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Mendelian Inheritance

Using the data generated from the fly lab, Compare all and match brown (eyecolour) gene to two its closest genes.

To fully understand genetics we need to know the various terminologies found in the unit. All living things have genes. They are the specific information encoded for a particular trait. Genes are located on the chromosomes. Each gene occupies a specific location known as Loci. Human beings have a total of twenty-three chromosomes. The first twenty-two is located in the somatic cells. They are known as autosomes. They are diploid in number. The twenty-third chromosome is found in the gametes. They are known as sex chromosomes. They are responsible for sex determination. They are haploid in number.

The outward appearance of an organism is known as its phenotype. The genetic composition of an organism is known as its genotype. Each individual organism has a specific genotype and phenotype throughout its lifetime. The specific form of a gene for studying genetics is known as alleles. For each trait, an organism has two forms of the allele. If the alleles are the same type then the organism is said to be homozygous. If the alleles are different types the organism is said to be heterozygous (Orel, 1984). There are two different types of genes, dominant gene, and recessive genes. Dominant genes are genes that are expressed irrespective of being homozygous or heterozygous. Recessive genes are genes whose effect is sometimes masked and not expressed. These genes are only expressed in the heterozygous state. The character of a gene is determined by the nature of the gene or the allele. A trait is a particular form of character. For example, a character could be eye color and the trait is the different forms of color, for example, white or red color (Orel, 1984).


Mendel, the father of genetics, developed a means to study genes through monohybrid crosses, dihybrid crosses, trihybrid cross and the use of a chi square. Mendel also discovered that a gene separates individually during a cross such that the offspring will have one gene from each parent. This is the law of segregation. He also discovered that a gene is expressed independently of other factors surrounding it. This is the law of independent assortment.  The law of probability states that the probability of simultaneous occurrence of two independent events equals to the product of the probability of their separate occurrence (Stern, 1966).

Experiments and Assignments

There are different methods of mapping. This includes the use of Mendel's cross, use of a punnet square and use of a chi-square. Mendel's cross is used in determining the monohybrid inheritance. They are simple and can the results are accurate. A punnet square is used for obtaining dihybrid inheritance. The punnet square drawn and using simple mathematics results of the offspring is obtained. The chi-square considers the possibility of any deviation of a ratio and the size of a given sample and expressed this data as a single value

  • To understand fully the relationship between the organisms phenotype and its genotype.
  • To be able to show monohybrid inheritance and dihybrid inheritance.
  • To use genetic crosses to locate the position of a gene.
  • To show the use of statistical methods to approve or disapprove a hypothesis
  • To stimulate practically the Mendel inheritance

Assignment 1: Monohybrid, Dihybrid, and trihybrid inheritance.

Monohybrid inheritance is an inheritance performed involving one gene loci. Two parents with particular traits are crossed and the result obtains results in offspring with certain traits. It is performed using Mendel's cross. A dihybrid cross refers to an inheritance involving two gene loci. The results are obtained using a punnet square. Dihybrid inheritance results in complete dominance, incomplete dominance, and codominance (Soudek, 1984).

In this experiment, cross a wild type female with a male with white eyes. Observe the F1 and F2 generation.

The results for the monohybrid inheritance after crossing the homozygous wild type red eye and the homozygous white eyes is that the F generation has red eyes. This means that the gene for red eyes is dominant over the gene for white eyes. The F2 generation obtained showed that the offspring that both red eyes and white eyes in the ratio of 3:1. The genotype however varied in the ratio of 1:2:1. This further proved that the gene for red eyes is dominant over the gene for white eyes. The results of the dihybrid inheritance are that the F1 generation is heterozygous for each of the two pairs of a gene. This is because both parents were homozygous. Selfing the F1 generation resulted in the F2 generation. The phenotypic ratio was 9:3;3:1. This ratio showed complete dominance. If there was incomplete dominance the phenotypic ratio would be 3:6:3:1:2:1. Co-dominance is when the crossing of two traits results in a completely new trait that is not found in either of the parents. In incomplete dominance, the phenotype is an intermediate of the two traits (Soudek, 1984). None of the alleles is expressed

This is a cross used to determine the genotype of an unknown crossing. In this type of cross, an individual of dominant phenotype is crossed with another of unknown genotype. It is used to determine whether an individual is heterozygous or homozygous for a particular character.

Conclusion

In this experiment, cross a female with brown eye color and a male with ebony body color. Note the results. Also, cross a male with brown eye color and ebony body color with a female that is wild type. Observe and record the data obtained.

The results show that if the unknown is homozygous the offspring have the same phenotype. If the unknown is heterozygous the resulting offspring have a phenotypic ratio of 1:2:1

These are genes which when expressed the effects kill the organism. The genes may be dominant or recessive. The organism hardly reaches maturity. Lethal genes result in the death of an offspring thus reducing the number of offspring drastically. The new phenotypic ratio is 2:1.

In this experiment, design a cross between two flies with asterapedia mutation for antennae. Observe and record. Set up a control using a fruit fly that is wild type and another that has asterapedia mutations for the antennae. This will help prove that the lethal gene is recessive in nature.

The experiment shows that the lethal gene is recessive in the fruit fly. The gene results in death. However, the offspring survive if the gene is heterozygous hence it is not expressed by the individual. The offspring can live a normal life. The phenotypic ratio of the experiment was 2:1. One offspring were homozygous for the lethal gene (Russell, 2006).

The effect of one gene is masked or not expressed due to the presence of another non-allelic gene. This is against Mendel's law of independent assortment. The gene may be affected due to the physical environment.

In this experiment, design a cross with a female fruit fly that has vestigial wing size and a male fruit fly that has incomplete wing vein mutation. Observe and note down the results. Also set up another cross with a female with apterous wing size mutation and a male fruit fly with radius incomplete wing vein mutation (Russell, 2006). Observe and compare the two crosses. Note down the results.

The result shows that one gene was not expressed due to the presence of a known allelic gene. The results are obtained using a punnet square. The phenotypic ratio is 9:3:4

An organism may be monoecious or dioecious. A monoecious is an organism which produces both the male and the female gametes in the same organism. A dioecious is an organism which the male and female gametes are found in different organisms. There are various sex systems. In the fruit fly, the have an XX-XY system the females are XX and the males are XY. This is similar to human beings and most mammals. Some gene may be carried on the sex chromosomes. This means that the genes will be inherited together with the genes for sex.

In this experiment, design a cross for a female fruit fly with tan body and wild type male. Observe and note down the results obtained. Design another experiment between a female fruit fly with vestigial wing size mutation and a white eye type male fruit fly. Observe and record the results obtained.

The result shows that some traits are found in females only and others in males only. Irrespective of whether the gene is dominant or recessive it will be expressed in the offspring. The result can be obtained using a simple Mendel’s cross.

During fertilization, the gene may undergo mutation and be recombined.

In this experiment, design a cross for a female with an eyeless mutation for eye shape and a male with shaven bristles. Observe the results and note down

The result shows that the genes underwent recombinatio

Conclusion

In conclusion, the various methods of genetic mapping yielded the same results. We use of Mendel's cross, use of punnet square or the chi-square depending on what results are needed. If statistical data is needed then a chi-square is used. We have also seen that two organisms can have the same phenotype but different genotype

Genetic mapping has proved to be very important in today’s  society. In the field of medicine genetic mapping has been used to discover drugs that can treat diseases that were once incurable. This has greatly advanced the field of medicine. Genetic mapping has also been used to understand the root causes of diseases, for example, the sickle cell. This has helped to reduce the spread of the disease and has created awareness of particular environmental factors leading to diseases.

Genetic mapping has also been used in developing strains of bacteria that can be used to generate bio fuel. Bio fuel is cheaper and easier to use. It is also environmentally friendly. The knowledge of genetic mapping is used by sewage companies and garbage disposal companies by using bacteria to degrade the waste resulting in an environmentally friendly of reducing the toxicity of waste. This is also a method of proper disposal of waste.

In agriculture, genetic mapping has been used in developing new crops that are resistant to drought, pest, diseases and so much more. The new crops yield higher produce that is of a much more higher quality than regular crops. The technique has also been applied in livestock resulting in much more healthier livestock. The livestock produces better produce in terms of quality and quantity. This has greatly improved the field of agriculture.

Genetic mapping has also been used in forensics. Today paternity test can be performed much more quickly and more efficiently due to the use of genetic mapping. It has also helped in identifying victims of burns. Sometime during a fire, some victims may be burned beyond recognition. Genetic mapping is used in identifying who the victims are and where are from.

There were no other experiments performed at the moment. Researchers are still trying to understand Mendel’s inheritance

In the fruit fly, primary on disjunction could occur. The gene responsible for the red eye color is found on the X chromosome hence it is a sex linked gene. The recessive gene for white eyes is found also on the X chromosome. white eyes are eyes with a bright red color and an ocellus that is colorless. A white female crossed with a red eye male will result in the F1 generation having red eyed females and white eyed males. This shows that the red eye is dominant.

However, in some rare cases due to nondisjunction, this was not the case.  Nondisjunction is the failure of the x chromosome to separate during fertilization. This results in an organism having an extra X chromosome or no X chromosome at all. Symbolizing X+ for red eyes and Xv for white eyes, and each of the autosomes as A. a cross between a white eyed female, AAXvXv and a red eye male AAX+Y, the result is as follows;

AAXvX+ - red female

AAXvY- white male

AAXvXvX+- metafemale, rare dies

AAXvXvY- white female (rare)

AAX+O- sterile red male, rare

AAYO- die early, rare

The metafemale is weak and survives beyond the pupal stage. In the fruit fly, the sex is determined by the ratio of X chromosomes to autosome cells. If there are more autosomes that X chromosomes the individual is male. If the X chromosomes are more or equal to autosomes the individual is female. The Y chromosomes have no function in gender and it only plays a role in fertility.

References

  1. Orel V. New york: oxford university press 1984
  2. Stern C and Sherwood E.R. the Origin Of Genetics. A Mendel source book. San Francisco: Freeman 1966
  3. Soudek D. ‘Gregor Mendel and the people around him.’ Am J. Hum Genet. May 1984
  4. Russell P.J iGenetic. A molecular approach San Francisco: Benjamin Cunnings. 2006
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