IMAT 2018 Q25 [Phenotypes]

Which of the following could explain why the phenotype of a healthy female child may show
differences from her healthy biological brother (apart from the gender)?

  1. independent assortment after the formation of the female zygote
  2. crossing over between a chromosome from the sperm and a chromosome from the egg
  3. random alignment of homologous chromosomes at the equator of the cell during
    metaphase I.

A. 1 and 2 only
B. 1 only
C. 2 only
D. 2 and 3 only
E. 3 only

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For this, we need to take into consideration not only the events but the TIMING of the events. Remember that the exam contains tricks and things that, although may be partially true, can be completely false based on the combination of different facts. What do I mean by this? Let’s look at 1.

1. independent assortment after the formation of the female zygote

Independent assortment contributes greatly to genetic variability, so the first half of this answer appears to be correct, but when does independent assortment occur? It does not occur AFTER the formation of the female zygote, it occurs before. A zygote is the fusion of the male and female gametes. Independent assortment occurs during the random lining up of chromosomes during meiosis 1, which happens in the female and male gametes before fertilization occurs (a process happening in the body without the presence of the gametes of the opposite sex). Therefore we can rule out 1.

2. crossing over between a chromosome from the sperm and a chromosome from the egg.

Recall: Crossing over is a process that occurs during meiosis when homologous chromosomes are aligned and then there is an exchange of genetic information of the same corresponding genes between these chromosomes. It occurs during prophase 1 when the chromosomes are joining to form a tetrad.

This contributes to genetic variability because there is an exchange of genetic information between homologous chromosomes, but what about the TIMING? When does crossing over occur? We said it occurs during prophase 1 of meiosis, which is during the development of gametes within the mother or father. Crossing over does not occur after the fertilisation of the egg, it occurs before they ever come into contact. Therefore 2. is incorrect as crossing over does not occur between the chromosomes of the sperm and the egg.

3. random alignment of homologous chromosomes at the equator of the cell during metaphase I.

We have eliminated the first 2 answers, now we are left with only this option. Let’s break it down. Recall what happens at metaphase 1, the tetrads randomly align in the centre and the orientation can face either pole. This randomises the process and increases genetic variability because it is unknown where the chromosomes are going to end up. This process does indeed occur in metaphase 1, so the timing is correct. Since it does increase genetic variability and the timing is correctly stated, 3 must be correct.