Genetics

Gregor Mendel– the “father” of genetics

A genetic characteristic generally has two (or more) possible varieties– known as traits.

Ex. Plant height: long or short. Flower color: purple or white

Along every chromosome, each specific location contains a specific gene for a specific characteristic.

Different varieties of each gene are called alleles.

Genes exist in pairs, like chromosomes. For every characteristic, you have two genes that determine how that characteristic will appear.

The dominant allele of the gene almost always completely masks the recessive allele of the gene.

Genotype: the combination of genes that the organism has

Phenotype: the actual appearance of the organism

Heterozygous: when the two alleles are different

Homozygous: when the two alleles are the same

If the pea plant is purple, purple is dominant over white in flower color.

Could be PP (homozygous dominant) or Pp (heterozygous).

If the pea plant is white, it must be pp (Homozygous recessive).

Exceptions to “normal” scenarios of inheritance

Incomplete dominance: in the heterozygous genotype, the dominant allele does not completely mask the recessive

Co-dominance: in the heterozygous genotype, two alleles are visible in the phenotype at the same time

Epistasis: one gene modifies the appearance of another gene

Incomplete dominance

Codominance

Sex-linked traits• The gene is usually located on the x-

chromosome. • Sex-linked traits are most common in

males. • Usually passed from mother to son. • Female carriers get one gene from either

mother or father. • Examples: Colorblindness, hemophilia,

Duchenne Muscular Dystrophy

Alternative Patterns of Heredity ( in addition to incomplete dominance, codominance, and

multiple alleles)

• Epistasis– a gene at one locus alters the expression of a separate gene

• Pleiotropy– a gene with multiple phenotypic effects

• Polygenic inheritance– an additive effect of two or more genes on a single phenotype

epistasisFur pigment color

B= black (complete)

b= brown

Pigment deposition gene

C= pigment deposited

c= pigment NOT deposited

The pigment deposition gene is epistatic to the pigment color gene.

Pleiotropy

• Cystic fibrosis– one gene codes for a defective protein in certain cell membranes; causes multiple effects: thicker & stickier mucus membranes, poor nutrient absorption, chronic bronchitis, recurrent bacterial infections

• Sickle-cell disease– one gene for a single amino acid is altered in the hemoglobin molecule; causes multiple effects: blood clots, physical weakness, pain, organ damage, paralysis, lowered blood oxygen, brain damage

Polygenic inheritance (simplified)

3 pairs of genes determine skin color and the combination of those genes determine the phenotype. The more dominant genes, the darker the skin.

Sample question: describe the pattern of inheritance of Huntington’s disease.

• Dominant, autosomal, lethal (presents at relatively older age, so inheritance is possible in younger generations)

• Any child born to a parent with Huntington’s has a 50% chance of inheriting the gene