Genetic basis for variation

Question 1

Defn. Gene

Answer 1

A specific nucleotide sequence in DNA which codes for a functional product that determines a particular characteristic in the organism

It is located at a specific locus on the chromosome

Question 2

Defn. gene locus

Answer 2

The specific position of a gene along the length of a chromosome

Question 3

Defn. allele

Answer 3

An alternative form of a gene which occur as a pair of alleles at the same gene locus in diploid cell

Different alleles of the same gene have a unique nucleotide sequence

Question 4

Defn. dominant

Answer 4

An allele that has the ability to manifest itself at the exclusion of the other allele

The dominant allele is fully expressed in the phenotype in a heterozygote

Question 5

Defn. recessive

Answer 5

Phenotypic effect of the recessive allele is completely masked in the presence of a dominant allele

The recessive allele is completely masked in the phenotype in a heterozygote

Question 6

Defn. codominant

Answer 6

Both alleles of a gene are fully expressed and manifested in the phenotype in a heterozygote

Question 7

Defn. incomplete dominance

Answer 7

The phenotypes of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele

Question 8

Defn. homozygote

Answer 8

an organism that has a pair of identical alleles for a gene at a particular locus on a pair of homologous chromosomes

Question 9

Defn. heterozygote

Answer 9

an organism that has two different alleles for a gene at a particular locus on a pair of homologous chromosomes

Question 10

Defn. phenotype

Answer 10

The observable physical and physiological trait of an organism. Phenotype is the result of the expression of an organism’s genotype and the influence of environmental factors

Question 11

Defn. genotype

Answer 11

The genetic constitution, or set of alleles, of an organism.

Question 12

Defn. linkage

Answer 12

When 2 genes are located on the same chromosome, they are described as being linked.

All genes on a single chromosome form a linkage group. Linked genes do not exhibit independent assortment

Question 13

Defn. monohybrid inheritance

Answer 13

involves the inheritance of one characteristic controlled by the alleles of one gene. Mendel conducted crosses or hybridisation between pure-breeding plants

Question 14

Defn. dihybrid inheritance

Answer 14

involves the inheritance of two characteristic controlled by the alleles of two gene. Mendel conducted crosses or hybridisation between pure-breeding plants

Question 15

How is genotype linked to phenotype

Answer 15

Phenotype is the result of an organism’s genotype and the influence of environmental factors

Question 16

Linked genes without crossing over

Answer 16

• Draw diagram
• Phenotypic ratio: 3:1
• The ratio of 3:1 is seldom achieved because total linkage is rare.

Question 17

Linked genes with crossing over

Answer 17

• Draw diagram
• Phenotypic ratio: 24:1:1:7
• Phenotypic ratio observed is not according to the expected ratio as the genes for flower colour and length of pollen are linked/ on the same chromosome
• Phenotypic classes which are under-represented are usually formed from recombinant gametes

Question 18

Genetic diagram

Answer 18

Let:
R represent.. Y represent..
r represent.. y represent..

Parental Generation (underlined)
Parental Phenotypes
Parental Genotypes

After meiosis,
gametes produced OO OO
Random fertilisation

Offspring genotype
Offspring phenotypic ratio

Question 19

Epistasis

Answer 19

a type of gene interaction in which the phenotypic expression of one gene alters the phenotypic effects of another gene that is independently inherited.

Any gene that masks the expression of another gene is epistatic to that gene

Question 20

How temp. affects fur colour of Himalayan rabbits

Answer 20

4 alleles:
1. C - wild type/agouti
2. c^(ch) - chinchilla
3. c^(h) - Himalayan
4. c - albino

Dominance Hierarchy:
C > c^(ch) > c^(h) > c

C allele codes for a functional enzyme involved in pigment synthesis which has an optimum temperature at body temperature of 25 degree celsius

• natural climate for Himalaya is cold. phenotype is white body with black ears, nose, feet and tail
• heat from the environment prevents the development of black pigment. Black fur only grows on parts of the body which is cold enough
• C^(h) allele in the Himalayan rabbits code for a temp-sensitive protein
• mutation of allele changed the enzyme such that the enzyme only functions at lower temps

Question 21

How diet affects the differentiation of honey bees

Answer 21

3 phenotypes:
1. Drones (male)
2. Queen
3. Workers

• drones arise from unfertilised eggs
• difference between phenotype of workers and queens arises due to the diet of the larvae during development
• on the third day, most larvae are switched to a diet of pollen and honey and develop into workers
• a larva is kept on the diet of “royal jelly” until she pupates. high protein content of royal jelly stimulates the formation and maturation of the female reproductive system, giving rise to the queen bee

Question 22

Discontinuous variation

Answer 22

• a phenotypic variation that falls into 2 or more distinct, non-overlapping classes with clear-cut differences and no intermediate between them
• Characteristics showing discontinuous variation are usually controlled by one or a few genes which may have two or more alleles for each genes and their phenotypic expression is relatively unaffected by environmental conditions

Question 22

Continuous variation

Answer 22

• certain characteristics within a population exhibit a complete range from one extreme phenotype to the other without any break
• characteristics showing continuous variation are produced by additive effects of many genes on a single phenotypic character (polygenic inheritance) and their phenotypic expression is influenced by environmental factors

Question 23

Chi-square test

Answer 23

x^2 = sigma [(O-E)^2] / E

• degree of freedom = no. of phenotypic categories - 1
• Probability is fixed at 5% level of significance
• if calculated x^2 is less than critical value, difference between observed and expected results is not significant and is due to chance vice versa