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Chapter 16 Blog: Simple Patterns of Inheritance (Kimberley)

Page history last edited by KimberleyHausheer 13 years, 3 months ago

A. Chapter 16 Summary

 

     Chapter 16 explores the simple patterns of inheritance. The first attempt to understand inheritance concluded in the idea each parent makes an equal contribution to  their offspring and their genetic material would be blended. It wasn’t until about 100 years later when Mendel started to study peas when the truth was discovered. 

            Mendel chose the garden pea to investigate the general laws of inheritance for a few very practical reasons.  First, it had many different traits or characteristics that could be studied and many of them came in only two varieties. Peas were also self fertilizing. An organism is self-fertilizing when it has both male and female sex organs. This was beneficial in his experiment because  it produces true breeding. Which is when a many generations have the exact traits as the one before. The third reason he chose to experiment on peas was because they were easy to manipulate. By cutting out the stamen, where the sperm are produced, he could easily stop the self-fertilization process and control it himself. He would do this by using a paintbrush to carry pollen from another plant and fertilizing the plant with absent stamen. 

            Mendel started his experimentation with pea plants with monohybrid crosses, also known as single-factor crosses. A monohybrid cross is when the experimenter follows the difference of only one variant. Mendel first crossed a dwarf plant and a tall plant in the P (parental) generation. The P generation produced all tall plants considered the F1 generation (first filial generation). Mendel allowed these plants to self-fertilize. They then produced the F2 generation, consisting of three tall plants and one dwarf plant. This outcome was generally consistent with all of the traits he tested. From analyzing his data he came out with the outcome that 1) traits exist in two forms: dominant and recessive; 2) an individual carries two genes for any given character and their different forms are called alleles; and 3) two alleles of a gene separate during gamete formation.

            The third point that he found, the idea that two alleles of a gene separate during the formation of eggs and sperm so that every gamete receives only one allele is know today as Mendel’s law of segregation. He contrived this idea by concentrating on the F2 generation, which consistently came up with a 3:1 ratio, 3 tall plants and 1 dwarf plant. How could this possibly happen if the parents were all tall? Knowing that the P generation had a dwarf plant he figured that the trait was silently passed down but no expressed, thus the dominant and recessive traits. But in order for it to be expressed in the F2 generation the F1 plant alleles must separate. Otherwise you would continue to get the exact same outcome. One way to predict the outcome of crosses is to use a punnett square. We are all a little too familiar with punnett squares I think because it is hard to use anything else.

            If you don’t know the genotype of an individual you can examine the data from a test cross. If the individual is displaying a recessive trait, like a dwarf plant, than the genotype must be tt, or both recessive alleles. However if the individual is displaying the dominant trait it can either be homozygous or heterozygous. Don’t worry- Mendel invented the method called a testcross. You would cross the unknown genotype plant with a homozygous recessive plant. If the results were all tall plants then the unknown plant must be homozygous. If there were two short plants than it must be heterozygous. This is because in order for a plant to be recessive it must have two recessive alleles. If the other plant was homozygous, there is no way that a plant could be recessive because it would only ever have one recessive allele.

            The next step in Mendel’s research was to follow the inheritance of two different characters, called a dihybrid or two-factor cross. He carried it out the same way as the monohybrid cross. He first crosses two true plants (p generation) one with yellow round seeds and the other with green wrinkled seeds. The F1 generation consisted of all yellow round seeds, proving that yellow and round were dominant. Then let them self-fertilize, producing the F2 generation. The F2 generation had a ration similar to 9:3:3:1.It had 9 yellow round seeds, 3 yellow wrinkled seeds, three green round seeds and 1 green wrinkled seed. His results supported the idea that each allele assorts independently during gamete formation. This means that the two genes are not linked and their inheritance is not affected by each other at all.

            In the early 1900s the idea that chromosomes held genetic material opened many doors and helped scientists make connections. Two biologists of the time proposed a chromosome theory of inheritance which consists of a few fundamental principles. We know all of them from the last chapter. Something new though is that the physical location of a gene on a chromosome is called the locus.

          Mendel’s law of segregation can be better understood when put into the process of meiosis. In meiosis the genetic material replicates. So, focusing on only height, the parent cell is heterozygous. The original cell has a genotype of Tt The sister chromatids pair up forming chromosomes. This causes one chromosome to be TT and the other to be tt. Then the homologues separate into separate cell. During meiosis II the chromosomes split, separating the sister chromatids creating four cells. Each cell has one allele. There are two with t and two with T. Then during fertilization, any of the egg cells can pair up with a sperm cell. This also helps explain the law of independent assortment. Instead of  having only one chromosome though there are two. The same things happen though.

          In humans, you cannot tell if you don’t know the genotype of a certain trait it is not ethical to simply reproduce with another person who is homozygous for the recessive and see what your children turn out to be. Instead, scientists use an approach called pedigree analysis. A pedigree analysis is when an inherited trait is analyzed ove the course of a few generations in one family. Pedigree analysis is often used to determine if an individual has a matant allele that would cause a genetic disease and if it is dominant or recessive. A male is represented by a square and a female is represented by a circle. If the individual is a carrier than the shape is half filled in. If the individual phenotypically displays the trait the entire shape is filled in. A trait is dominant when none of the shapes are half filled in.

          In the last chapter we learned that humans have sex chromosomes that are not homologous. Males have and X and a Y and females have two Xs. However it is not that simple. In mammals the Y chromosome determines the sex, because it has the SRY gene. So when a mammal has genetic abnormalities the Y chromosome, or more specifically the SRY gene is the deciding factor. Many insects operate on the X-O system, which depends more on ratio. If the ratio of X chromosomes to number of sets of chromosomes a cell has is 1 than the individual is female. If the ratio is less than 1 than the individual is male. For example if a haploid cell only has one X chromosome than the ratio would be ½ so the individual would be male. some animal species like birds and fish have a Z-W system where the male carries two similar chromosomes. The male is ZZ and the female is ZW.

          Some chromosomal mechanisms do not even involve a special pair of chromosomes. Bees have a haplodilpoid system in which male bees are from eggs that were never fertilized. IN certain reptiles and fish sex is controlled by environmental factors. In alligators, when eggs are incubated at 33oC they all become male. When eggs are incubated at a temperature significantly below 33o they become nearly all females. When the eggs are incubated above 33o it is a mix.

In humans some genes are sex-linked,  which are genes found on one sex chromosome but not the other. Sex-linked chromosomes are usually found on the X chromosome because it is much larger. A male is said to by hemizygous about a X-linked gene because it only has one copy.

          In 1910 the American geneticist Thomas Hunt Morgan first linked genetic traits and inheritance of a sex chromosome. He crossed a mutated white eye male with a normal red eye female. The F1 generation all had red eyes, indicating that red was dominant. The F1 generation were than mated to each other to produce the F2 generation. The F2 generation produced 1,011 red-eyed males, 782, white-eye males, and 2,459 red-eyed females. Because there were no white-eyed females Morgan realized their much be a connection between the certain alleles and the sex of the offspring. The mutant allele for white eyes is recessive and carried on the X chromosome.

          Their were many ways however where Mendel's principals were too basic and do not apply. Mendel studied seven characteristics that only seemed to affect one trait, color, height, pea shape, etc. However many single-gene disorders are pleiotropy, which means that a mutation in a single gene can have multiple effects on an individual’s phenotype. The expression of certain traits is also influenced sometimes by the sex of an individual. One example is pattern baldness. In males, it acts dominant but in females it acts recessive. the environment can also play a role in the creation of a phenotype in addition to the genotype, like talked about earlier. The norm of reaction has to be taken into consideration which is the range of phenotypes a certain genotype can exhibit.

 

B. Useful Materials

 

 

This video explains Morgan's experiment with flies that lead to the discovery of sex-linked traits.  Morgan, analyzing his results, hypothesized that while there is a gene for eye color located on the X chromosome, there is no corresponding gene on the Y chromosome.  Thus females with two X chromosomes receive two genes for eye color, whereas males only receive one.  Genes that are located on one gene but not the other are known as sex-linked genes. Many diseases are carried only on the X chromosome, making males as a much better risk.

Added: 2/20/11 Source: Youtube

 

 

This video explains codominance and incomplete dominance. This is an area I was weak in. There seemed to be a lot of questions in the homework about it and although I understood some of them others I know I got wrong. This video really helped me fully understand the topic. Codominance is when you can see both alleles showing their effects but not blending. Whereas in incomplete dominance you see both alleles blending their effects.

Added:2/20/11 Source: Youtube

 

Mapping of a novel locus associated with autosomal recessive congenital cataract to chromosome 8p.

This article is about the search for a locus associated with autosomal recessive congenital cataract. One of the things that was done in order to find the location of the locus was that blood samples were taken from everyone in the family.  An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop. They then created a pedigree. The pedigree helps to track the disease through the family. It helps to find out things about the disease by comparing different outcomes and crosses. They can help predict the probability of future offspring having the disease.a

Added: 2/20/11 Source: PubMed

Comments (1)

Derek Weber said

at 2:00 am on Apr 2, 2011

Great

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