Question

A cell has two pairs of submetacentric chromosomes, which we will call chromosomes Ia, Ib, IIa, and IIb (chromosomes Ia and Ib are homologs, and chromosomes IIa and IIb are homologs). Allele M is located on the long arm of chromosome Ia, and allele m is located at the same position on chromosome Ib. Allele P is located on the short arm of chromosome Ia, and allele p is located at the same position on chromosome Ib. Allele R is located on chromosome IIa and allele r is located at the same position on chromosome Ib. Allele G is located on the long arm (q arm) of chromosome Ia, and allele g is located at the same position on chromosome Ib. Allele N is located on the q arm of chromosome IIa and allele n is located at the same position of chromosome IIb.
Required:
a. Draw these chromosomes, identifying genes R, r, G, g, N, and n as they might appear in metaphase I of meiosis. Assume there is no crossing over.
b. Taking independent assortment and the subsequent random separation of chromosomes in anaphase I, draw the chromosomes, with genes identified, present in all possible gametes that could result once this cell completes meiosis. As above, assume there is no crossing over. Explain in your own words what is meant by independent assortment.

Answer 1

Answer:

Note: You will find the chromosomes and the gametes in the attached files

Chromosomes:

• IA    ----------G-----о---------
• IB     -----------g-----о----------

• IIA    --------N--------------о------R----
• IIB   ---------n--------------о-------r----      

Gametes:

• Gamete 1:  GNR          
• Gamete 2: Gnr            
• Gamete 3: gNR
• Gamete 4: gnr

Independent assortment occurs when the alleles of two or more different genes distribute in the gametes independently from each other.

Explanation:

Available data:

two pairs of submetacentric chromosomes

IA     -------------------о-----------

IB     -------------------о-----------

IIA    ----------- ------------о------------

IIB    ------------------------о------------

chromosomes Ia and Ib are homologs

IA     -------------------о-----------

IB     -------------------о-----------

chromosomes IIa and IIb are homologs

IIA    -----------------------о------------

IIB    ------------------------о------------

Allele M is located on the long arm of chromosome Ia, and allele m is located at the same position on chromosome Ib

IA     -------M----------о-----------

IB     -------m----------о-----------

Allele P is located on the short arm of chromosome Ia, and allele p is located at the same position on chromosome Ib.

IA     -------M----------о------P---

IB     -------m----------о------p----

Allele R is located on chromosome IIa and allele r is located at the same position on chromosome IIb.

IIA    ------------------------о------R----

IIB    ------------------------о-------r----

Allele G is located on the long arm (q arm) of chromosome Ia, and allele g is located at the same position on chromosome Ib.

IA     -------M---G-----о------P---

IB     -------m----g-----о------p----

Allele N is located on the q arm of chromosome IIa and allele n is located at the same position of chromosome IIb.

IIA    --------N--------------о------R----

IIB    ---------n--------------о-------r----

A). Draw these chromosomes, identifying genes R, r, G, g, N, and n as they might appear in metaphase I of meiosis. Assume there is no crossing over.

IA     ----------G-----о---------

IB     -----------g-----о----------

IIA    --------N--------------о------R----

IIB    ---------n--------------о-------r----

During metaphase I, homologous pairs together migrate to the equatorial plane, where they randomly aline with their kinetochores facing opposite poles. The random arrangement of tetrads is different in every cell going through the meiosis process. There is no equal alinement between two cells. When tetrads aline in the equatorial plane, there is no predetermined order for each of the homologous chromosomes of each tetrad to face one of the poles and then migrate to it while separating. Any chromosome of the homologous pair might face any of the poles and then migrate to it. Each of the chromosomes has two possibilities for orientation at the plane. When the new haploid cells are formed, the number of variations in each cell is also different and depends on the chromosomes that form that cell. This random order in the equatorial plane is what introduces variation into the gametes. It is almost impossible that two gametes resulting from meiosis will get the same genetic charge.

B). Taking independent assortment and the subsequent random separation of chromosomes in anaphase I, draw the chromosomes, with genes identified, present in all possible gametes that could result once this cell completes meiosis. As above, assume there is no crossing over. Explain in your own words what is meant by independent assortment.

Independent assortment occurs when the alleles of two or more different genes distribute in the gametes independently from each other. In other words, a gamete receives an allele from a gene that does not depend on nor influence the allele of another gene in the same gamete. This can only be applied to independent genes. An organism gets only one of the members of the chromatids pair of each parent.

During gamete formation in meiosis, the pairs of homologous chromosomes separate after crossing over, and then chromatids sisters also separate, resulting in the formation of four gametes. Each gamete has different information from the combination of homologous chromosomes belonging to the father and to the mother.

There are four possible combinations, considering that there is no crossing over.

• Gamete 1:   GNR
• Gamete 2: Gnr
• Gamete 3: gNR
• Gamete 4: gnr