Genetic Basis for Variation

Question 1

define genotype

Answer 1

made up of genes which are sequences of deoxyribonucleotides and is in turn determined by the alleles of the different/specific genes

Question 2

define phenotype

Answer 2

physical manifestation/trait resulting from a specific genotype and its interaction with the environment

Question 3

define locus

Answer 3

position of a gene on a chromosome

Question 4

define allele

Answer 4

alternate forms of a gene, each coding for a different trait

Question 5

define linkage

Answer 5

genes coding for (first trait) and (second trait) are found on the same chromosome

Question 6

define dominant allele

Answer 6

• an allele that is expressed in the phenotype whether it occurs in the homozygous or heterozygous condition
• one copy of the dominant allele is sufficient to mask the effect of a recessive allele

Question 7

define recessive allele

Answer 7

• an allele that is expressed in the phenotype of a diploid organism only in the homozygous state
• its effect is not seen in the presence of a dominant allele

Question 9

define co-dominance

Answer 9

• phenomenon in a heterozygote in which the effects of both alleles of the gene pair can be observed to produce the phenotype, each allele of the pair is expressed equally

Question 10

define incomplete dominance

Answer 10

occurs when the phenotype of the heterozygote is distinct from and often intermediate to the phenotypes of the homozygotes

Question 11

define homozygous

Answer 11

a diploid organism has two identical alleles at a given gene locus

Question 12

define heterozygous

Answer 12

a diploid organism has two different alleles at a given gene locus

Question 13

explain why there is a range of phenotypes for continuous variation

Answer 13

• several genes on multiple loci/polygenes determine (trait)
• the effect of these genes is similar and additive to result in the range of variation in the phenotype
• presence of multiple alleles at each locus, which gives rise to a greater combinations of genotypes
• expression of phenotype is strongly affected by environmental factors

Question 14

explain how genotype is linked to phenotype (10pts)

Answer 14

• definition of genotype and phenotype
• when different alleles are transcribed to form mRNA, and mRNA is translated into polypeptides/proteins, different alleles will result in the production of different polypeptides/proteins, which have specific functions/roles
• different gene products/proteins may affect different metabolic pathways, hence resulting in different phenotypes
• gene interaction can also result in specific traits to be expressed
• gender determined by sex chromosomes inherited
• presence of a dominant allele will mask the effect/influence of recessive alleles, only the dominant trait is expressed
• both copies of the alleles must be recessive to express the recessive trait

Question 15

explain how genes are inherited from one generation to the next

Answer 15

• genes on DNA are replicated by semi-conservative DNA replication before cell and nuclear division
• DNA associates with proteins to give a condensed chromosome, which ensures genetic integrity during the division process
• in meiosis, reduction division occurs to halve the ploidy level, resulting in haploid gametes
• crossing-over between non-sister chromatids of homologous chromosomes takes place in prophase I gives different combinations of alleles on each chromatid
• independent assortment of homologous chromosomes takes place in metaphase I, to give different combinations of paternal and maternal chromosomes in each gamete, resulting in genetic variation in the gamete
• random fusion of gametes during fertilisation allows the offspring to inherit a set of genes from both parents
• offsprings are genetically different from each other, and from the parents
• parental cells divide by mitosis/binary fission
• daughter cells have the same number of chromosomes and are genetically identical to their parents in asexual reproduction

Question 16

explain using the himalayan rabbit how the environment may affect the phenotype

Answer 16

• fur colour in himalayan rabbits
• himalayan rabbits are homozygous for the c^h allele of the gene, which codes for a heat-sensitive form of an enzyme, tyrosinase
• heat-sensitive form of tyrosinase is active when the air temperature is below 33 degrees Celsius
• hair-producing cells of these Himalayan rabbits will not produce the melanin pigment when exposed to warmer temperatures hence, the hair of rabbits apperas white
• in the extremities the cooler temperature allows the growth of black fur

Question 17

explain how environment affects phenotypes using bees

Answer 17

• drones (male) develop from unfertilised/haploid eggs and hence their phenotype is unaffected by environment
• both queen and worker bees develop from fertilised/diploid eggs
• phenotypic difference between queens and workers are due to diet
• larvae who become queens are fed on a diet of royal jelly, while larvae becoming worker bees are fed on worker joelly
• high sugar content stimulates the formation/maturation of the female reproductive system in queen bees
• different jelly changes the level of expression of different genes via DNA methylation, leading to different proteins expressed in cells of the queens and worker bees, resulting in different phenotypes – worker bees are sterile vs queen bee is fertile/worker bees smaller in size

Question 18

describe how phenotypic variation may result from gene interactions at one locus

Answer 18

• gene interaction at one locus refers to the interaction between the alleles of a single gene
• complete dominance whereby the single copy of the dominant allele will mask the effect of the recessive allele as it is sufficient for phenotypic expression of the dominant trait while recessive allele is only able to express itself in the phenotype in the absence of the dominant allele/when present in 2 copies/homozygous
• incomplete dominance whereby heterozygotes have a phenotype which is intermediate to those individuals which are homozygous for either allele as a single copy of an allele gives insufficient transcripts and gives rise to the intermediate phenotype
• codominance whereby 2 alleles of a single gene will produce 2 distinct gene products resulting in heterozygotes having a phenotype where both alleles are equally expressed
• multiple alleles whereby 2/3 or more alleles of the same gene exist resulting in different combinations of alleles which increases the number of possible phenotypes

Question 19

describe how phenotypic variation may result from gene interaction between loci

Answer 19

• gene interaction between loci refers to the non-allelic gene interaction between 2 genes
• non-epistatic gene interaction whereby 2 independently assorting genes can affect a single characteristic, different combinations of alleles from the 2 genes cause different phenotypes due to interaction of their gene products at the biochemical/cellular level
• epistasis: an allele of the epistatic gene masks the phenotypic expression of the hypostatic gene
• dominant epistasis occurs when a dominant epistatic allele at 1 locus inhibits the phenotypic expression of all alleles of the other hypostatic gene
• recessive epistasis occurs when 2 recessive alleles at 1 locus inhibits the phenotypic expression of all alleles of the other hypostatic gene
• autosomal linkage where the positions of the genes/loci resulted in variation in phenotypic ratio as the genes on the same chromosome tend to be inherited together as a single unit

Question 20

differences between discontinuous and continuous variation

Answer 20

• discontinuous variation is variation showing different phenotypes with no intermediates while continous variation is variation showing graded phenotypes
• discontinuous variation is qualitative inheritance while continuous variation is quantitative inheritance
• discontinous variation is usually controlled by one or two major genes which may have two or more alleles, presence of multiple alleles will result in more than 2 categories of phenotypes, nonetheless still discrete while continuous variation is affected by polygenes (usually controlled by several genes i.e. polygenes – genes which each have a small and additive effect, collectively producing a phenotypic trait) and multiple alleles (gives rise to multiple combinations of genotypes)
• discontinuous variation relatively unaffected by environmental conditions while continuous variation is affected by the environment