1 INCOMPLETE DOMINANCE The phenotype of heterozygotes is

1. INCOMPLETE DOMINANCE The phenotype of heterozygotes is intermediate between homozygotes

2. CODOMINANCE The heterozygote shows the phenotypes of both homozygotes (dominant and recessive)

Blood groups Phenotype 0 A B AB Genotipo i/i IA/IA o IA/i IB/IB o IB/i IA/IB 3 allelic variants: IA; IB (dominant); i (recessive)

The ABO locus codes for a glycosyltransferase

Phenotype M N MN Genotype LM /LM LN/LN LM/LN Two allelic variants: LM ; LN (codominant) Phenotype Genotype Rh+ D /D o D/d Rhd/d Two allelic variants: D (Dominant); d

Fetal Erythroblastosis Mother Rhdd Father Rh+ DD Offspring Rh+ Dd Antibody production against D Possible anemia of the offspring

Interactions between genes Epistasis: Form of interaction between genes, in which the activity of one gene masks the phenotypic contribution of a second gene. The 9: 3: 3: 1 ratio is modified in a dyhybrid cross The gene masking the expression of the second gene is called epistatic, the masked gene ipostatic.

C controlla la formazione del colore del pelo C/ -neri c/c albini A determina l’aspetto aguti A/- aguti a/a non aguti Phenotypes: A/- C/a/a C/A/- c/c or aguti black a/a c/c albinos 9 3 3+1

DOMINANT EPISTASIS Color of the pumpkin fruit: allele W prevents color formation allele w does not interfere with color formation allele Y yellow allele y green

F 2: P: white pumpkins X green pumkins F 1: 100% white (W/w Y/y) W/w Y/y X W/w Y/y 3/4 W/- 1/4 w/w 3/4 Y/- 9/16 W/- Y/- 9/16 white 1/4 y/y 3/16 W/- y/y 3/16 white 3/4 Y/- 3/16 w/w Y/- 3/16 yellow 1/4 y/y 1/16 w/w y/y 1/16 green 12/16

Gene expression and environment Penetrance Not all individuals with a specific genotype express the corresponding phenotype. The frequency of expression of a dominant gene, or a recessive one in homozygosity, in a population is called PENETRANCE.

The expressivity is the degree at which a gene or genotype is phenotypically expressed in a given individual.

Human polydactily Autosomal dominant 90% Penetrance Example: of 42 individuals with Polydactily allele, 38 are affected. 38/42= 0. 90= 90%

Neurofibromatosis Autosomal dominant Penetrance 50 -80% variable expressivity

Environmental effects Sex Gene expression is influenced by sex. Autosomal genes control traits manifest only in males OR females. Sex controlled traits: Ex: humans: facial hair distribution, development of mammary glands Sex inflenced traits: Present in males and females with different degrees of expressivity. Ex: humans: baldness.

L’allele b è influenzato dagli ormoni con espressività variabile.

Ex: Gotta Rheumatoid arthritis Osteoporosis Males/females = 8: 1 Males/females = 1: 3 Temperature sensistive enzymes Ex: Cvoat color of himalayan rabbits

Chemicals Phenylchetonuria (PKU): autosomal recessive; defective del metabolic pathway of Phenylalanine (Phe). Homozygotes show mental retardation in early age. Accumulation of Phenylpiruvate (toxic). Diet drives the severity of the symptoms: Phe-poor diet limits the severity of the degree. Phenylalanine hydroxylase Ritardo mentale; crescita ridotta; morte prematura