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Chapter 16 (blog): Simple Patterns of Inheritance (Semon)

Page history last edited by Semon Rezchikov 13 years, 5 months ago

A.  Daily Blog

Through the quantative experiments of Gregor Mendel, a 19th century monk, much of the inheritance of traits was explained through a model called Mendelian genetics. The model postulates that each organism has 2 "alleles" (variants) of each gene, but only one variant is expressed as a trait in the actual organism (i.e. expressed phenotypically). Some alleles dominate other aleles, so that even though the recessive allele is present in the organism's genome, only the dominant allele is expressed. Each gamete of the organism contains one allele of a gene, with a 50% chance of containing one of the organism's alleles and a 50% of containing the other. These rules allow for probabilistic prediction of the phenotypic outcomes of a cross if the genotype of the parents is known; it also allows for the indirect determination of a parent organism's genome through observation of the results of crosses. In the canonical example, if a line of pure breeding tall pea plants (dominant allele, genotype TT) gets crossed with a line of pure breeding short pea plants (recessive allele, genotype tt), the new plants will all be tall - their genotype is Tt, and the T allele is dominating the t allele. Crossing two plants from this new generation will create 75% tall pea plants (25% TT, 50% Tt) and 25% short plants (tt). By examining ratios of offspring of crosses in which two genes are examined (of the form AaBb x AaBb), Mendel formulated his Law of Independent Assortment, that alleles from different genes segregate independently into gametes.


Chromosomes are the physical basis behind Mendel's laws. Humans have two chromosomes - so each person contains two alleles of each gene, one on each chromosome. Dominance occurs when, for instance, one allele produces a protein while the other does not; if even one copy of a functional allele is present, the protein is produced and the allele's phenotype is expressed! As mentioned in a previous blog, paired chromosomes segregate independently across the metaphase plate in meiosis; thus, genes on different chromosomes segregate independently into gametes. When two genes are on the same chromosome, they tend to be segregate less than independently; a larger distance between the genes on the chromosome indicates a higher chance of crossover during the tetrad pairing in meiosis and thus a more "independent" segregation.


A pedigree is a family tree with different people shaded differently depending on whether they express or carry an allele or not. It can be used to determine whether the allele for a disease is dominant or recessive - recessive diseases skip generations while dominant ones do not.


Animals determine sex in many ways. In some, like humans, one pair of homologous chromosomes is significantly different in males while fairly similar in females (females are XX, males are XY). In others the same occurs females. In some the female has two X chromosomes while the males have only one. In some males are haploid while females are diploid! Some traits demonstrate sex-linked inheritance because they are on a sex chromosome. For Drosophilia, the eye-color trait is on the X chromosome, so the cross of a heterozygous female (with red eyes, since that's the dominant allele) and a red-eyed male will produce white-eyed males (since they have a chance of inheriting the white-eye allele from the mother on their X chromosomes) but no white-eyed females (since one of their X chromosomes will have come from the father, who is red-eyed and has only that allele as he has only one X chromosome).


Mendel's theory only deals with discrete traits, like tall/short, rather continuous traits like height. This is because continuous traits are the result of the interactions of many alleles, and the environment to boot. Likewise, not all genes segregate independently, as mentioned in paragraph 2. Furthermore, dominance and recession are not the only two interactions alleles can have with each other - in incomplete dominance, the phenotypes of the alleles blend (red flower + white flower = pink flower), while in codominance both phenotype are expressed in different locations (red flower + yellow flower = patchy red and yellow flower). 


B.  Useful Materials



A short, interesting biography of Mendel. Apparently he at one point fell ill from test anxiety. I'm glad he wasn't born in the modern day and age.



Wow. I've actually never seen this. This is a massive resource dedicated for teaching genetics. This is really cool, and really well done. Dr. Weber, you might find some of the stuff in here useful.



This goes into detail explaining how to analyze pedigrees.



Comments (1)

Derek Weber said

at 2:37 am on Apr 2, 2011

I have used the utah site for other courses. I like the pharmacogenomics game.

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