Genetics and Society - Essay Example

Genetics The chromosomes are the basic genetic composed of a long strand of DNA and present inside the nucleus of the cell. Human cells contain 46 chromosomes in total in which half is inherited from each parent. Each cell contains 22 pairs of autosomes or non-sex determining chromosomes and one pair of sex chromosomes which determine the sex of the individual. Chromosomal pairs that are similar are called homologous chromosomes. The body cells or somatic cells carry all the 46 chromosomes and are called diploid cells while the germ cells that produce the sperm and the egg cells contain only half the number of chromosomes and are called haploid cells.

Diploid cells which are destined to produce the male and female reproductive cells undergo a type of cell division referred to as meiosis by which the chromosomes present in the parent cell is reduced to half to form four reproductive haploid cells called gametes. This process of separation and assortment of the homologous chromosomes occurs at random and the resulting haploid cells do not contain only paternal or maternal chromosomes but a mixture of genes from both parental chromosomes. This independent assortment of genes was first observed by Mendel when he was studying the genetics of pea plants and from his study he formulated the principle of independent assortment.

The process of meiosis which is responsible for the production of gametes occurs via this principle of independent assortment. Meiosis begins with the replication of a pair of chromosomes in the diploid germ cells which are destined to become male or female gametes. The replicated chromosomal pairs are called sister chromatids which are initially joined at the central point called the centromere. As the meiotic spindle forms at each pole of the cell with the help of microtubules, the two pairs of homologous chromosomes form tetrads within which any two chromosomes can overlap or recombine to form recombinant DNA which gives rise to new genetic combinations.

This process is called crossing-over. Following this the pair of chromatids are pulled towards each pole of the cell and the chromosomal pair is enclosed within a nuclei. The cell then undergoes cytokinesis during which the cytoplasm divides into two giving rise to two daughter cells each containing a haploid chromosome. Thus meiosis is an important process during which pairs of homologous chromosomes are independently assorted to give rise to haploid gametes and also the chromosomes in the tetrad stage recombine to form new combination of genes which is responsible for differences in the genetic make-up of humans.

Variant forms of a gene can exist in the same genetic loci within the chromosome and these are referred to as alleles. In case of humans two alleles of a gene exist within a given locus with one allele inherited from the father and one from the mother. The genotype of a specific gene is represented by each pair of alleles. If the alleles are identical the genotype is referred to as homozygous and in case of non-identical alleles they are called heterozygous. Alleles are also responsible for the organism’s outward appearance or the phenotype.

Autosomes are the somatic chromosomes which are present in 22 pairs of genes and these are responsible for the body or somatic characteristics of the individual. The presence of one dominant mutant allele within a genetic loci results in the expression of that allele while the other allele remains unexpressed or recessive. The recessive trait is expressed only when both the alleles are identical or homozygous. In case of true-breeding organisms the alleles are identical or homozygous for a particular trait which is always passed on to the offspring.

When the genotype or the alleles are heterozygous they cannot be definitely passed on to the offspring. In his experiments Mendel used true-breeding plants that showed variation for a particular character. When he crossed two heterozygous pea plants that carried two different alleles for a given trait he observed that the offspring did not always match the traits of the parents. New traits were also observed in addition to the expression of the parental traits which proves that segregation of the genes had occurred during the formation of daughter cells.

The monohybrid cross is carried out for one character which has different traits. The mating between the alleles from both parents can be represented in a Punnett square in which the dominant allele is expressed in the uppercase and the recessive allele in the lowercase. The alleles from each parent are given in bold. R rR RR Rrr rR rrA test cross is done between a homozygous recessive genotype and an unknown genotype for the same character. This cross will show whether the unknown genotype is homozygous or heterozygous for that particular trait.

Works citedHunter, Jill and Mitchell, Rita. “Genetics and Inheritence” Dartmouth.edu. n.d. Web. 2 March 2014. https://www.dartmouth.edu/~cbbc/courses/bio4/bio4-1997/01-Genetics.html