Question

Graded for correctness: In humans, the ability to digest lactose beyond childhood is determined by a single gene on chromosome 1. L denotes the allele that gives the ability to digest lactose and l denotes the inability to digest lactose. On chromosome 3 is the gene for widows peak. A denotes the allele for no widows peak and a denotes a widows peak. A woman volunteers to be a participant in a genetic research study. Her genotype is LlAa. A doctor harvests a single egg from her body. The genotype of her egg is LA. How did her chromosomes line up at the metaphase plate during meiosis

Answer 1

Answer:

Metaphase I:    

• Homologous chromosomes are placed in the equatorial plane
• Chromosomes carrying the dominant alleles, L and A, face one of the poles
• The homologous chromosomes, carrying the recessive alleles, l and a, face the opposite pole.

Metaphase II:  

• Chromosomes carrying the dominant alleles, L and A, are placed in the equatorial plane
• One of the chromatid sisters of each chromosome faces one of the poles
• The other chromatid sisters of each chromosome face the opposite pole.

You will find the image in the attached files.

Explanation:

During metaphase I, homologous pairs migrate to the equatorial plane. 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. 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.

During metaphase II, fibers of the spindle apparatus take chromosomes toward the equatorial cell plane, where they line up. Sister chromatids are holden together until they reach the Anaphase, during which specialized enzymes break the bonds between chromatids and separate them. Each chromatid migrates to one of the poles. In telophase, the new chromosomes are already in the corresponding poles, and the nuclear membrane forms again. Finally, cytokinesis occurs.

In this example, we will assume no crossing-over in the prophase. I will propose the two metaphase stages.

Metaphase I:                                   Pole 1

        Chromosome 1   ---------L----                -----------A---------    Chromosome 3

                                    ----------L----               -----------A---------

Equatorial plane.....................................................................................................  

        Chromosome 1   ---------l----                  -----------a---------    Chromosome 3

                                     ---------l----                  -----------a---------                      

                                                           Pole 2

In this scheme of Metaphase I, homologous chromosomes are already aligned in the equatorial plane. Each homologous chromosome is facing a pole. So, in the superior part of the scheme, we have chromosomes 1 and 2 carrying the dominant alleles L and A. Both chromosomes are facing pole 1. Then, we can recognize the equatorial plane, and on the other side, we find the homologous chromosomes 1 and 2, facing pole 2, and carrying the recessive alleles, l and a.

During anaphase I, homologous chromosomes will separate and migrate to different poles. In this example, we are interested in chromosomes carrying the dominant alleles that migrate to pole 1. LL and AA.

Metaphase II:                                 Pole 1

        Chromatid 1   ---------L----                    -----------A--------  Chromatid 3

Equatorial plane.....................................................................................................  

        Chromatid 1   ----------L----                   -----------A---------  Chromatid 3

                                                         Pole 2

During metaphase II, each chromatid sister carrying the dominant alleles faces a different pole. During anaphase II they separate and migrate again.

The total result of meiosis in this particular cell is the formation of 4 haploid cells -gametes-: LA, LA, la, la