 10

1. a recessive allele produces its effects only in homozygous conditions
2. an organism homozygous for a recessive allele is known as homozygous recessive

Question 11

what is the definition of true-breeding?

Answer 11

true-breeding is when an organism gives rise to all offspring of the same phenotype

Question 12

what is the definition of a carrier?

Answer 12

a carrier is an 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

Question 13

what is the definition of monohybrid inheritance?

Answer 13

monohybrid inheritance is the inheritance of a single character of contrasting traits

Question 14

how does monohybrid inheritance suggest that alleles come in pairs?

Answer 14

1. in a monohybrid cross, a specific allele exists for each trait respectively
2. each diploid individual receives one allele from each parent, where every individual possesses either one of 3 combinations which determine stem height

Question 15

how does monohybrid inheritance suggest that in heterozygous genotypes, one allele is dominant to the other, which is said to be recessive?

Answer 15

1. in a monohybrid cross, trait 1 is expressed in the heterozygous F1 generation results from the presence of the dominant allele
2. trait 2 which is not expressed in the F1 generation but reappears in the F2 generation, is under the influence of the recessive allele. this is because in the F1 generation, the recessive allele is masked by the dominant allele, and the recessive allele is only able to express itself in the outward appearance of the plant in the absence of the dominant allele

Question 16

what does mendel’s first law of segregation say?

Answer 16

1. mendel’s first law of segregation states that during the formation of gametes, the paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood
2. when the pair of homologous chromosomes segregate from each other during anaphase I of meiosis, they take their genes with the, resulting in each gamete receiving only one of each type of chromosome, and thus one allele instead of the normal two

Question 17

what is the purpose of a test cross?

Answer 17

1. this involves crossing the organism of the dominant phenotype but unknown genotype with a homozygous recessive individual. the phenotype of this offspring indicates the genotype of the organism instead
2. if the organism displaying a dominant phenotype is homozygous, all the offspring will express the dominant phenotype

Question 18

what is the definition of incomplete dominance?

Answer 18

incomplete dominance is a condition where neither of the two alleles is completely dominant to the other, so that the heterozygote has a phenotype which is intermediate

Question 19

what happens when a trait exhibits incomplete dominance?

Answer 19

1. a cross between two heterozygotes in the F1 generation produces a 1:2:1 phenotypic ratio in the F2 progeny
2. the 1:2:1 ratio for the F2 generation is identical to that of mendel’s monohybrid crosses, but the phenotypic ratio is not

Question 20

what is the biochemical explanation for incomplete dominance?

Answer 20

1. each specific allele allows the production of a functional enzyme required for the synthesis of a particular pigment
2. heterozygotes possess only one copy of the allele per cell and hence produces insufficient enzyme to synthesise enough of the pigment as compared to a homozygote that makes twice as much of the pigment
3. the other allele codes for a non-functional enyme

Question 21

what is the definition of codominance?

Answer 21

codominance is a phenomenon in which both alleles are equally expressed in the phenotype of the heterozygote. the heterozyote simulatenously expresses the phenotypes of both types of homozygotes

Question 22

what is the biochemical explanation for codominance?

Answer 22

both alleles of a particular gene code for functional products. both products appear in the phenotype of the heterozygote

Question 23

what is the definition of multiple alleles?

Answer 23

it is by which a gene controlling a characteristic has three or more alleles

Question 24

how does the ABO blood groups in humans illustrate the concept of multiple alleles?

Answer 24

1. the ABO system is characterised by the presence of antigens on the surface of RBCs. the A and B antigens are under the control of a gene located on chromosome 9
2. the three alleles responsible for the production of antigens are designated IA, IB and i, where I stands for isoagglutinogen
3. the IA and IB alleles are codominant to each other, while the i allele is recessive to both, where IA and IB code for the production of antigen A and B respectively, and i codes for neither antigen A or B

Question 25

give examples of recessive lethal genes

Answer 25

1. the snapdragon plant antirrhinum which 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 during the embyronic stage 2. coat colour in mice is also linked to a lethal gene. the mutant yellow allele Y is dominant to the wild-type agouti allele y, hence heterozygous mice will have yellow coats. during the cross breeding of heterozygous yellow mice, the atypical mendelian ratio is due to the fatal death of homozygous yellow coat mice. animals homozygous for the wild-type y allele have yellow pigment deposited as a band on the otherwise black hair shaft, resulting in the agouti phenotype. heterozygotes deposit yellow pigment along the entire length of hair shafts as a result of deletion of the regulatory region preceding the DNA coding region of the Y allele. without any means to regulate gene expression, one copy of the Y allele is always turned on in heterozygotes, resulting in this gain of function leading to the dominant effect. extensive deletion of genetic material characterising the Y allele extends into the coding region of an adjacent gene, rendering it non-functional

Question 26

what is the definition of dihybrid cross?

Answer 26

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

Question 27

what is mendel’s second law of independent assortment?

Answer 27

1. mendel’s second law of independent assortment states that the segregation of one pair of alleles is independent of the segregation of other pairs 2. as such, 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

Question 28

describe the production of gametes and sex determination in humans

Answer 28

1. in humans, the female produces an egg, which carries an X chromosome. the female is described as homogametic 2. the male produces a sperm, where half carry an X chromosome and the other half carries a Y chromosome. the male is heterogametic 3. thus, a zygote may have either 2 X chromosomes and will develop into a female, or may have 1 X and 1 Y chromosome and will develop into a male

Question 29

describe sex-linkage

Answer 29

1. sex linkage refers to the carrying of genes on the sex chromosome 2. the genes are mostly located on the X chromosome, as the Y chromosome is shorter than the X chromosome and carries very few genes as compared to the X chromosome. the x chromosome contains many loci that are required in both sexes, whereas the Y chromosome contains only a few genes

Question 30

what are the principles of sex-linked disorders

Answer 30

1. sex-linked inheritance mainly affects males, as males has only one sex-linked gene locus and is neither homozygous nor heterozygous for his sex-linked loci. 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 of recessive 2. 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 the mother 3. if the mother is homozygous dominant, the daughters will have the normal phenotype but will be carriers of the mutation 4. 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

Question 31

what is the definition of haemophilia and how does it manifest?

Answer 31

1. haemophlia refers to the reduced ability of blood to clot, due to deficiency of one of the blood clotting factors 2. it is caused by the X-linked recessive allele represented by h, while the normal dominant allele is represented by H 3. haemophlia is more common in males than in females, as males need only one copy of the defective recessive allele to suffer from haemophilia while females require two copies of the defective recessive allele

Question 32

what is the definition of red-green colour blindness and how does it manifest?

Answer 32

1. red-green colour blindness is the inability to distinguish between red and green 2. it is caused by the X-linked recessive allele represented by b, while the normal dominant allele is represented by B

Question 33

what is the definition of duchenne muscular dystrophy?

Answer 33

1. duchenne muscular dystrophy is an X-linked, recessive condition which affects muscle development. progressive weaknes develops as the muscles of the body are replaced by fibrous tissues 2. it is believed that the allele for DMD codes for an enzyme that induces the replacement of muscle by fibre 3. the gene for DMD is sex-linked, and found on the X chromosome

Question 34

what is the purpose of a reciprocal cross

Answer 34

1. a reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome or on an autosomal chromosome. 2. a reciprocal cross is a pair of crosses in which the traits of the two parents are reversed

Question 35

how does a reciprocal cross work?

Answer 35

1. since the male transmits the X chromosome only to his female offspring, and the female transmits an X chromosome to both male and feamle offspring, the male does not contribute to the appearance of their X-linked phenotypes 2. hence, reciprocal crosses involved X-linked traits yield non-identical results in constrast to reciprocal crosses involving autosomal traits which yield identical results

Question 36

what are the principles of autosomal recessive inheritance?

Answer 36

1. the gene of interest for the trait is carried on an autosome 2. a recessive trait only becomes phenotypically apparent when the subject is homozygous for the trait

Question 37

what are the characteristics of an autosomal recessive inheritance?

Answer 37

1. this mode of inheritance is recessive, as if both parents are affected, all children should be affected. when at least 1 child is unaffected, indicating that the unaffected parent is heterozygous, approximately half the children should be affected. unaffected parents can produce affected individuals, and if both parents are affected, all children should be affected 2. this mode of inheritance is autosomal, as both males and females are afffected with equal probability

Question 38

what are the principles of autosomal dominant inheritance?

Answer 38

a single copy of the dominant allele is sufficient to allow the mutant phenotype to be expressed

Question 39

what are the characteristics of autosomal dominant inheritance?

Answer 39

1. this mode of inheritance is dominant, because unaffected parents do not have affected children. traits should not skip generations. when an affected individual mates with an unaffected individual, 50% of their offspring should be affected 2. this mode of inheritance is autosomal, as both males and females are affected with equal probability

Question 40

what are the principles of X-linked recessive inheritance?

Answer 40

1. the gene of interest for the trait is carried on the sex chromosome 2. males have only one X chromosome, thus a single recessive allele on the X chromosome will cause the male to display the disease phenotype 3. females will require two recessive alleles, one of each of their X chromosomes, to display the disease phenotype

Question 41

what are the characteristics of X-linked recessive inheritance?

Answer 41

1. this mode of inheritance is X-linked because most affected individuals are males, affected mothers always produce affected sons, and affected females are the result of a mating between affected fathers and affected or carrier mothers. approximately half of the sons of carrier females should be affected 2. this mode of inheritance is recessive becasuse if fathers are not recessive, daughters will not be affected but may be carriers

Question 42

what are the principles of X-linked dominant inheritance?

Answer 42

1. the gene of interest for the trait is carried on the sex chromosome. 2. a single abnormal allele on the X chromosome can cause a sex-linked dominant disease

Question 43

what are the characteristics of a X-linked dominant inheritance?

Answer 43

1. this mode of inheritance is X-linked because affected mothers produce affected sons, affected females are the offspring of affected mothers or fathers, and all daughters but none of the sons of an affected father are affected, assuming mother is normal 2. this mode of inheritance is dominant because this trait does not skip generations, and approximately half of the children of the affected heterozygote females are affected

Question 44

why do linked genes 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?

Answer 44

1. linked genes are genes that control different characters and are situated on the same chromosome at different loci 2. linked genes located on the same chromosome tend to be inherited together in genetic crosses because the chromosome is passed together as a unit, and thus do not show independent assortment

Question 45

what is the definition of complete linkage?

Answer 45

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

Question 46

what are the principles of incomplete linkage?

Answer 46

1. 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 2. during prophase I of meiosis, chiasmata may form between non-sister chromatids of homologous chromsomes 3. at a chiasma, the non-sister chromatids break at corresponding points, exchange genetic material and rejoin to form recombinant chromatids 4, if the crossing over occurs between the 2 gene loci, there will be genetic recombination leading to a new combination of alleles in gametes 5. 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 6. 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 combinantions and hence recombinant phenotypes

Question 47

how can a test cross be used to detect if genes are unlinked, completely linked or incompletely linked?

Answer 47

1. if the genes are present on different chromosome, 4 different phenotypes are produced, and offspring should consist of 4 phenotypes with a ratio of 1:1:1:1 2. if the genes are completely linked, only 2 different phenotypes are produced, and offspring should consist of 2 phenotypes with a ratio of 1:1, with no recombinant phenotypes 3. if the genes are incompletely linked, 4 different phenotypes are produced, and two of them are parental and two are recombinant. there is a larger percentage of parental phenotypes are smaller percentage of recombinant phenotypes

Question 48

what is the difference between a coupling and a repulsion arrangement?

Answer 48

1. coupling is when two dominant alleles are on one chromosome and the two recessives are on the homologous partner 2. repulsion is when the dominant allele is linked with a recessive allele on one chromosome

Question 49

what are the principles of chromosome mapping?

Answer 49

1. crossing over can occur at any point along the chromosome 2. the chance of crossing over occuring between two linked genes on the chromosome is proportional to the distance between them 3. 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 4. the distance between genes can therefore be determined by the proportion of recombinants 5. 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 chromosome and there is a certain probability of a crossover occuring between them 6. therefore, the percentage of crossing over between the two linked genes can be taken as the arbitrary units to represent the relative distance between the genes concerned

Question 50

how is the crossover value counted?

Answer 50

the number of individuals showing recombinanation, over the total number of offspring

Question 51

what are the components of phenotypic variance?

Answer 51

1. genotypic variance is due to the genotype of organisms, including the effects of additive genes, dominant genes and epistasis. the contributions to each of the phenotype can be estimated from crosses involving homozygous varieties and their F1, F2 and backcross progeny 2. environmental variance is most easily determined when homozygous organisms are studied

Question 52

how do gene mutations introduce genetic variation in a population?

Answer 52

gene mutation, identifed as an inheritable change in the nucleotide sequence of DNA, occurs at a single gene locus on a chromosome, resulting in the formation of new alleles, and can be brough about by a deletion, insertion or substitution mechanism

Question 53

how do chromosomal mutations introduce genetic variation in a population?

Answer 53

chromosomal mutations are defined as a change in the structure of a chromosome, involving several gene loci, or a change in the number of chromosomes

Question 54

how does meiosis and sexual reproduction introduce genetic variation in a population?

Answer 54

1. crossing over of non-sister chromatids of homologous chromosomes during prophase I results in new combinations of paternal and maternal alleles in each chromatids 2. independent assortment of homologous chromosomes during metaphase I results in random distribution of paternal and maternal chromosome in each gamete 3. in sexual reproduction, the fusion of two haploid gametes to form a diploid zygote restores the diploid number, and also results in genetic variation due to random fusion of gametes from each individual, as well as random mating between individuals in a population

Question 55

give an example of the effect of temperature on phenotype

Answer 55

1. 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 begins to grow below the ice pack 2. this is because all 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 33 degrees 2. in drosophila, vestigal wings are recessive to that of long wings. the allele for vestigal wings is expressed at low temperatures, and drosophilas which are homozygous recessive for vestigal wings will develop vestigal wings at 21 degrees, intermediate wings at 26, and long wings at 31

Question 56

give an example of the effect of elevation on phenotype

Answer 56

1. geneticaly identical cuttings from yarrow plants grown at three elevations grew differently at different altitudes 2. cuttings from one plant grew tall at the lowest and highest elevation, but a third cutting remained short at mid-elevation, even though these plants were genetically identical

Question 57

give an example of the effect of soil acidity on phenotype

Answer 57

1. colour variation in hydrangea is due to the differences in soil acidity in which the plants grow, affecting the availability of aluminum to the plants 2. in soils of pH 5.5 or lower, aluminum assumes a form that is easily absorbed by plant roots, and thus flowers are predominantly blue 3. in soils pH 6.5 or higher, aluminum is unavailable and flower colour is pink

Question 58

give an example of the effect of diet on phenotype

Answer 58

1. the phenotypic differences between the queen and the worker bees, despite having the same amount of genetic material, is due to the diet of the bee larvae 2. after hatching, all the larvae are initially fed with royal jelly, and larvae destined to be worker bees are switched to a diet containing honey and pollen, while the larvae destined to be the queen are fed with royal jelly 3. the higher protein content in royal jelly stimulates the formation and maturation of the female reproductive system

Question 59

what is the definition of gene interaction?

Answer 59

1. gene interaction is used to describe the idea that two or more genes influence one particular character 2. non-epistatic gene interaction is used to describe the idea that two independently assorting genes may interact to influence a single character 3. epistatic gene interactions is inferred when the expression of an allele ofone gene supresses the expression of alleles in a different gene 4. recessive epistasis is inferred when two recessive alleles at the epistatic gene locus will suppress the effect of either allele of the hypostatic gene at a different locus 5. dominant epistasis is inferred when one dominant allele at the epistatic gene locus will suppress the effect of both alleles of the hypostatic gene at a different locus 6. duplicate recessive epistasis is inferred when two recessive alleles at either of the two gene loci will supress the effect of the dominant allele at the other locus

Question 60

what is the evidence for non-epistatic gene interactions?

Answer 60

1. chicken comb shape is determined by interactions between 2 gene loci, where the dominant P allele results in a pea-shaped comb, the dominant R allele results in a rose comb, and the recessive p and r alleles result in a single comb. when both dominant alleles are present, a walnut comb is observed 2. 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 level

Question 61

what is the evidence for recessive epistatic gene interactions?

Answer 61

1. 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 2. the dominant E allele allows deposition of dark pigment, while the recessive e allele prevents deposition of dark pigment, causing the hair to be yellow. e is hence a recessive epistatic allele to alleles B and b at a different gen locus 3. the dominant B allele codes for black pigment, while the recessive b allele codes for brown pigment 4. genotype ee is epistatic to B and b, because ee supresses the expression of the alleles for the black and brown pigments in the phenotype, and thus alleles B and b are hypostatic to e 5. however, e is a recessive epistatic allele because two copies of the recessive allele ee must be present to suppress the expression of the alleles B and b

Question 62

what is the evidence for dominant epistatic gene interactions?

Answer 62

1. 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 2. the dominant W allele codes for an inhibitor for pigment production and is a dominant epistatic allele to alleles Y and y at a different gene locus, while the recessive w alleles allows pigment production 3. the dominant Y allele codes for yellow pigment production, while the recessive y allele codes for green pigment production

Question 63

what is the evidence for duplicate recessive epistatic gene interactions?

Answer 63

1. sweet pea flowers are either purple, containing anthocyanin pigment or white without pigment. 2. each dominant allele encdes 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 3. C is the dominant allele for the production of the intermediate pigment, and c is the recessive allele for no producion of the intermediate pigment. P is the dominant allele for production of anthocyanin, and p is the recessive allele for no production of anthocyanin. C and P belong to two independently assorting gene loci 4. 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

Question 64

what are the differences between discontinuous and continuous variation?

Answer 64

1. in discontinuous variation, phenotypes are definite and clear cut, and can be divided into distinct phenotypic classes with no intermediates. in continuous variation, phenotypes are not clear cut and a rangeof phenotypes are observed, with intermediates 2. discontinuous variation is controlled by a single or a few genes, which may have two or more alleles. continuous variation is controlled by the combined effect of multiple additive genes, and is known as polygenic inheritance. 3. in discontinuous variation, there is little or no environmental effect on the phenotypic expression of the gene. in continuous variation, phenotypes can be modified by the cumulative effect of varying environmental factors acting on the different genotype 4. phenotypic measurements in discontinuous variation are normally represented on bar graphs, while phenotypic measurements in continuous variation form a normal distribution curve, normally represented on histograms 5. discontinous variation is presented as qualitative data, while continuous variation is presented as quantitative data

Question 65

what are the causes of genetic variation in an asexually reproducing population?

Answer 65

any apparent variation between asexually reproducing organisms is almost always the result of environmental influence, as DNA replication is so highly accurate and almost error-free that there is little possibility of genomic variation

Question 66

what are the principles of the chi-squared test?

Answer 66

1. the chi-squared test is a statistical test for the significance of data that consists of discontinuous variables 2. 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 or due to an incorrect prediction of expected ratios 3. the larger the difference between observed and expected results, the more likely that the hypothesis is incorrect and not due to chance alone 4. the smaller the difference between observed and expected results, the more likely that the hypothesis is correct and is due to chance alone

Question 67

what are the steps in a chi-squared test?

Answer 67

1. state the null and alternative hypotheses 2. calculate the expected number of individuals for each phenotypic class, using mendelian ratios 3. calculate the chi-squared value 4. compare the chi-squared value against a probability table 5. calculate the degree of freedom, which is n-1, where n is the number of phenotypic classes 6. state the level of significance 7. obtain the critical theoretical chi-squared value 8. write a conclusion