Genetic Inheritance has Rules - but is not an exact science
A male Budgerigar has two X chromosomes, and the female Budgerigar has just one X chromosome which is paired with one Y chromosome. The two X chromosomes of the male are derived from a different parents, ie., 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 on one X chromosome from one parent, a Normal gene, and on the other X Chromosome an opposing Opaline gene. This Opaline gene is the result of the mutation of the original Normal gene many generations ago (in 1933) which has been transferred from parent to offspring throughout the generations and can be traced by the analysis of pedigrees.
X chromosome carries sex-linked genes
It is essential to understand that the Y chromosome contains little, if any, useful genetic information, and therefore does not contain any of the sex-linked genes that are carried on the X chromosome. In the case of the female Budgerigar, if the X chromosome carries a sex-linked gene, then it will be displayed in that bird. The male carrying an Opaline gene can, of course inherit the gene from either mother or father if it was inherited from the mother, since the mother possessed only one X chromosome. She would have also displayed the same sex-linked Opaline character. If the Opaline gene had been inherited from the father, then the cock could have been either- i. a homozygous Opaline (both X chromosomes the same) or
- ii a heterozygous recessive ie., a Normal/Opaline.
Since all the sex-linked genes are recessive then, in the male only, the sex-linked genes operate as do all other non sex-linked recessive genes, whereas in the female, the sex-linked genes always act as though they are dominant. If we now examine A Collins birds' breeding pattern, the crossing of a Normal/Opaline cock with a Cinnamon hen operates as in Diagram 1:
Diagram 1 illustrates the fact that there are four possible genetic outcomes from this mating and each has a 25% chance of occurring. However, there are only two possible phenotypes (displayed genetic characters) although the phenotypes have different genotypes (genetic composition) from this mating - Normals (cocks and hens) 75%, and Opaline hens 25%.
In the case of A Collins's birds, the above result did not occur, because another genetic type appeared, namely, a Cinnamon Opaline hen. For this to happen, the above diagram gives no clues; in order to understand the occurrence of the Cinnamon Opaline hen from the above mating, we need to examine the mechanics of reproductive cell division - meiosis. A simple diagram (Diagram 1), can only properly explain the inheritance pattern of single genes, and in the above diagram we have considered two genes (Opaline and Cinnamon) and a third genetically inherited character, sex, all of which characters are determined by the X chromosome.
In reality, where the cell, with paired chromosomes, divides to produce two gamete cells (sperm or eggs), the pairs of chromosomes do not pass unchanged to the gametes, they undergo a mixing process, by which the single chromosome passed to the gamete is in fact, a mixture of genetic material that was contained on both the 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 of its parents, and not just two grandparents.
For the sake of identification, the paired chromosomes of the parent are shown as one black chromosome and one white chromosome, the black chromosome inherited from, say it's own father, and the white chromosome inherited from the mother, or vice versa. During meiosis, the phenomenon of crossing over occurs, when the chromosomes join together at points called chiasmata and on later separating, the two new chromosomes in gametes A and B are inverted opposites to each other and totally different to the parental chromosomes from which they are derived. When the chromosomes of the pair 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: this gives endless different chromosome combinations after meiosis and no two chromosomes will be identical in the gametes. Diagram 3 illustrates this point: the same pair of chromosomes, one black and one white, have formed chiasmata at different sites, producing different chromosomes to those in Diagram 2.
For the sake of correcting the popular misconception
that an individual possesses 25% of each of its grandparents' genes, 12.5% of
each of its great-grandparents' genes and so on, I shall use Diagram 4.
This diagram shows that the sperm carrying chromosome A (as produced in
Diagram 2) has fertilized the egg containing chromosome Z to produce
a new individual - Parent 2. If parent 2's gametes are considered,
there are of course, endless recombination possibilities of the genes on the
chromosomes of the sperm or eggs. In Diagram 4 chromosome T possesses
no genetic material derived from the grandparent that produced the chromosome,
whereas chromosome S possesses genetic material inherited from both grandparents
via the father through his sperm. Thus, all offspring of a parent do not carry
the same genetic information, and hence the wide variation often seen in siblings
(brothers and sisters).
So far, we have only considered the behaviour of the autosomes, (all the chromosomes
except the sex chromosomes). With specific regard to the behaviour of the two sex
chromosomes during meiosis, they differ somewhat, from that described so far.
The fundamental difference between the sex chromosomes and the autosomes, is that
crossing over during meiosis does not occur between the X and Y chromosome in the
ovaries of the female, because the X and Y combination is not an equal pair, but does
take place between the two X chromosomes in the testes of the male during
spermatiogenesis (sperm production). In the case of the female, the X chromosome is
passed to the gamete containing the X chromosome unchanged, as in Diagram 5.
Nature has devised a most elegant method of ensuring just two different sexes, by the
use of a "dummy" chromosome, the Y chromosome. As we all know, the
expectation of male to female offspring is 1:1 (50% males, 50% female), when you
compare this it the expectations for all other genetic characters, the rest follow
the&nsp;Mendalian Ratios, none of which produce phenotypes in the 1:1 ratio.
With reference to Diagram 6, it can be seen that the X chromosome of the grandchild is inherited from the grandfather, via the mother, in an unchanged state.
It is interesting to note that only the male grandchildren inherit the unchanged X chromosome from the maternal grandfather.
Now that we have discussed crossing over and sex-linked genes, we are now ready to consider the effects of crossing over on sex-linked genes. Diagram 7 shows part of a chromosome where parts A B C D E and F represent sites at which genes are found - Loci and the spaces in between are the positions along the chromosome at which the chiasmata may occur. It is obvious that the closer two different genes are together, the less the chances of chiasmata occurring between the two genes. If the Cinnamon and Opaline genes are positioned at, say, sites C and D on the chromosome, then the frequency of chiasmata occurring between the two genes is much less than if the Cinnamon and Opaline genes are positioned at, say, sites A and F on the chromosome, where, in this example, there would be five possible sites for chiasmata to occur.
The fact that a Cinnamon Opaline hen has appeared in A Collins's birdroom demonstrates the fact that during spermatogenesis in the Dark Green Normal/Opaline/Cinnamon cock, crossing over occurred or the X chromosomes at at a chiasmata between the locus of the Cinnamon and Opaline genes The fact that crossing over between the Cinnamon and Opaline genes is not a very common occurrence, indicates that the loci of the Cinnamon and Opaline genes on the X chromosome are extremely close together (almost immediately adjacent) and hence chiasmata rarely form between the two loci of these genes. After crossing over, the Cinnamon and Opaline genes are now almost side by side with each other and in Diagram 1, the Normal cock split Cinnamon and Opaline (number 3), now has one X chromosome with both Cinnamon and Opaline genes, whilst the other X chromosome has neither gene. This is the origin of the Cinnamon Opaline variety; it is not a form of mutation. The Cinnamon Opaline variety will follow the same pattern of heredity as any of the sex-linked varieties considered individually, but rarely, crossing over may occur, in reverse to the formation of the Cinnamon Opaline variety, during which the Cinnamon and Opaline genes may part company, when one of the genes is transferred to the other X chromosome.
Another variety which demonstrates crossing over as its origin, is the Lacewing. This variety is the close linkage of the Ino (Albino and Lutino) and Cinnamon genes on the same X chromosome. Ino, Cinnamon and Opaline are all sex-linked genes, and so, individually, follow the usual pattern of sex-linked inheritance; 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 observe that the bird that provoked A Collins to write the letter was in fact Dark Green; unfortunately we do not know the colour of the hen to which he was paired or the colour of any of the offspring, since the Dark factor also shows a more common form of linkage with the Green/Blue gene, on the same autosome, hence the different pattern of inheritance shown by Type 1 and Type 2 Dark factor birds.
Original text Copyright 1998, Dr John Pilkington
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