Lecture 8-Mendel to Mapping

Lecture 8-Mendel to Mapping - -Each individual carries only...

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Lecture 8- Mendel to Mapping Dominance -Dominant alleles are those that specify the phenotype of a heterozygote -Not due to any physical interaction between DNA of the two alleles -Results from relative biochemical activity of gene products (e.g. proteins) -Nothing to do with relative frequency of alleles -Incomplete dominance- one allele is unable to make enough product to compensate for the other defective one -Codominance- no dominance; two different alleles, both expressed equally Probability -The randomness inherent in chromosome segregation at meiosis allows probability theory to predict the outcome of crosses -Probability of both of two independent events occurring is the product of their individual probabilities -Probabilities of either of two independent events occurring is the sum of their individual probabilities Histocompatability Locus (HLA) -Several different alleles among humans, each giving rise to a distinct cell surface protein
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Unformatted text preview: -Each individual carries only two alleles but displays both respective proteins -Phenotype of heterozygote results from equal expression of both alleles: codominance Chimeras -HLA alleles in various tissues-DNA test revealed presence of 4 different alleles-Patient is two people! -Tetragametic chimera-2 balls of cells fused, became part of the same embryo Significance?-Chimerism may be widespread-Fetal and maternal cells cross the placenta; we may all have some maternal cell lines-In vitro fertilization increases incidence of twinning (and chimerism?)-Epistasis occurs when two genes affect the same trait. The genes are each inherited as usual, but then interaction of their products (for example, enzymes) results in modified phenotypic ratios. (Example: puppies fur colours) -If there are many genes, you get continuous variation, polygenic inheritence. (skin colour)...
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