Lecture 3: Jan. 25

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Lecture 3 Jan. 25

• Transmission genetics independent assortment
• Human pedigrees

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The 7 traits in garden pea studied by Mendel
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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.
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Results of a monogenic genetic cross (involving 2
alleles of the same gene)
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Genetic and molecular explanation of dominance
in the F1 and 31 segregation in the F2
generation
A diagram like this is called a Punnett square
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The smooth F2 plants segregate in F3 while the
wrinkled ones breed true
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Genetic and molecular explanation of a testcross
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Results of a dihybrid cross (involving alleles of
2 genes)
The coat color gene and seed shape genes assort
(segregate) independently
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Independent segregation of alleles of 2 genes
leads to 4 kinds of gametes in equal proportions
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Random fertilization of the 4 kinds of gametes
generates the 9331 phenotypic ratio
Punnett square for a dihybrid cross
Genotype ratio
Phenotype ratio
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Genotype and phenotype ratios in the F2 of a
dihybrid cross
The W gene is segregating 1 WW 2 Ww 1ww
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A backcross of the F1 to the double recessive
parent (a testcross) yields a 1111 ratio
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Results of a trihybrid cross - 1 (involving 3
genes, W, G and P)
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Results of a trihybrid cross
Punnett square for a trihybrid cross would have
64 boxes. Here it is broken up into 3 dihybrid
diagrams, each with 16 squares
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Symbols used in a human pedigree diagram
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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)
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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.
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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)
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Independent assortment of alleles for 2 different
genes yields 4 kinds of gametes in 1111 ratio
in all organisms (peas and humans, for example).
Mendels laws apply to all organisms because the
mechanisms of meiosis and fertilization are the
same
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Incomplete dominance between 2 alleles of the
same gene yields a 121 phenotypic ratio in F2
(not 31)
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