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BIOL 1020 (293)
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Lecture 25

BIOL 1020 Lecture 25: Lecture 25
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Department
Biological Sciences
Course
BIOL 1020
Professor
Joy Stacey
Semester
Winter

Description
Lecture 25 9:3:3:1 ratio o 9 plants are purple and tall o 3 plants are white and tall o 3 plants are purple and short 1 plant that is white and short if just looking at tall trait obeys 3:1 ratio (12 are tall, 4 are short) law of segregation shows up in law of independent assortmentinheritance of one trait does not at all affect inheritance of another trait when on separate chromosomes Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles Eg. Human growth hormone determines whether we are tall or short dietary factors affect and many other factors Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: o When alleles are not completely dominant or recessive o When a gene has more than two alleles o When a gene produces multiple phenotypes Degrees of Dominance 1. Complete Dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical (RR Rr are the same) what Mendel looked at 2. Incomplete Dominance, the phenotype of F1 hybrids is somewhere between the phenotype of the two parental varieties (red and white flower cross to make a pink flower) crossing the F1 pink flowers produces F2: red (1) and white (1) and pink flowers (2) 1:2:1 ratio o 3. Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Relationship between dominance and phenotype A dominant allele does not subdue a recessive allele; alleles dont interact Alleles are simply variations in a genes nucleotide sequence For any character, dominancerecessiveness relationships of alleles depend on the level at which we examine the phenotype o Phenotype is the outward expression of the genotype
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