Genetic basis for variation Flashcards
Defn. Gene
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
Defn. gene locus
The specific position of a gene along the length of a chromosome
Defn. allele
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
Defn. dominant
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
Defn. recessive
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
Defn. codominant
Both alleles of a gene are fully expressed and manifested in the phenotype in a heterozygote
Defn. incomplete dominance
The phenotypes of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele
Defn. homozygote
an organism that has a pair of identical alleles for a gene at a particular locus on a pair of homologous chromosomes
Defn. heterozygote
an organism that has two different alleles for a gene at a particular locus on a pair of homologous chromosomes
Defn. phenotype
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
Defn. genotype
The genetic constitution, or set of alleles, of an organism.
Defn. linkage
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
Defn. monohybrid inheritance
involves the inheritance of one characteristic controlled by the alleles of one gene. Mendel conducted crosses or hybridisation between pure-breeding plants
Defn. dihybrid inheritance
involves the inheritance of two characteristic controlled by the alleles of two gene. Mendel conducted crosses or hybridisation between pure-breeding plants
How is genotype linked to phenotype
Phenotype is the result of an organism’s genotype and the influence of environmental factors
Linked genes without crossing over
- Draw diagram
- Phenotypic ratio: 3:1
- The ratio of 3:1 is seldom achieved because total linkage is rare.
Linked genes with crossing over
- 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
Genetic diagram
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
Epistasis
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
How temp. affects fur colour of Himalayan rabbits
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
How diet affects the differentiation of honey bees
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
Discontinuous variation
- 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
Continuous variation
- 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
Chi-square test
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
