BIOL239 Study Guide - Midterm Guide: Dihybrid Cross, Epistasis, Mendelian Inheritance
4 Dec 2018
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BIOL 239 Practice Midterm Notes
Allele:
- One of two or more alternative forms of a gene that arise by mutation and are found at the
same place on a chromosome
- Different alleles: different gene products being made and different phenotypes being
expressed
Recessive Alleles:
- Not always mutant alleles (some mutant alleles act dominantly)
- Expressed as inactive products
- Look at phenotypic expression of alternative alleles in heterozygotes identifies which
allele is contributed to phenotype
Mendel’s Crosses:
- Started with true breeding homozygous parents and reciprocal crosses
- F2 had 3:1 ratio
- Proved that genes of each parent contributed equally to the next generation
Alleles from two genes:
- Dihybrid cross
- Phenotypic ratios involving 9:3:3:1 or derivatives
- Show that TWO GENES ARE INVOLVED
- Ex. 9:3:4 = recessive epistasis
- Ex. 9:3:3:1 = two genes acting on two different traits or two genes acting on a single trait
(ex. comb shape in chickens)
- Ex. 12:3:1 = dominant epistasis
Law of segregation = single gene
Law of independent assortment = 2 + genes
Bombay allele:
- Homozygous for this allele, cannot express ABO genotype
Restrictive = restricts life
- Organism can function normally until a temperature is reached where the gene product is
no longer functional
Heterogenous trait:
- Genes acting in a pathway
- Two diff mutants give rise to same mutant phenotype, they are mutations on different
genes
- Individual heterozygous for one gene will have a normal (wild-type) copy of an allele for
each gene and will express normal phenotype (assuming wild type is dominant)
New Phenotype:
Document Summary
One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Different alleles: different gene products being made and different phenotypes being expressed. Not always mutant alleles (some mutant alleles act dominantly) Look at phenotypic expression of alternative alleles in heterozygotes identifies which allele is contributed to phenotype. Started with true breeding homozygous parents and reciprocal crosses. Proved that genes of each parent contributed equally to the next generation. 9:3:3:1 = two genes acting on two different traits or two genes acting on a single trait (ex. comb shape in chickens) Law of independent assortment = 2 + genes. Homozygous for this allele, cannot express abo genotype. Organism can function normally until a temperature is reached where the gene product is no longer functional. Two diff mutants give rise to same mutant phenotype, they are mutations on different genes.
