Single and multi factor inheritance.

Single factor inheritance results in phenotype due to single gene which may be in autosomal or sex chromosome. phenotypes are influenced by alleles of that gene which may be dominant co dominant recessive etc. Follow mandolin pattern of inheritance

Autosomal dominant. One copy of gene required to express phenotype. Huntington's disease. Polycystic kidney disease. Carriers have 50% chance of passing it on.
Autosomal recessive. Express 22 copies of recessive alleles are present. Otherwise mild or no effect observed. Careers have 75% chance of giving to offspring if both parents are heterozygous for that gene. 25% chance if only one parent is heterozygous. Albinism and cystic fibrosis
X linked recessive. when gene is on X chromosome and recessive. Expressed in males carrier and females if homozygous. Haemophilia duschene muscular dystrophy
X linked dominant. Dominant allele on X chromosome. Rett syndrome. Hypo phosphatic rickets
Y linked inheritance. Only from Father to son. Daughters never affected. Hairy ears and webbed toes.

Multi factor inheritance

If three genes dominant (.) and three recessive (-)

...... 1.6% ------
.....- 9.3% .-----
....-- 23,4% ..----
...--- 31.25%

Traits inherited via multiple genes
Skin colour eye colour hair height weight
Each gene has some contribution towards the phenotype. moreover it is influenced by environment through temperature nutrition exercise social economic condition
Features of polygenic inheritance as per h h green

Most metric and meristic traits are controlled by number of genetic loci
Additive effect of genes buy small contribution of alleles
Effect of allele substitution are relatively small and may be interchangeable - different gene combinations may give same phenotype
Considerable modification by environment
Continuous distribution
Balanced systems of polygenic inheritance contain great potential variability and heterozygosity.

Case study. Type 2 diabetes is polygenic which increases and where is its intensity and complications. Can be managed by altering environment that is insulin injection.

One gene may have multiple effects. Plieotropy

Phenylketonuria causes this amino acid to increase in amount in the body, which can be very dangerous. The disease is caused by a defect in a single gene on chromosome 12 that codes for enzyme phenylalanine hydroxylase, that affects multiple systems, such as the nervous and integumentary system.^[2]
Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favors one allele, while selection on other traits favors a different allele

Epistasis

One gene depends on presence of one or more other genes.
Additive. Positive or negative magnitude effect. Inversion of other gene expression. Suppression of other gene.
Balding gene trumps hair colour genes.

Single and multi factor inheritance 2 - 1