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Typed answer preferred :) (please answer all, not partially. If you do not know theses all, Please don\'t answer)

1.Define the following terms: gene, allele, phenotype, genotype, law of segregation, and law of independent assortment.

2.How do complete dominance, incomplete dominance, and codominance differ? Give human examples of each.

3.Describe what is meant by sex-linked inheritance.

Solution

Gene: Gene is a region of DNA which is composed of three parts (eukaryotic) two regulatory sequence and a open reading frame. Open reading frame is the middile portion and both end of this frame is attached with regulatory sequence. The regulatory sequence contains enhancer, promoter, starter and terminator sequence region. Open reading frame contains transcribing sequence. The regulatory sequence regulate the process of protein synthesis while transcribing portion forms m-RNA. Hence gene is the unit of heridity.

Allele: A pair of characters which is controlled by a single gene. For example colour of eye that is brown and black. The gene which produces black colour of eye, can produce brown colour after mutation.

Genotype and Phenotype: The genotype is the set of genes in our DNA which is responsible for a particular trait. The phenotype is the physical expression, or characteristics, of that trait. For example, two organisms that have even the minutest difference in their genes are said to have different genotypes. These two mice may have different genotypes and both can still be white if they share the particular phenotype for white fur.

Law of segregation: States that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. According to this law:

Example: The gene for seed color in pea plants exists in two forms. There is one form or allele for yellow seed color (Y) and another for green seed color (y). In this example, the allele for yellow seed color is dominant and the allele for green seed color is recessive. When the alleles of a pair are different (heterozygous), the dominant allele trait is expressed and the recessive allele trait is masked. Seeds with the genotype of (YY) or (Yy) are yellow, while seeds that are (yy) are green.

Law of independent assortment: This law was given by Mendel and according to this law, the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.

2. Dominance is the relationship between different alleles at the same locus on homologous chromosomes, in terms of how those alleles interact to produce a phenotype. In general, a dominant allele is an allele that only needs one copy of itself in order to be expressed in the phenotype. Meanwhile, a recessive allele is only expressed in an organism that carries two copies of itself (i.e. when the allele is homozygous).

Co-dominance is the property where two different alleles are both fully expressed in the phenotype, in spite of the other allele\'s presence. So, for example, in an individual heterozygous for blood group - genotype I(A)I(B) - there is no dominant or recessive allele because both alleles express themselves fully in the phenotype to provide A and B antigens, and give an AB blood group.

Sometimes there may be a phenotype in the heterozygote that is intermediate between those of the homozygotes. For example, a red snapdragon flower and white snapdragon flower may be crossed to give one that is pink. This is called incomplete dominance because neither allele is completely dominating the phenotypic expression of the other; there is a \'compromise\', almost. This is different from co-dominance, because in co-dominance the two alleles have full phenotypic expression, while in incomplete dominance neither allele is being fully expressed in the phenotype; they are both \'mixing\' expression to give a different phenotype.

3. Sex-linked inheritance: Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia. Such genes which are always associated with sex chromosomes are called sex-linked genes.

Characteristics of Sex Linked Inheritance:

- It is a criss-cross inheritance as the father passes its sex-linked character to his daughter who in turn passes it to the grandson.

- Daughter does not express the recessive trait but act as carrier in the heterozygous condition.

- Female homozygous for recessive trait expresses the trait.

- Any recessive gene borne by the X chromosome of male is immediately expressed as Y chromosome has no allele to counteract.