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Chapter 16: Simple Patterns Of Inheritance (Nicole Lee)

Page history last edited by Nicole Lee 13 years, 3 months ago

A.  Chapter Blog

     Gregor Johann Mendel was an extremely important figure in the science world.  He had created an experimental system in which he took pea plants and analyzed them.  This led to the formulation of the basic generic principles known as Mendel's Laws.  Not only did Mendel's studies give people insight on the genetics of pea plants, but it also gave the scientific world information on a large variety of sexually reproducing organisms. Mendel studied the pea plants and found that when two individuals with different characteristics are mated, this is known as hybridization.  The offspring to the experiments Mendel created are known as hybrids (like the cars).  As Mendel was studying this phenomenon he also reasoned that the characteristic of an organism, such as the appearance of seeds, pods, flowers, or stems are known as characters.  He also found from his studies that a trait is an identifiable characteristic; in which this usually refers to a variant. Mendel found that using ea plants for his experiments was beneficial.  For starters, pea plants are a low budget and morally acceptable way to explore genetics rather than breeding humans for the sake of science.  Another reason is because pea plants are not only easy to take care of, but they grow fast, usually within a few days rather than waiting a few years to observe a trait (like in humans).  Pea plants could also self- fertilize.  I know that may sound gross to some, but self- fertilization is a crucial part in plants.  This mechanism occurs from the fertilization that involves the union of a female gamete and male gamete from the same individual.  Self-fertilization makes it easier to produce plants that breed true for a specific trait.  This basically means the trait does not vary from generation to generation.  

     Mendel also studied the cross fertilization mechanism between pea plants.  He found that cross- fertilization involves the union of a female gamete and a male gamete from different individuals.  This is also important because Mendel found a way he could cross- breed any two of his true breading pea plants and create any type of hybrid that he pleased!  Although this is ethically immoral for humans and other animals these days, it is extremely important to the scientific world.  

     During Mendel's historical experiments, he investigated the inheritance patterns of pea plants that differed with regard to a single character.  Monohybrid crosses involves the union of a female gamete and a male gamete from different individuals.  Mendel had followed the tall and short variants for height in pea plants.  When he cross bred true- breeds of tall and short plants, he discovered from this that the plants in the next generation came out to be tall.  From this experiment Mendel founded the idea that there are dominant and recessive traits.  Now what are dominant and recessive traits you may ask.  Well in a nut shell, they are the displayed trait in a heterozygote. Heterozygote meaning an organism having two different alleles of a particular gene and so giving rise to varying offspring.  He also found that recessive traits are a trait that is masked by the presence of a dominant trait in a heterozygote.  In Mendel's pea plant experiments he found that tall stems and purple flowers were the dominant traits however the dwarf stems and the white flowers are recessive.

     When Mendel regenerated the offspring he made from his first batch of pea plants, he created the F2 generation.  The F2 generation showed that the offspring exhibited both dominant and recessive traits (rather than the first generation only showing the dominant traits).  Mendel concluded from this experiment that each parent carries two versions of a gene (versions meaning the allele) and that the two alleles that were carried from the F1 plant will segregate.

     Mendel's principle of segregation meant that the two alleles were going to separate, as in chromosomes during mitosis. The law of segregation states that two copies of a gene segregate from each other during gamete formation and during transmission from parent to offspring.  Mendel used punnett squares (A common method for predicting the outcome of simple genetic crosses) to predict the outcome of simple genetic crosses.  To make a punnett square, you have to know the genotypes of the parents.  Genotypes are the genetic composition of an individual.  An individual with a homozygous genotype has two identical copies of an allele (TT or tt).  An individual with a heterozygous genotype is an individual with two different alleles of the same gene (Tt).  The phenotypes are the characteristics of an organism that are the result of the expression of the genes (through the alleles).  After finding the genotype for the parents, you would draw a square out, put the father's genotypes on the top part of the square and put the mother's of the left side of the square.  From there you can almost multiply like the way you did in 4th grade with the squares :).  

     From the outcome, you can see all of the possibilities for the offspring to show which trait before the offspring is put to life.  Mendel's studies also included when a character has two variants, one of which is dominant over the other, the individual with a recessive phenotype is homozygous for the recessive allele.  To distinguish between the two possibilities, Mendel created the idea of a test cross.  A test cross is used to determine if an individual with a dominant phenotype is a homozygote or a heterozygote. Also, a cross to determine if two different genes are linked.

Mendel performed crosses that he continuously followed the inheritance of two characteristics.  A cross of that nature is known as the dihybrid cross.  A dihybrid cross is a cross in which the inheritance of two different traits it followed.  Mendel calculated from his earlier experiments that the dominant traits would show over the recessive no matter what, so from that information he assumed the outcome of the new gametes.  The offspring from these are called dihybrids.  

     Mendel found that from the majority of his experiments, he got the same results he had originally analyzed.  In later years, the law of independent assortment would be founded from most of Mendel's work.  The law of independent assortment states that the alleles of different genes assort independently of each other during gamete formation.  

     In the next section of this chapter, the Chromosome Theory of Inheritance is discussed.  This theory is an explanation of the steps of meiosis account for the inheritance patterns observed by Mendel.  


  1. Chromosomes contain DNA, which is the genetic material. Genes are found in the chromosomes.

  2. Chromosomes are replicated and passed from parent to offspring. They are also passed from cell to cell during the development of a multicellular organism.

  3. 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.

  4. 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.

  5. Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.

These 5 bullet points were from the book.  Although I did not word them to be in my own context, I kept them the way they are because I think they are set out perfectly and easier to understand.


The Chromosome Theory of Inheritance in a nut shell is an explanation of how the steps of meiosis account for the inheritance patterns observed by Mendel.  Another thing discussed in this chapter is independent assortment.  Independent assortment is another topic that this section reached upon just like it was in the first section.  The section also was based on comparing the segregation of alleles with the behavior of homologues in meiosis.  When meiosis begins, each of the chromosomes have already replicated.  They consist of two sister chromatids. At metaphase I of meiosis, the two pairs of chromosomes randomly align themselves along the metaphase plate.  When these cells undergo the cell cycle, there are 4 allele combinations in equal numbers.  Segregation is when two copies of a gene segregate from each other during gamete formation and during transmission from parent to offspring however the chromosome theory states otherwise.

     The next section is an overview on the pedigree analysis of human traits.  Pedigree analysis is an examination of the inheritance of human traits in families.  A pedigree is used to analyze the transmission of an inherited trait over the courses of several generations.  The results of this method may be less accurate than the results of breeding. Because of the small size of human families, this may lead to large sampling errors.  Despite the set back, a pedigree analysis can provide important clues from human inheritance.  A particular trait is can be seen based on pedigree analysis because by looking at the chart, you can see the dominant and the recessive traits and therefor you are able to see whether or not the trait is going to show up in the offspring or not. 

     In the next section, the class discussed sex chromosomes and X- linked inheritance patterns.  There are the X-Y system, the X-O, and the Z-W.   The X-Y system is when the somatic cells of males have one X and one Y chromosome, whereas female somatic cells contain two X chromosomes.  The X-O system operates in insects.  Unlike the X-Y,  the presence of the Y chromosome in the X-O system does not determine maleness. In X-O, the insect's sex is determined by the ratio between its X chromosomes and its sets of autosomes.

     Through more study, scientists discovered that there are genes on chromosomes.  They observed this from the fact that when the chromosomes in thecell were put through cell division, the genes were carried along with them.   

          There were many ways when Mendel's principals could not apply to certain topics in the scientific world.  Mendel studied seven characteristics that only seemed to affect one trait.  There were many variations in the medical community that were observed in later years.  Genetics is an extremely important part of life and studies in the scientific community!  Without it, people could not predict the outcome of offspring.  



B.  Useful Materials

This first useful material is a video that covers the basis on how to create punnett squares.  This introductory video is extremely helpful because it uses a real life situation of determining the possible blood types of an offspring.  Not only was this video easy to understand and follow, but it makes the process of understanding the importance of Punnett squares better.


I LOVE this video!  I remember watching this is middle school and it was stuck in my head.  This song is catchy and helps understand the basis of genetics.  This is related to the topic because it describes recessive and dominant traits.


C. Article


This article is about how scientists are trying to identify the immunogenetic factors that trigger the beginning of preeclampsia and eclampsia through methods in genetics.  This article describes the study of 142 pregnant women in Obstetrics and Gynecological Hospital Isidro Ayora in Quito.  The women were put into two differed groups, diseased and healthy.   This study was used to predict the outcome of the children and test genetics by looking at traits like height, eye color, you name it.  This article was very interesting and helped me understand the chapter better with a real life situation.  





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