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)