Lecture 3: Jan. 25Lecture 3: Jan. 25

Transmission genetics: Transmission genetics: independent assortmentindependent assortment

Human pedigreesHuman pedigrees

The 7 traits in garden pea studied by MendelThe 7 traits in garden pea studied by Mendel

Molecular basis of the wrinkled seed coat mutationMolecular 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 Results of a monogenic genetic cross (involving 2 alleles of the same gene)alleles of the same gene)

A diagram like this is called a Punnett square

Genetic and molecular explanation of dominance Genetic and molecular explanation of dominance in the Fin the F11 and 3:1 segregation in the F and 3:1 segregation in the F2 2 generationgeneration

The smooth FThe smooth F22 plants segregate in F plants segregate in F33 while while the wrinkled ones breed truethe wrinkled ones breed true

Genetic and molecular explanation of a testcrossGenetic and molecular explanation of a testcross

Results of a dihybrid cross (involving Results of a dihybrid cross (involving alleles of 2 genes)alleles of 2 genes)

The coat color gene and seed shape genes

assort (segregate) independently

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

Random fertilization of the 4 kinds of gametes Random fertilization of the 4 kinds of gametes generates the 9:3:3:1 phenotypic ratiogenerates the 9:3:3:1 phenotypic ratio

Punnett squarefor a dihybrid

cross

Genotype ratio

Phenotype ratio

Genotype and phenotype ratios in the FGenotype and phenotype ratios in the F22 of of a dihybrid crossa dihybrid cross

The W gene is segregating 1 WW : 2 Ww : 1ww

A backcross of the FA backcross of the F11 to the double recessive parent to the double recessive parent (a testcross) yields a 1:1:1:1 ratio(a testcross) yields a 1:1:1:1 ratio

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

Symbols used in a human pedigree diagramSymbols used in a human pedigree diagram

A human pedigree showing the inheritance of a A human pedigree showing the inheritance of a dominant disease gene.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 Inheritance of an autosomal recessive disease genedisease 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 human pedigree showing the inheritance of a polymorphic DNA markera polymorphic DNA marker

There is no masking of one allele by another allele for DNA markers (codominance of alleles is seen)

Independent Independent assortment of assortment of alleles for 2 alleles for 2 different genes different genes yields 4 kinds of yields 4 kinds of gametes in 1:1:1:1 gametes in 1:1:1:1 ratio in all ratio in all organisms (peas organisms (peas and humans, for and humans, for example).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 Incomplete dominance between 2 alleles of the same gene yields a 1:2:1 phenotypic ratio in Fsame gene yields a 1:2:1 phenotypic ratio in F22

(not 3:1)(not 3:1)