Quiz 5: Genetics: a Review

The relationship between the homologous chromosomes, copies of a gene and alleles is as follows: Each organism has two pairs of chromosomes. Each chromosomal pair consists of two chromosomes. These two chromosomes are known as homologous chromosomes. Each member of a pair is called a homolog. One homolog comes from mother and the other homolog comes from the father. As there are two sets of chromosomes [2n] the other set is a copy. Each cell has two copies of each gene on each of the homologous chromosomes. If an alternative form of the gene for the same trait is present, it is known as an allele. Alleles are alternative forms of a gene for the same trait. Only one allele can occupy a chromosomal locus at a time. Alternative alleles for the locus may be on homologous chromosomes of a single individual, making the individual heterozygous for a given gene.

Meiosis in eukaryotic animals occurs in ovaries in females and testes in males to produce haploid eggs and sperms. At the end of meiosis, four daughter cells with haploid set of chromosomes are produced from the parent cells. Meiosis in eukaryotes is characterized by two divisions: • Meiosis I • Meiosis II Meiosis I: It is divided into the following sub-stages: • Prophase I • Metaphase I • Anaphase I • Telophase I (a) Prophase I - It is very elaborate and longer than prophase of mitosis. It is divided into the following five stages: 1. Leptotene - Chromosomes condense. 2. Zygotene - Synapsis occurs by the formation of synaptonemal complex. Synapsis is the pairing of two homologous chromosomes. DNA strands of non-sister chromatids begin the process of recombination. 3. Pachytene - Synapsis is complete. Non-sister chromatids of homologous chromosomes randomly exchange segments. This is called chromosomal crossover. 4. Diplotene - DNA recombination is complete. Synaptonemal complex begins to break. Chromatids start to pull apart. 5. Diakinesis - It is the final stage of the prophase in meiosis characterized by shortening and thickening of the paired chromosomes, formation of spindle fibers, and disappearance of the nucleolus and degeneration of the nuclear membrane. Chiasmata are clearly visible. (b) Metaphase I - Bivalents start to lineup on the equatorial plate in the middle of the meiotic spindle. The chromosomes are attached to the spindle by their centromeres where centromere of one homolog is towards one pole while the centromere of the other homolog is towards the other pole. (c) Anaphase I - It refers to the point at which homologous chromosomes pairs separate and move to opposite poles. Cell elongation starts. (d) Telophase I: The chromosomes arrive at the poles of the cell. The microtubules disappear. A new nuclear membrane surrounds each haploid set of chromosomes. Chromosomes turn into chromatin. Cytokinesis occurs, creating 2 new cells. Sister chromatids remain attached to each other. Meiosis II - This process consists of four stages: • Prophase II • Metaphase II • Anaphase II • Telophase II 1. Prophase II: The disappearance of the nucleoli occurs. The chromatids start to thicken and shorten once again. The Centrioles move to the opposite poles and spindle fibers are ready for another cell division. 2. Metaphase II: The centromeres contain 2 Kinetochores that attach to spindle fibers. The equatorial metaphase plate formed is rotated in meiosis II. 3. Anaphase II: In this stage the centromeres are cleaved. This step allows the microtubules attached to the Kinetochores to pull apart the sister chromatids. The sister chromatids are pulled towards opposite poles of the cell. 4. Telophase II: The chromosomes uncoil and lengthen to become chromatin. The spindle apparatus disappears. Four cells are formed with the formation of nuclear membrane and cytoplasmic membrane.

Males in humans have XY chromosome, while females have XX chromosomes. Hence this type of sex determination is XX-XY type. Butterflies: Males have chromosomes called ZZ and females have a single Z chromosome. Hence the genotype of females is ZO. So this type of sex determination is called ZZ-ZO type.