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Chapter 16 Blog: Simple Patterns of Inheritance

Page history last edited by Larissa-Helen Mahaga-Ajala 12 years, 9 months ago

A.  Daily Blog


16.1: Gregor Mendel did experiments on pea plants to see how a handful of traits was passed down through generations.  The advantages of his experimental system were that pea plants were easy to purchase, the plants didn't take long to reproduce, there are many different characteristics available to study in pea plants, pea plants are easy to take care of, and all of the characteristics and phenotypes were evident immediately.  Mendel did monohybrid crosses which look at the inheritance of one trait.  For example, Mendel studied the traits for tall pea plants and short pea plants.  He crossed a tall plant with genotype TT with a short plant with genotype tt in the P generation.  This would yield all heterozygotes (Tt) that would all be tall in the F1 generation.  Then, the heterozygote offspring would be self-fertilized and yield offspring with the genotypic ratio of 1 TT: 2 Tt: 1 tt.  Mendel's law of segregation states that two copies of a gene segregate from each other during gamete formation and during transmission from parent to offspring.  This means that each gamete will have only half of what will make its offspring's genotype.  One of Mendel's tools to predict the genotypic outcomes of the offspring of pea plants was the Punnett Square.  Basically, you have a grid that you can use to figure out the different outcomes from different combinations of the parents' gametes.  Then you can use that information to interpret the different outcomes phenotypically.  Genotypes for one gene are expressed as either XX, Xx, or xx.  Capital letters always represent dominant traits and lowercase letter represent recessive traits.  A genotype can be homozygous like XX and xx.  This means that the alleles from the homologous chromosomes are the same.  Also, a genotype can be heterozygous.  This means that the alleles from the homologous chromosomes are different.  In the case of a heterozygous genotype, the dominant trait is always expressed over the recessive trait.  Mendel also did dihybrid crosses which examine the inheritance of two traits.  Each potential gamete carries an allele that pertains to the one trait and an allele that pertains to the other trait.  It must be understood that these genes still assort independently and that dominant traits don't have to go with dominant traits, and maternal traits don't have to go with maternal traits.  Mendel's Law of Independent Assortment states that the alleles of different genes assort independently of each other during gamete formation.  


16.2: The Chromosome Theory of Inheritance is an explanation of how the steps of meiosis account for the inheritance patterns observed by Mendel.  The basic tenets of this theory are: 


  • Chromosomes contain DNA which is genetic material.  Genes are found in chromosomes.
  • Chromosomes are replicated and passed on from parents to offspring.  They are also passed on from cell to cell in the development of multi-cellular organisms.  
  • The nucleus of a diploid cell contains two sets of chromosomes, which are found in homologous pairs.  The maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes.
  • At meiosis, one member of each chromosome pair segregates into one daughter nucleus, and its homologue segregates into the other daughter nucleus.  During the formation of haploid cells, the members of different chromosome pairs segregate independently of each other
  • Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.  

Independent assortment in meiosis occurs during Metaphase I.  When the homologous pairs of chromosomes line up on the metaphase plate, it can occur in varying ways.  This allows for many different possibilities in gametes, and therefore many possibilities for the genotype of the resulting offspring.  Basically, the way the homologous chromosome pairs line themselves up during Metaphase I determines the alleles that belong to each gamete.  


16.3: Pedigrees are basically family trees that are used to trace certain traits through the family's bloodline.  You can determine whether the trait is sex-influenced, codominant, incompletely dominant, simple Mendelian inheritance, or X-linked by looking at the genders of those with a certain trait and the pattern the trait has within the pedigree.  


16.4: The mechanisms for sex determination are X-Y, X-O, Z-W, and haplodiploid in mammals, certain insects, birds and bees respectively.  Sex-linked inheritance in fruit flies can be found in the determination of eye color.  Let's say that XW+ represents red eye color, which is normal, and that XW represents white eye color, which is not normal.  If a male with genotype XW+Y mates with a female with genotype XW+XW, then the expected offspring would be XW+XW+, XW+Y, XW+XW, and XWY.  The only fly that would have white eyes is the last one.  Even though one fly has the genotype XW+XW, the phenotype would still be red eye color because the normal W+ gene would overpower the mutant W gene.  In XWY, there is no other W+ gene that could overpower the mutant W gene.  This may be why X-linked diseases are found in men most often.  Though it is possible for a woman to have an X-linked disease, it is less likely.  I'm not totally sure how people figured out that genes are on chromosomes, but I suppose that people looked at what differed between two organisms that exhibit different traits.  Then they could've noticed that the way the chromosomes lined up on the metaphase plate and all that.  I'm not totally sure.  


16.5: People always look at Mendel's experiments and assume that genetics is a pretty black-white game.  You either exhibit something or you don't, and maybe you carry it.  Done.  Well that's not it.  If genetics were that simple, then there would be two hair colors, two eye colors, two heights, and two skin colors.  Since that is not the case, we must recognize the other inheritance patterns in order to look at genetics properly.  Continuous variation has to do with traits that do not reside in the realm of clear types.  Skin color, height, and weight are examples of continuous variation.  Basically, there are many varieties and one's offspring may not have the exact same height, weight, or skin coloring as oneself.  Though, Mendel's ratios are correct, the traits he studied are not as intricate as some of the traits being studied in human beings.  His ratio isn't wrong, however it is too simple to be used to study the inheritance of more complex traits.  Below is a table of the molecular basis of inheritance patterns with relevant examples from the medical community.

Type   Molecular Basis 
Simple Mendelian Inheritance 

In many cases, the recessive allele is nonfunctional, though heterozygote may produce 50% of the functional protein compared to a dominant homozygote, this is sufficient to produce the dominant trait. 

Ex: Huntington's Disease

X-linked Inheritance 

In a female with one recessive X-linked allele (a heterozygote), the protein encoded by the dominant allele is sufficient to produce the dominant trait.  A male with a recessive X-linked allele (a hemizygote) does not have a dominant allele and does not make any of the functional protein.   

Ex: Hemophilia

Incomplete Dominance 

50% of the protein encoded by the functional (wild-type) allele is not sufficient to produce the normal trait.  

Ex: Hair type (as in straight, wavy, curly)


The codominant alleles encode proteins that function slightly differently from each other.  In a heterozygote, the function of each protein affects the phenotype uniquely.   

Ex: The ABO blood system

Sex-influenced Inheritance 

Sex hormones affect the molecular expression of genes which can have an impact on phenotype

Ex: Baldness


16.6: The product rule states that the probability that two or more independent events will occur equals the product of their individual probabilities.  The sum rule states that the probability that one of two or more mutually exclusive outcomes will occur is the sum of the probabilities of the possible outcomes.  



B.  Useful Materials


A picture of a monohybrid cross with a Punnett Square.  A picture of a dihybrid cross with a Punnett Square. 



The ABO Blood Group and Plasmodium falciparum Malaria in Awash, Metehara, and Ziway Areas of Ethiopia

This article is pretty interesting.  The harmfulness of Plasmodium falciparum is measured by the capacity of infected red blood cells to adhere to uninfected red blood cells.  This process is called rosetting.  Scientists looked at rosetting in people with different blood groups in the endemics in the areas mentioned above.  People with blood group O were less prone to having rosetting and were therefore less likely to develop severe Plasmodium falciparum.

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