Chapter 14 – Mendel and the Gene Idea

Gregor Mendel

• Mid 19th century Austrian monk

Garden pea

• Self fertilize or cross fertilize

• True breeding or hybrids

Garden pea traits

Experimental design – monohybrid cross

Terminology • “factors” = genes

– Alleles

• Dominant

• Recessive

Terminology cont

• Phenotype

• Genotype – Homozygous

– Heterozygous

Interpretation of monohybrid cross

• Law of Segregation

Testcross

• To determine genotype of dominant individual

Experimental design – dihybrid cross

• Law of independent assortment

Extensions of Mendelian inheritance

• All of Mendel’s traits were coded for by a single gene and had only two alleles – with one being completely dominant over the other

• Not all traits are that simple

Incomplete dominance

• Heterozygote has an intermediate phenotype

Epistasis

• One gene masks the effects of another gene

Environmental effects

• Phenotype is affected by environmental influence of gene expression

Multiple alleles & Codominance

• Some genes have more than two alleles present in a population

• Codominance – heterozygote expresses both alleles

Blood type cont

• Rh factor – Mendelian inheritance

• “+” completely dominant over “-”

– Erythroblastosis fetalis

Polygenic inheritance

• Continous traits

Pleiotropy

• One gene having multiple effects

Pedigree analysis

Recessive – albinism Dominant – Achondroplasia

Prenatal testing