The Secrets of Life (5 of 5) - Crossover and Chromosome Inversion

An ordinary human has 46 chromosomes in each cell made up of 23 pairs; one half of the chromosomes are inherited from the mother and the other half from the father. In each chromosome, there are hundreds or thousands of genes. But to simplify our discussion, let's assume there are only four genes in a particular pair of chromosome; A, B, C, D in one copy inherited from the mother and A', B', C', D' in the other copy inherited from the father as shown in Figure 1. During the formation of the egg or the sperm, the chromosome pair separates so that the egg or sperm cell contains only one copy of the chromosome. If this egg or sperm combines with a sperm or egg of a partner at time of conception, an embryo of a child will be formed with a full pair of chromosomes. As we can see in this way, genes from only the mother or the father will be inherited to the child (i. e. either the chromosome with genes A, B, C, D or the chromosome with genes A', B', C', D' will be passed down to the child). So there is a possibility that a child inherits characteristics from the mother or the father only, and not from both parents.
Nature plays a wonderful role in this regard. In practice, before the pair of chromosome separates, the two legs of the chromosome line up very closely with each other. Genes in one leg of the chromosome may exchange with the corresponding genes in the other leg of the chromosome (e.g. genes C, D may swap with genes C', D' as in Figure 2). The chromosomes then separate to form the egg or sperm. This process is called crossover which enables the egg or sperm to carry genes from both the mother and the father before conception, and hence enables the child to inherit genes (and hence characteristics) from both parents. With similar reasoning, we can see that a child can inherit genes from all his grandparents, all his great grandparents and great great grandparents back in many generations. Thus, with crossover, a child is able to inherit a little bit of each from any of his ancestors. The exact mechanism of crossover (i.e. when and how crossover will take place and which genes will exchange etc.) is not known. Hence, the exchange of genes is considered at random.

An individual with chromosome inversion has a normal chromosome and a chromosome with some genes inverted as shown in Figure 3. The individual is completely normal as he/she has no extra or missing genes. It does not really matter which order the genes are as long as they are all present. During the formation of the egg or sperm, the two chromosomes line up very closely with each other, and crossover of genes may take place.
If the crossover occurs not in the inverted region (e.g. genes A, D swap with A', D' as in Figure 4) before the chromosomes separate, both the normal leg and the inverted leg of the chromosomes still has the right number of genes. If the egg or sperm formed from the normal leg of the chromosome combines with a sperm or egg of the partner at conception, an embryo with normal chromosomes will be produced. Alternatively, if the egg or sperm formed from the inverted leg of chromosome combines with a sperm or egg of the partner, an embryo with inverted chromosomes will be produced instead.
If crossover occurs in the inverted region (e.g. gene B swaps with C' as in Figure 5) before the chromosomes separate, both the normal leg and the inverted leg of the chromosomes will have an imbalance of genetic material. If the egg or sperm formed from either of these legs combines with a sperm or egg of the partner at conception, an embryo with a faulty pair of chromosomes will be produced.

In summary, when an individual with chromosome inversion has baby, one of three things might happen:
1. the individual passes his/her normal chromosome on in the egg or sperm. The baby will have a completely normal chromosome pattern with the correct number of genes in the correct order.
2. the individual passes his/her inverted chromosome on in the egg or sperm. The baby will inherit the inverted chromosome. However, since the correct amount of genetic material are present, the baby will be perfectly normal and healthy.
3. the individual passes his/her chromosome with an imbalance of genetic material on in the egg or sperm. The baby will inherit a chromosome which carries an incorrect number of genes. This may result in an early miscarriage or stillborn (usually in the first 3 months of pregnancy). The baby, if born, may have problems in his/her physical or intellectual development.
Given the facts that there are hundreds or thousands of genes in a chromosome and inversion usually occurs in a small portion of the chromosome, the probability of crossover in the inverted region is relatively small compared with crossover not in the inverted region. Hence, the probability of having the baby with genetic problem is relatively small. Nevertheless it is not possible to predict which of the three events would occur in a particular pregnancy. It is therefore important to have prenatal testings during the early stage of pregnancy to examine if the baby has inherited the correct amount of chromosome material, and thus to determine if the baby is genetically healthy before his/her birth.