Inheritance

Chromosome

Consists of DNA molecule associated with proteins and condensed into a highly compact structure

Homologous chromosomes

Pair of chromosomes of same size, centromere position and contain genes coding for same character at corresponding loci

Gene locus

Specific location along length of chromosome where gene is found

Gene

Specific nucleotide sequence in DNA that codes for a functional product

Allele

Alternative form of a gene, have unique nucleotide sequences and results in different traits

Homozygous

Diploid condition where alleles at a given locus on a pair of homologous chromosomes are identical

Heterozygous

Diploid condition where alleles at a given locus on a pair of homologous chromosomes are different

Phenotype

Observable trait of organism

Genotype

Genetic constitution of an organism

Dominant allele

Allele that is fully expressed in the phenotype of a heterozygote

Recessive allele

Allele that is not expressed in the phenotype of a heterozygote, 2 recessive alleles needed for expression

Codominant alleles

2 alleles that are both expressed in the phenotype of a heterozygote

Incomplete dominant allele

2 alleles that when expressed together produces a phenotype that is intermediate to the phenotypes of individuals who are homozygous for either allele

Linkage

2 or more genes on the same chromosome, located close together hence less likely to cross over during meiosis thus more likely to be inherited together

How to determine homozygous dominant or heterozygous?

TEST CROSS, cross with homozygous recessive

Monohybrid:

• HD will give all dominant phenotype

• HT will give 1:1 dominant and recessive phenotype

Dihybrid:

• AABB will give all dominant phenotype

• AaBB or AABb will give 1:1 ratio of phenotypes

• AaBb will give 1:1:1:1 ratio of phenotypes

How to determine is gene is sex-linked?

RECIPROCAL CROSS

• Performed 2 times

• Female w trait X Male wo trait, Female wo trait X Male w trait

• If phenotypes differ, gene is sex-linked

Autosomal Linkage

• All genes on a chromosome is known as linked genes

• The closer the gene loci of 2 genes on the same chromosome, the less likely crossing over will be of both genes, hence higher chance alleles will be inherited together

Diet on differentiation of honeybees

• All larvae fed with royal jelly initially

• On 3rd day, most larvae switched to a diet of pollen and nectar, however queen larvaes are bathed in royal jelly throughout larval development and feed on it during adulthood

• High protein content of royal jelly stimulates formation and maturation of female reproductive system, giving rise to the queen bee

Temperature on fur colour of Himalayan rabbits

• c^h allele codes for temperature sensitive enzyme

• Enzyme functions at low temperatures

• Rabbits with c^h allele are white because heat produced by body inactivates the enzyme and pigments are not produced

• However at Himalayas where it is cold, the extremities of the rabbit turn black as enzyme is at functional temperature and codes for black pigment

Epistasis

• A form of gene interaction in which a gene at one locus alters the phenotypic expression of a gene at another locus

• Causes deviation from standard 9331 ratio

Types of genetic variation

Discontinuous:

• Phenotypic variation that falls into 2 or more distinct classes (eg blood group)

Continuous:

• Phenotypic variation that exhibits a range (eg mass, height)

How to conduct Chi-square Test

1. State null hypothesis — There is no significant difference between observed and expected values. Any difference is due to chance

2. Determine chi square value

3. Determine degrees of freedom: n-1

   1. Calculated chi squared value is larger than critical (at p=0.05), reject null hypo

   2. Calculated p value is less than 0.05, reject null hypo