Lecture 3: Jan. 25 Transmission genetics: independent assortment Human pedigrees

The 7 traits in garden pea studied by Mendel

Molecular basis of the wrinkled seed coat mutation The wrinkled seed coat mutant is due to the insertion of a foreign sequence in the wild type smooth seed coat gene. As a consequence, the mutant gene is longer and runs slower in a size-separation gel.

Results of a monogenic genetic cross (involving 2 alleles of the same gene)

A diagram like this is called a Punnett square Genetic and molecular explanation of dominance in the F 1 and 3:1 segregation in the F 2 generation

The smooth F 2 plants segregate in F 3 while the wrinkled ones breed true

Genetic and molecular explanation of a testcross

Results of a dihybrid cross (involving alleles of 2 genes) The coat color gene and seed shape genes assort (segregate) independently

Independent segregation of alleles of 2 genes leads to 4 kinds of gametes in equal proportions

Random fertilization of the 4 kinds of gametes generates the 9:3:3:1 phenotypic ratio Punnett square for a dihybrid cross Genotype ratio Phenotype ratio

Genotype and phenotype ratios in the F 2 of a dihybrid cross The W gene is segregating 1 WW : 2 Ww : 1ww

A backcross of the F 1 to the double recessive parent (a testcross) yields a 1:1:1:1 ratio

Results of a trihybrid cross - 1 (involving 3 genes, W, G and P)

Punnett square for a trihybrid cross would have 64 boxes. Here it is broken up into 3 dihybrid diagrams, each with 16 squares Results of a trihybrid cross

Symbols used in a human pedigree diagram

A human pedigree showing the inheritance of a dominant disease gene. The diseased individuals are present in every generation (indicates a dominant disease) and males and females are both about equally affected (indicates autosomal inheritance)

Inheritance of an autosomal recessive disease gene The heterozygous individuals are phenotypically wild type. In this pedigree, there are only 3 affected individuals (III.2, III.4, IV.5). Mating of two heterozygotes is required to produce an affected child.

A human pedigree showing the inheritance of a polymorphic DNA marker There is no masking of one allele by another allele for DNA markers (codominance of alleles is seen)

Independent assortment of alleles for 2 different genes yields 4 kinds of gametes in 1:1:1:1 ratio in all organisms (peas and humans, for example). Mendel’s laws apply to all organisms because the mechanisms of meiosis and fertilization are the same

Incomplete dominance between 2 alleles of the same gene yields a 1:2:1 phenotypic ratio in F 2 (not 3:1)