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Mendelian Genetics 2
Transcript of Mendelian Genetics 2
Influence Incomplete Dominance CoDominance Multiple Alleles More Than 2 Choices Pleiotropy The interactions among genes and the organism can be highly variable. Variable Effects Penetrance Expressivity Sex-Linkage Gene Linkage Epigenetics Extra-nuclear genes Epistasis Polygenic Inheritance When the heterozygous condition expresses a third phenotype, different from either homozygous condition. When the heterozygous condition expresses both of the homozygous conditions simultaneously. Example: Snapdragon Color C C w w C C w R C C R R How does this alter ratios? Example: Human A, B, & AB Bloodtypes Very few traits are expressed in simple Dominant/Recessive relationships The body makes antibodies that are found in the plasma. Antigens are proteins found on the surface of red blood cells. If an antibody comes into contact with a matching antigen, it causes carbohydrates to agglutinate (clump).
Transfusion a person with type A has A antigens and B antibodies. This is a problem if given blood with B antigens present. Many traits are controlled by more than two alleles or one locus When there are more than two alleles for a gene. When more than one gene contributes to the expression of a particular trait Example: Human A,B, & O alleles Three blood type alleles = six possible genotypes, and four different phenotypes. Example: Human Skin Color Interactions at three(?) different loci leads to a wider variation for the trait (distributed along a bell curve). When one gene has multiple effects on the physiology of an organism When two or more genes control the expression of a single trait. Example: Sickle-cell anemia Example: Coat color in Laborador Retrievers One locus has wide ranging effects on the physiology of the organism. The E/e allele controls whether pigment will be produced or not.
E allele = the B/b locus determines the color
__ee - golden lab
bbE_ - chocolate lab
B_E_ - black lab Some genes travel together Some traits aren't on chromosomes Linkage was first studied in the early 1900's by Thomas Hunt Morgan, a biologist who worked at Columbia University. Refers to genes that are on a sex chromosome.
Males will show sex-linked traits at a higher frequency than females (why?)
First discovered in fruit flies. The white eye mutation is on the fruit fly X chromosome The X chromosome in humans has many genes. The Y chromosome has very few Map of Y Chromosome... Different modes of sex determination (fruit flies are essentially like us) Drosophila melanogaster The "Fruit Fly": An awesome model organism for genetics (why?) Refers to any genes that are on the same chromosome
Usually linked genes will travel together during meiosis.
When might they separate? How linkage affects transmission of genes Linkage Mapping Since linked genes are only separated by crossing over events, the higher the frequency of recombinant offspring for any two genes, the greater the likelihood of a crossing over event.
This correlates to a greater distance between the genes on a chromosome.
This information can be used to generate a "linkage map" of genes on a chromosome.
"Map units": indicate distance between genes on a linkage map.
1 map unit = 1% chance of crossover (and recombinant offspring) A linkage map of a Drosophila chromosome Mitochondria and Chloroplasts have their own chromosomes.
These are inherited "matrilineally"
In males, the Y-chromosome is inherited "patrilineally"* * not technically "Extra-nuclear", but still... Variegated Leaves: A trait conferred by a chloroplast gene Mapping of Y-Chromosome mutations informs our understanding of historical human migration. Refers to any heritable trait that is not determined by DNA sequences Genomic Imprinting Methylation Some traits are only expressed if the gene is inherited from one parent or the other. Methyl groups (-CH3) are added to DNA to inactivate it.
Some evidence to suggest this methylation pattern may be heritable, and affect phenotype.
Might explain phenomena like genomic imprinting Big Questions Make Sure You Can "Dominance Hierarchy": I = I > i A B ratios? ratios? How does this alter ratios? The likelihood of a genotype expressing a phenotype in an organism (or population) The extent to which a trait is expressed in an organism. Down's Syndrome is a genetic condition with total penetrance, but highly variable expressivity. Several possible sex-linked transmission situations The interaction of an organism's genome and its environment leads to complex patterns of gene expression. The flower color of some species of hydrangea depends upon the pH of the soil (more acidic = bluer) The coat color of Arctic Hare's changes on a seasonal basis. How much of a human is a product of genes? How able are scientists to separate themselves from their culture and biases? Summing It Up: How are traits inherited?
How are traits expressed? Note: Gigantificated! Describe how each of the situations that is discussed in this presentation extends Mendelian understanding of genetics.
Use your understanding of these situations to analyze/predict data from experiments and genetics problems.
Provide examples of each of the situations described in this presentation.
Use data from crosses involving linked genes to determine the distance between linked genes on a chromosome (in map units).
Explain why most traits result from the interplay between an organism and its environment.
Illustrate how genetics illuminates the relationship between science and society. Actual Size Meiosis...makes gametes