Genetic Inheritance Has Rules – But Is Not An Exact Science

A letter from A Collins of Tasmania came in the September issue of The Budgerigar, expressing amazement at his/her production of a Cinnamon Opaline hen from one pairing. I am delighted to report that, as I shall demonstrate, the laws of genetics do not need to be revised.

Male Budgerigars have two X chromosomes, whereas female Budgerigars have one X chromosome coupled with one Y chromosome. The male’s two X chromosomes are derived from different parents, i.e., father and mother, and the genetic information contained on the two different X chromosomes may not necessarily be the same; for example, a Normal/(split) Opaline cock has a Normal gene on one X chromosome from one parent and an opposing Opaline gene on the other X chromosome from the other parent. This Opaline gene is the consequence of a mutation in the original Normal gene many generations ago (in 1933), which has been passed down down the generations and may be tracked via pedigree analysis.

X chromosome carries sex-linked genes

It is critical to recognize that the Y chromosome carries little, if any, relevant genetic material and so lacks the sex-linked genes found on the X chromosome. In the case of the female Budgerigar, if the X chromosome has a sex-linked gene, it will be expressed. Because the mother had only one X chromosome, the male having an Opaline gene may, of course, receive the gene from either mother or father if it was acquired from the mother. She would also have shown the same sex-linked Opaline character. If the Opaline gene was acquired from the father, the cock may have been either i. a homozygous Opaline (both X chromosomes identical) or ii. a heterozygous recessive, i.e., Normal/Opaline.
Because all sex-linked genes are recessive, the sex-linked genes work as do all other non-sex-linked recessive genes in males exclusively, however in females, the sex-linked genes always behave as if they are dominant. If we look at the breeding pattern of A Collins birds, we can see that the crossing of a Normal/Opaline cock with a Cinnamon hen works as shown in Diagram 1:

Diagram 1 depicts the fact that this mating has four different genetic consequences, each with a 25% probability of occuring. However, there are only two conceivable phenotypes (displayed genetic features) from this mating, despite the fact that the phenotypes have distinct genotypes (genetic makeup) – Normals (cocks and hens) 75% and Opaline hens 25%.

The aforesaid outcome did not occur in the instance of A Collins’ birds because another genetic kind, namely a Cinnamon Opaline hen, arose. The preceding figure provides no hints as to how this may happen; to explain the occurrence of the Cinnamon Opaline hen from the aforementioned mating, we must investigate the mechanics of reproductive cell division – meiosis. A simple figure (Diagram 1) can only fully depict the inheritance pattern of a single gene, and in the diagram above, we have studied two genes (Opaline and Cinnamon) and a third genetically inherited feature, sex, both of which are decided by the X chromosome.

When a cell with paired chromosomes divides to produce two gamete cells (sperm or eggs), the pairs of chromosomes do not pass unchanged to the gametes; instead, the single chromosome passed to the gamete is a mixture of genetic material that was contained on both chromosomes of the pair in the parent cells; in this way, every individual receives a mixture of genetic information that was derived from both grandparents.

For identification purposes, the parent’s paired chromosomes are displayed as one black chromosome and one white chromosome, with the black chromosome inherited from, say, the father and the white chromosome inherited from the mother, or vice versa. Crossing over happens during meiosis when the chromosomes come together at locations called chiasmata and subsequently separate, resulting in two new chromosomes in gametes A and B that are inverted opposites to each other and completely distinct from the parental chromosomes from which they are derived. When a pair’s chromosomes cross over, the chiasmata do not always occur at the same points along the lengths of the chromosomes; there are different sites at which chiasmata may occur, resulting in an infinite number of different chromosome combinations after meiosis, and no two chromosomes will be identical in the gametes. Diagram 3 exemplifies this point: the identical pair of chromosomes, one black and one white, have produced chiasmata at different locations, resulting in distinct chromosomes from those in Diagram 2.

I’ll utilize Diagram 4 to disprove the widely held belief that a person has 25% of each of its grandparents’ genes, 12.5% of each of its great-grandparents’ genes, and so on. This figure demonstrates that the sperm with chromosome A (as formed in Diagram 2) fertilized the egg with chromosome Z, resulting in the birth of a new human – Parent 2. When considering parent 2’s gametes, there are of course infinite recombination possibilities of the genes on the chromosomes of the sperm or eggs. In Diagram 4, chromosome T has no genetic material obtained from the grandmother who created the chromosome, but chromosome S has genetic material received from both grandparents via the father’s sperm. As a result, all kids of a parent do not have the same genetic information, explaining the enormous variance found among siblings (brothers and sisters).

So far, we have only looked at the behavior of autosomes (all the chromosomes except the sex chromosomes). They vary somewhat from what has been described thus far in terms of the behavior of the two sex chromosomes during meiosis. The fundamental difference between sex chromosomes and autosomes is that crossing over during meiosis does not occur between the X and Y chromosomes in female ovaries because the X and Y combination is not an equal pair, but it does occur between the two X chromosomes in male testes during spermatiogenesis (sperm production). In the case of females, the X chromosome is transferred unaltered to the gamete carrying the X chromosome, as shown in Diagram 5. Nature has evolved a very ingenious means of insuring just two distinct sexes by using a “dummy” chromosome, the Y chromosome. When comparing the predictions for all other genetic traits, the remainder follow the Mendelian Ratios, none of which yield phenotypes in the 1:1 ratio, as we all know.

With reference to Diagram 6, it can be observed that the grandchild’s X chromosome is inherited intact from the grandpa through the mother.

It’s worth noting that only the male descendants inherit the maternal grandfather’s unaltered X chromosome.

After discussing crossing over and sex-linked genes, we may now investigate the impact of crossing over on sex-linked genes. Diagram 7 depicts a section of a chromosome in which portions A, B, C, D, E, and F represent gene loci and the gaps in between are the regions along the chromosome where chiasmata may form. It is evident that the closer two distinct genes are together, the less likely chiasmata will form between them. If the Cinnamon and Opaline genes are located at, say, sites C and D on the chromosome, the frequency of chiasmata occurring between the two genes is much lower than if the Cinnamon and Opaline genes are located at, say, sites A and F on the chromosome, where chiasmata can occur at five different locations.

The appearance of a Cinnamon Opaline hen in A Collins’ birdroom reveals that during spermatogenesis in the Dark Green Normal/Opaline/Cinnamon cock, crossing over of the X chromosomes happened at a chiasmata between the Cinnamon and Opaline genes. Crossing over between the Cinnamon and Opaline genes is uncommon, indicating that the Cinnamon and Opaline gene loci on the X chromosome are quite near together (almost directly adjacent), and so chiasmata seldom arise between the two loci of these genes. The Cinnamon and Opaline genes are now practically side by side after crossing over, and in Diagram 1, the Normal cock split Cinnamon and Opaline (number 3) now has one X chromosome containing both Cinnamon and Opaline genes, while the other X chromosome has none. This is where the Cinnamon Opaline variation comes from; it is not a result of mutation. The Cinnamon Opaline variety will follow the same pattern of heredity as any of the sex-linked varieties considered individually, but crossing over may occur in reverse to the Cinnamon Opaline variety’s formation, during which the Cinnamon and Opaline genes may part company when one of the genes is transferred to the other X chromosome.

The Lacewing is another variation that shows cross-over as its origin. This variation is the result of a tight relationship between the Ino (Albino and Lutino) and Cinnamon genes on the same X chromosome. Since the Cinnamon Opaline variety and the Lacewing (properly, the Cinnamon Ino) have occurred, it is possible to say that the genes for Cinnamon, Ino, and Opaline all have their loci close to each other on the X chromosome, and it is most likely that the Ino gene is in the middle of the three genes. It is interesting to note that the bird that prompted A Collins to write the letter was Dark Green; unfortunately, we do not know the color of the hen with which he was paired or the color of any of the offspring, because the Dark factor also shows a more common form of linkage with the Green/Blue gene, on the same autosome, explaining the difference in inheritance pattern shown by Type 1 and Type 2 Dark factor birds.

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