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Basic Concepts of Genetic Inheritance

Discuss about the Case Study of Genetic Inheritance for Managing Host Genes.

The Gene is the basic functional and physical unit of heredity. Genes are composed of DNA which acts as an instruction in the production process of different proteins. The human gene varies in size it can be composed of a few hundred DNA bases or it can be made up of more than two million DNA bases. It has been estimated in the Human Genome Project that humans have 20,000 to 25,000 genes present in them. In every individual, the gene is inherited from his or her parents (Raff 2012).

The Alleles are the forms of a same gene with a very minor or little difference in the sequences of their DNA bases. This minor difference in the alleles of the individual person contributes to every individual person’s unique physical features. The alleles determine different traits that can be transferred from the parents to their offspring through sexual reproduction (Crews et al. 2012).

The organism which have identical pairs of alleles or genes for a particular trait gene each one is inherited from each parent. When both the two gametes fuse together during fertilization and they carries similar form of the gene for a particular trait. Then the organism forming due to this fusion is said to be homozygous for that trait. If the homozygote organism is dominant homozygote then the dominant characteristic will be expressed and if it is recessive homozygote then the recessive characteristics will be expressed (Raff 2012).

The organism that has two different forms of a particular gene, each one inherited from each parents. The organism having heterozygote genotype has relatively higher fitness than the homozygous organism. If the selection is made by favoring the heterozygote, polymorphism can be maintained and this mechanism can be used to explain the occurrence of several genetic variability. In a heterozygote individual the only the phenotypic features of the dominant trait is expressed and the phenotypic characteristics of the recessive trait remain suppressed (Kaelin et al. 2012).

In genetics, there are two kinds of trait the dominant trait and the recessive trait. A dominant trait is that trait which is dominant or can be seen in the phenotypic features of the offspring that has a heterozygote genotype. The dominant trait expresses its phenotypic characteristics and suppresses the phenotypic features of the recessive trait (Delgado et al. 2012).

Family Tree Diagram of Autosomal Recessive Pattern of Inheritance

In genetics, two terms are very common they are dominant trait and recessive trait. A trait is said to be recessive when the phenotypic features of that trait cannot be seen in the offspring having heterozygote genotype. The trait remains present in the genotype of the individual but does not gets expressed in the phenotypic characteristic of the organism (Delgado et al. 2012).

The complete heritable genetic identity of an individual is referred to as the genome of that individual. Sometime the genotype is also considered as the particular set genes carried by an individual. A little change or mutation in the genotype gives out drastic changes in the phenotypic characteristics of the individual (Lyons 2012).

The phenotype of an individual is referred to as the actual physical characteristics of that individual. These physical characteristics includes eye colour, height, some disease and even certain behavior of the individual. Most of the phenotypic characteristics are influenced by the genotype of an individual (Browning et al. 2012).

2.

family tree diagram

This is a family tree diagram that displays autosomal recessive pattern of inheritance in a family. In the diagram the circles represent the females and the square represent the males and the darken circle and square represent the individual who are affected by the trait. The digits represents the individual within the generation. A male and a female directly connected by a straight line states that they have mated and have children. The pairs which have mated in this tree are 1and 2, 3 and 4. The founding parents in this tree were female-1 and male-2 in the first generation at the top. They have two children female-4 and male-5 the vertical line connects the parents with the children. The founding parents of this family are the carrier of the trait and they have two child one girl and one boy. Both the children are carrier and when the girl of this family get married to a carried male-3 they got five children. Among these children, three are female and two are male.  The boy-6 and girl-8 are affected offspring whereas the girl-7 and girl-10 are unaffected and the boy-9 is carrier.

Both the foundation parents have one copy of affected gene so they were carrier of the trait and none of them will show any sign or symptoms of the disease. Likewise  their children both the girl-4 and boy-5 are also carrier and there will also be no observation of the symptoms and sign of the disease  in them. In the second generation as the girl-4 and the boy-5 have one copies of affected gene and when this girl-4 mated with a carrier male-3 both of them were not showing any symptom of the disease but in the third generation they got the children girl-7 and the girl-9 who don’t have any copies of  affected genes, so they are unaffected offspring, the boy-9 has one copy of affected gene and is considered affected offspring  but the boy-6 and girl-8 have both the copies of affected gene. Therefore they are affected with the trait and they will show the sing and symptom of the disease respective to the trait of the gene.

Punnett Square Example of Monohybrid and Dihybrid Crosses

The risk of having affected child is 25 percent when both the individuals in the couple are carrier. The chance of having child carrier is 50 percent and unaffected child is 25 percent when the parents are carrier.

3. a) Red hair (b) is recessive trait and black hair (B) is dominant trait. Therefore, when a woman has black hair, it can be said that, the genotype of the woman would be either homozygous dominant (BB) or heterozygous dominant (Bb). The woman marries a man with red hair. Therefore, the genotype of her husband is homozygous recessive (bb). Here, the condition says that, the woman is BB and the man is bb. Using a punnett square, the genotype of their children can be determined.

                         Male

Female

b

b

B

Bb

Bb

B

Bb

Bb

b) From the punnett square, it has been revealed that all the children of the above couple would have black hair and would carry heterozygous dominant trait for black hair. In the above genetic cross, the male parent has two recessive genes, whereas, the female parent is carrying two dominant genes. According the knowledge of genetic inheritance, the dominant genes always shed the character of recessive genes (Veltman & Brunner, 2012). Thus, if the offspring obtain a single dominant allele along with a recessive allele from other parent, the child would show the phenotypic character of black hair. In this cross, all the offspring obtained dominant allele from mother and recessive allele from father. Thus, all of them obtained the gene of heterozygous black hair. Therefore, 100% offspring would have phenotypic characteristics of black hair.

Dihybrid cross

Dihybrid cross

Here, a yellow (YY) and round (RR) seed plant has been crossed with a green (yy) and wrinkled (rr) seed plant. This is a dihybrid cross. Here the ‘R’ allele is dominant over ‘r’; whereas ‘Y’ is dominant over ‘y’ allele. Thus, in first generation, all the plants would have the heterozygous genotype YyRr, with a yellow round seed phenotypic characteristic. When two offspring of the first generation would be fertilized, from the punnett square, offspring in 9 (yellow round):3 (yellow wrinkled):3 (green round):1 (green wrinkled) ratio has been found.  

4. a) The female cat has two alleles for black coat color. On the other hand, the male cat has one allele for ginger coat color. It is because, the male cat has single X chromosome and the coat color is determined by the gene in X chromosome. The female has a coat color for homozygous black (X B X B ) and the male has a gene for ginger coat color (X G Y). Therefore, with the help of the following punnett square, the possible coat color of their offspring can be determined.

                         Male

Female

X G

Y

X B

X B X G

X B Y

X B

X B X G

X B Y

Both the coat colors, black and ginger are dominant, thus none of the phenotypic characteristic would shed the other’s characteristics. As a result, a mixed variant of offspring are found, who have the heterozygous genotype (X B X G ), which is known as the co-dominance. Therefore, from the above punnett square, it has been revealed that, 50% offspring would be female and 50% would be male. All the females would be tortoiseshell female, which are the showing co-dominance; whereas all the male offspring would be pure black.

b) Here, one offspring tortoiseshell female (X B X G) has been crossed with an unknown male. Within the offsprings, there is only one ginger male.

                         Male

Female

X G

Y

X B

X B X G

X B Y

X G

X G X G

X G Y

From the above punnett square, it has been revealed that for obtaining single ginger female kitten, within four possible offspring, the male parent have to be ginger colored. Therefore, the genotype of the unknown male cat is X G Y. It is because, to get a ginger female offspring, the offspring should get two XG alleles, within which one is coming from female parent (X B X G); if the male is ginger (X G Y), then only the offspring would get the other  X G allele and become a homozygous ginger female. From the above punnett square, it can be said that, the coat color of cats is an X-linked dominant trait and the gene for female parent’s coat color determines the coat color of a male offspring.

5. a) The gene that causes hemophilia is embedded in X chromosome (Franchini & Mannucci, 2012). Thus, in case of female, they have two X chromosomes, if one is mutated for hemophilia, the other wild type allele can compensate the function of the mutated allele. Thus, with one mutated gene (X H), the female becomes heterozygous carrier, for being affected, two chromosomes have to be mutated. On the other hand, as males carry single X chromosome, mutation in single chromosome would show diseased phenotype.

Punnett Square Example of Codominance for Coat Color in Cats

Here, the condition states that, the female parent is the carrier of hemophilia and the male parent is unaffected. The following family tree can help to reveal the genotypes of children of the above couple.

Family tree of carrier female and non-hemophilic male

Figure: Family tree of carrier female and non-hemophilic male

(Source: Dunlap et al. 2012)

From the above family tree, the following results have been revealed:

Hemophilic son = 25 % ; genotype = XHY

Non-hemophilic son = 25 %; genotype = XY

Total male offspring = 50 %

Carrier daughter = 25 %

Non-hemophilic daughter = 25 %

Total female offspring = 50 %

b) As discussed above, mutation of the gene, which causes hemophilia is embedded in the X chromosome. Female has two X chromosome, but male has a single X chromosome. Thus, when a carrier female is being met with a healthy male, the defective gene is present in a single gene in one of the female gamet, whereas the others are wild type. Thus, when the offspring is female, she will get one defective or one wild type X chromosome, along with a wild type X chromosome from the male parent. Thus, the female offspring would be either carrier (XHX) or non-hemophilic (XX) (Klug 2012). In contrast, when the offspring is male, the child will get a Y chromosome from the male parent and one X chromosome from the female parent. As the mother has two X chromosome, there are two genotypic possibilities of the male offspring. The female parent is carrier, thus, she has one wild type X chromosome and a defective X chromosome. Now, there are two genotypic possibilities of a male offspring; either a Y from father and XH from mother or a Y from father and a wild type X from mother. In the first case, the son will be a hemophilic son, because there is no wild type X chromosome to compensate the function of the defective gene. In the second case, the son will be healthy (XY). This is why the male offspring would have 50 % chance of being hemophilic.

6. a) The small differences which exist between the individuals is called as variation. This variation can be of two types continuous heritable variation and discontinuous heritable variation.

Continuous heritable variation is also known as the polygenic inheritance. It  is the  combined effect  of many genes and also sometime get significantly affected by the environmental factors like the milk production in cows it is not only determined by the genotype of the animal but it is also significantly affected by the environmental factors like weather, quality of their diet, and the comfort available to them. A complete range of measurement that extends from one extreme to the other extreme is always present in continuous variations. The best examples of continuous variation are the height, shoe size, and weight of humans (Mode and Sleeman 2012). All these things can vary from generation to generation.

Alleles of a small number of genes or a single gene is responsible for the discontinuous heritable variation. The effect of environment over discontinuous heritable disorder is a very less. The features of discontinuous heritable variation cannot be measured across a complete range. These are the characteristics that you either have or you don’t have. Blood group is the best example of Discontinuous Heritable variation because an individual can be only of one blood group no one can posses two blood group (Relethford 2012).

These variation are caused by either environmental factor or by genetics or by combination of the both.  The example of genetically effected variation are down syndrome which is caused due to a trisomy in the chromosome number 21, and sickle cell anaemia which is caused when a base substitution occurs that is the replacement of one base by another. This only effect a single amino acid which normally remains silent but if an active site of an enzyme is affected then the effect is drastic (Brookfield 2012).

Many different factors contribute to obesity. Recent studies show that obesity is caused approximately 40 percent due to genetic and 60 percent due to the environmental factors.  Environmental factor like amount of food intake, types of food, amount of drinks consumed, and physical activity level have a vast effect on the obesity of an individual (Snustad and Simmons 2012).

The eye colour of an individual is determined by genetics. A specific region on the chromosome 15 plays the major role in formation of eye colour. OCA2 and HERC2 are the two genes that are responsible for the eye colour in humans (Avise 2012).

Tallness of individual person is fully dependent on genetics. There is only one third chances that a tall dominant couple have a dwarf child.

The ability of singing depends on both genetical and environmental factors. Though some musical understanding talent must be present in the individual but still to make someone proficient in singing, the individual should practice very hard to reach that position (Tollefsbol 2012).

Maleness:

Maleness is fully dependent on gene of the individual.

Masculinity:

Masculinity of an individual depends on genetics

Blood Group:

Blood Group is dependent fully on gene of the individual.  A particular individual should have a single blood group it cannot posses two blood group. The blood group also gets transmitted to the off springs.

Natural hair colour:

There are two types pigment present that gives hair colour. These are pheomelanin and eumelanin. However, the genetics of hair colour is still not firmly established. Some theory states that two pairs of genes are responsible for the colour of the human hair.

Sickle cell anemia:

Sickle cell anemia occurs due to mutation in the gene. This states that sickle cell anemia is caused by genetic variation.

Agility:

Agility is the ability of the body to change the position of the body efficiently. This condition can be achieved by regular practice and is not related to any genetic variation. it is dependent totally upon the environmental factors (Temelkova-Kurktschiev and Stefanov 2012).

Reference:

Browning, J. A., Simons, M. D., & Torres, E. 2012. Managing host genes: epidemiologic and genetic concepts. Plant Disease: An Advanced Treatise: How Disease Is Managed, 191.

Crews, D., Gillette, R., Scarpino, S.V., Manikkam, M., Savenkova, M.I. and Skinner, M.K., 2012. Epigenetic transgenerational inheritance of altered stress responses. Proceedings of the National Academy of Sciences,109(23), pp.9143-9148.

Delgado, M. M., Munera, J. D., and Reevy, G. M. 2012. Human perceptions of coat color as an indicator of domestic cat personality. Anthrozoös, 25(4), 427-440.

Dunlap, J., Goodwin, S. and Friedmann, T., 2012. Advances in genetics. Amsterdam: Elsevier.

Franchini, M., & Mannucci, P. M. (2012). Past, present and future of hemophilia: a narrative review. Orphanet journal of rare diseases, 7(1), 1.

Kaelin, C. B., Xu, X., Hong, L. Z., David, V. A., McGowan, K. A., Schmidt-Küntzel, A., ...and Manuel, H. 2012. Specifying and sustaining pigmentation patterns in domestic and wild cats. Science, 337(6101), 1536-1541.

Klug, W., 2012. Concepts of genetics. San Francisco: Pearson Education.

Lyons, L. A. 2012. Genetic testing in domestic cats. Molecular and cellular probes, 26(6), 224-230.

Mode, C. and Sleeman, C., 2012. Stochastic processes in genetics and evolution. Hackensack, N.J.: World Scientific.

Raff, R.A., 2012. The shape of life: genes, development, and the evolution of animal form. University of Chicago Press.

Relethford, J., 2012. Human population genetics. Hoboken, N.J.: Wiley-Blackwell.

Snustad, D. and Simmons, M., 2012. Genetics. Singapore: Wiley.

Temelkova-Kurktschiev, T. and Stefanov, T., 2012. Lifestyle and genetics in obesity and type 2 diabetes. Experimental and clinical endocrinology & diabetes, 120(01), pp.1-6.

Tollefsbol, T., 2012. Epigenetics in human disease. Amsterdam: Elsevier/Academic Press.

Veltman, J. A., & Brunner, H. G. (2012). De novo mutations in human genetic disease. Nature Reviews Genetics, 13(8), 565-575.

Brookfield, J.F., 2012. Heritability. Current Biology, 22(7), pp.R217-R219.

Avise, J.C., 2012. Molecular markers, natural history and evolution. Springer Science & Business Media.

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