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Sexual reproduction

How do living things evolve?

Today we know that genes, passed from parent to offspring, determine all hereditary characteristics, such as the colour of feather, size and deportment. 

Genes in turn are composed of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules, organized into structures called chromosomes. lt carries in coded form information that determines these traits, just as a score contains instructions for performing a piece of music. 

In bugerigars, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen- containing chemicals called bases. 

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Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base (Fig. 1). Four different bases are present in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). 

The particular order of the bases arranged along the sugar- phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits. 

The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs (bp). Genome size is usually stated as the total number of base pairs; the human genome contains roughly 3 billion bp Each time a cell divides into two daughter cells, its full genome is duplicated; for budgerigars and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. 

Strict base- pairing rules are adhered to adenine will pair only with thymine (an A- T pair) and cytosine with guanine (a C- G pair). Each daughter cell receives one old and one new DNA strand. The cells adherence to these base- pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring. 

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Each DNA molecule contains many genes, the basic physical and functional units of heredity.

A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions.

All genes are arranged linearly along the chromosomes and operates in pairs (fig. 2).

Genes vary widely in length, often extending over thousands of bases, but only about 10% of the genome is known to include the protein- coding sequences (exons) of genes. Interspersed within many genes are intron sequences, which have no coding function. 

46 pairs of chromosomes, are present in virtually every human cell (in budgerigars the exact number has not been found yet, as some of the pairs are very tiny).The major exception is the reproductive cells, also called germ cells or gametes (ova and spermatozoa). These contain only one copy of each gene contrary to normal cell which contain the genes in pairs. A new pairing is established when gametes meet at fertilisation. One of the genes in each pair comes from one parent the other from the other parent.

In Budgerigars, contrary to humans, the male has two X chromosomes and the female one X and one Y chromosome. These sex chromosomes carry genes for other traits too, which are therefore said to be sex-linked (more about this later). 

Meiosis is the stage during the development of germ cells at which the chromosomes are halved in number from the complete (diploid) set to the half (haploid) set (see figure 4, that shows the meiosis of one pair of chromosomes). 

Mitosis is the corresponding process when diploid cells divide and their full complement of chromosomes (and thus genes) is copied and maintained. 

This is what happens during the growth of animal and plant tissues, and in the development of a new organism from a fertilized egg cell.

Alternative traits such as tallness and shortness are determined by genes that occupy the same site or locus on a particular chromosome. These are called alleles

Many loci have several different alleles specifying alternative characteristics. 

An animal or plant carrying two identical alleles (e.g. a pea plant with two tallness genes) is said to be homozygous for that trait (A and B in figure 5). One with two different alleles is heterozygous (C in figure 5).

Working in the monastery garden at Brno, the munk Mendel crossed various varieties of edible peas, examined the offspring, and realised that certain traits sometimes eclipsed others.

When he crossed tall and short plants, for example, all of the progeny were tall (see figure 6). He described tallness as dominant over shortness. But a cross between two of these tall offspring yielded about a quarter of short peas in the next generation. The hidden, recessive gene for shortness had reasserted itself. Repeating these crosses hundreds of times, together with other crosses between plants with other pairs of qualities, Mendel found that the ratios of the progeny became extremely precise. This led him to one of the central tenets of genetics. 

Every plant (and every animal) carries a double dose of each of its genes. If both members of a pair of genes are dominant, then that trait (e.g. tallness in peas) is expressed. lt is also expressed if one gene is dominant and the other recessive. If both genes are recessive, then that trait (shortness in peas) is expressed instead. 

In Mendel's experiments, these combinations produced an overall ratio of 1 short to 3 tall plants. But 2 out of every 3 tall plants carried a masked, unexpressed gene for shortness. Although all 3 had the same phenotype (appearance), 2 had a different genotype (genetic make-up) from the other. 

More complex patterns

Dominance is not always complete. Red campions crossed with white campions produce pink progeny. At first sight, this looks suspiciously like blending inheritance. But if the pink progeny are crossed one with another, there will be an average ratio of 1 red flower, to 1 white and 2 pink in the next generation. 

The true explanation is that the red plants carry 2 genes for redness, the white flowers 2 white genes and the pink flowers 1 of each. The pink flowers arise because neither of the genes is dominant over the other. With dominance, the 1:2:1 ratio becomes 3:1, as in the case of the tall and short peas. 

When Mendel studied the inheritance of two pairs of characteristics together a more complex ratio emerged. He crossed a variety of peas whose seeds were yellow and round with another variety having green and wrinkled seeds. All of the progeny produced yellow, round seeds. 

When he crossed these plants, however, Mendel found a ratio of 9 yellow round, 3 yellow wrinkled, 3 round green and 1 green wrinkled seeds. This occurred because the two pairs of characters (green-yellow and round-wrinkled) segregated independently of each other, yellow and round being dominant. 

Not all traits are determined by the sort of alternative genes that are apparent in simple Mendelian crosses. Often, many different genes interact together to produce the end-result.

In contrast to Mendel's short and tall peas, height in humans is one example of such polygenic inheritance. It is reflected in the normal curve for height in the population, with few very short and few very tall people and a bulge of average individuals in the middle.

Genes, that do not has their loci on the same pair of chromosomes, are obviously not linked.

As an example one can take a normal green cock and pair it to a greywing blue hen. The gene pair for bodycolour, which may be located on chromosome pair no. 1, may be called AA, Aa or aa (for green, green/blue or blue).

The gene pair for wingcolour, which may be located on chromosome pair no. 4, may be called BB,Bb or bb (for normal, normal/greywing or greywing).

In the first generation the progeny will be as shown below.

aabb: blue greywing 

ab  AaBb:
GREEN/blue NORMAL/greywing 
GREEN/blue NORMAL/greywing 
ab  AaBb:
GREEN/blue NORMAL/greywing 
GREEN/blue NORMAL/greywing 

Table 1

If the progeny are paired together, we will have GREEN NORMAL, GREEN greywing, blue NORMAL
and blue greywing as shown below.
AaBb: GREEN/blue NORMAL/greywing 
AaBb: GREEN/blue NORMAL/greywing 
AB Ab aB ab
GREEN/blue NORMAL/greywing
Ab AABb:
GREEN greywing
GREEN/blue NORMAL/greywing 
GREEN/blue greywing
aB AaBB:
GREEN/blue NORMAL/greywing
blue NORMAL/greywing
ab AaBb:
GREEN/blue NORMAL/greywing
GREEN/blue greywing
blue NORMAL/greywing
blue greywing

Table 2

We can see, that blue are not linked with greywing, and green are not linked with normal wingcolour.

Sexlinked traits.
Table 3
A special linkage is the one where the traits are linked to the sex-chromosomes. In budgerigars the cock has as mentioned two socalled X-chromosomes, and the hen has one X-chromosome and one Y-chromosome. The traits (genes) that are linked with the sex-chromosomes are only expressed in the cock if they are present at both chromosomes. If the gene(s) is only present on one of the cocks sex-chromosomes, he is split for that trait. In hens the trait is expressed when the gene is present at her X-chromosome. 
*X  *X
Table 4
The Y-chromosome can't carry these genes. So, a hen can never be split for any sexlinked traits.

The easiest way to see how the rules for sex-linked heredity work is to put the pairings into tables as shown to the left. If the gene are present at a X-chromosome, it's marked with the special character *. In these examples it's the sex-linked variety OPALINE that is gone through. There are three more combinations - try to make them yourselves.

*XX: NORMAL/opaline 
*X  X
Table 5
The most common varieties with sex-linked heredity are opaline, cinnamon and inos. However there are undoubtedly some other traits connected to the sex-chromosomes. It's difficult to find any written facts about these traits. Have you ever thought about the heredity of size and shape of spots? Does cinnamons and opalines has special feathertypes? Only years of observations in your birdroom can bring you the answer.
Another sort of connexion is when the genes has their loci on the same pair of chromosomes, they are linked.

We'll imagine, that the genes A,B and C are three dominant linked genes for the traits: BIG HEAD, GOOD STANCE and SHORT BODY, and the genes a,b and c are the corresponding recessive linked genes for the traits: SMALL HEAD, BAD STANCE and LONG BODY.

When two homozygous birds AABBCC and aabbcc (see figure 7) are paired together, the progeny are all heterozygous AaBbCc, which are birds with BIG HEAD, GOOD STANCE and SHORT BODY - and with heredity for small head, bad stance but long body.

If this heterozygous offspring is paired together, we'll get AABBCC, AaBbCc - birds with big head, good stance but short body - and aabbcc - birds with small head, bad stance but long body.

Apparently it's impossible to get offspring with BIG HEAD, GOOD STANCE and LONG BODY.

However one will often see that the linking between some genes are not completely. Offspring with BIG HEAD, GOOD STANCE and LONG BODY will emerge. The explanation is, that the chromosome pairs during the phase of meiosis where they are very close sometimes exchange larger or smaller pieces. This phenomenon is called crossingover(see figure 8).

Notice that we also got birds with SMALL HEADS, BAD STANCE and SHORT BODY!

Linked genes, which are located very near each other (here A and B), are rarely separated by crossing. They are completely linked, whereas genes that are not located as close (here C and A/B) often are separated by crossingover.

Interestingly, if two genes are very far apart on the same chromosome pair, there is so much crossingover that the results obtained look like a regular dihybrid cross between unlinked genes.

Do we have an interest in knowing if some traits are linked?

Yes! - Imagine that the traits, that gives you a budgerigar with a good make up are more or less linked.

If skilful breeders in the course of time has realised this and been able to select the proper "batch", they has done themselves and us a big favour.

Let me illustrate it by an example.

For the sake of convenience we'll only take the three traits mentioned above: AA, Aa and aa - BB, Bb and bb - CC, Cc and cc, where AA and Aa = big head and aa = small head, BB and Bb = good stance and bb = bad stance, CC and Cc = short body and cc = long body.

Obvious there will be far more genes involved to express these traits, but for the convenience ....

I want to have birds with , BIG HEADS, GOOD STANCE and LONG BODIES. I can have this from the following combinations of gene pairs:

1) AABBCC - 2) AaBBcc - 3) AABbcc - 4) AaBbcc.

In addition there will be 55 combinations, that don't give the results wanted, (see the table below).

If I have a cock and a hen with gene pair combination AABBcc, it would be perfect, as it's impossible to make any birds with the 3 unwanted traits: SMALL HEAD, BAD STANCE or SHORT BODY - even if crossingover takes place. The three traits are purebred

ABC  ABc  AbC  Abc  abc  abC  aBC  aBc 
ABc  AABBCc  AABBcc AABbCc  AABbcc AaBbcc AaBbCc  AaBBCc  AaBBcc
AbC AABbCC  AABbCc  AAbbCC  AAbbCc  AabbCc  AabbCC  AaBbCC  AaBbCc 
Abc  AABbCc  AABbcc AAbbCc  AAbbcc  Aabbcc  AabbCc  AaBbCc  AaBbcc
abc  AaBbCc  AaBbcc AabbCc  Aabbcc  aabbcc  aabbCc  aaBbCc  aaBbcc 
abC  AaBbCC  AaBbCc  AabbCC  AabbCc  aabbCc  aabbCC  aaBbCC  aaBbCc 
aBC  AaBBCC  AaBBCc  AaBbCC  AaBbCc  aaBbCc  aaBbCC  aaBBCC  aaBBCc 
aBc  AaBBCc  AaBBcc AaBbCc  AaBbcc aaBbcc  aaBbCc  aaBBCc  aaBBcc 

Table 6

No matter what gene material I'll purchase, the gene pair combination for these 3 traits will be one the 64 in the table above, and I can use 9 out of 64, but only 1 out of 64 will be purebred (in the cell with black background).

If I purchase a bird with gene pair combination aabbCC, believing that the desired traits are hidden in the genome, we'll see, that even with crossing over it's impossible to pick any good traits out of this material.

Something that looks like the right "batch" can be seen at breeders, who by means of planned breeding has succeeded in creating and maintaining the right gene combinations.

You can see it by the uniform appearance of the stud.

On the other hand some breeders have the uniformity, but "the batch" isn't put together in the right way. Keep clear! It's devastating to cross such birds into your stud.

How are purebreeding done to make the right "batch" ?

First we'll have to look at the terms outcrossing, inbreeding, linebreeding, depression and heterosis.

Outcrossing is when you are pairing two unrelated birds together. If you have no information about the two lines, you can compare it with the chance to win the pools with one coupon, no warranty and no knowledge about football.

If you are "pairing" gene pairs from the cells with white background in the table above, you'll see, that you have to be lucky to hit the right combination in first round.

In figure 9 I have tried to illustrate the terms inbreeding and linebreeding very simplified. Here inbreeding means backcrossing. As a matter of fact linebreeding is also inbreeding. As the exhibition budgerigar usually only breeds in a small number of years, we can see that it's very theoretically. By a line is ment a population, where the individuals' genome are relatively similar. Other words for a line are family or strain.

By linebreeding you constantly tries to maintain the wanted traits to a certain degree. 

Here you can compare it with the chance to win the pools with many coupons, high warranty and good knowledge about football. Try to pair the individuals with the 9 marked genotypes in table 3 above. Here we have 3 traits that are almost purebred. The chance of having the combination you want is obvious great. 

What does happen when inbreeding ?

Let me illustrate it by the so called dominance theory. The theory is that size and growth are determined by a large number of more or less dominant linked genes.

An example:

A pair of chromosomes in a non-inbred individual could have the following combination of genes for size and growth:
aBCdEf gH

It's implied that the genes represented in capitals are promoting size and growth, and that they are dominant. One dose of a dominant gene is just as effective as two (Aa are just as effective as AA).

If we give the effect of every size and growth promoting gene pair the value 5, the total size and growth effect for this individual will be 8 x 5 = 40. If self-fertilization are carried out on this individual the progeny (no crossingover taking place) can have the three different gene pairs below:

AbcDeFGh AbcDeFGh aBCdEfgH
AbcDeFGh aBCdEf gH aBCdEfgH
Number of dominant gene pairs 
Effect per individual 
Total effect 
80 (2x40)
The effect in first generation was 40 against 30 in second generation. 

A reduction in size and growth will allways be the result of inbreeding. It's called depression. Why then use inbreeding at all? If you want to purebreed one ore several traits you have to pair related birds together to see which 

Average: (20+80+20)/4=30.

combinations that can be made out of the family's genome. By keeping the birds which has the traits that you want and cull the rest, you have taken the first step. However a lot of things can go wrong. If you don't have the eye to recognise an exhibition budgerigar, it's rather in vain, as you can't select the right birds.

One shall also be careful not to lose good traits, i.e. parental care. An example on loss of traits have recently been described here in Denmark during farmers process of changing-over from egg production from hens in small cages to hens living in open air. The latter has lost the ability to build a nest and to take care of the eggs. The original high yielding farm hen are long gone (extincted), or described in another way; the valuable genes that those hens had have been bred out!

As is the case with good traits you can also purebreed the bad traits - so be careful.

After some generations with careful inbreeding and selection, you will probably find, that you can't find additional good stuff in the genome of your line.

You have to go out to purchase some of the traits that your line is deficient in, if you isn't so lucky that you have a big stud with several unrelated purebred lines. If you have run the inbreeding consequently a degree of inbreeding depression is emerged. It can be seen by reduction in size or reduction in the number of chicks per clutch.

To neutralize this effect and at the same time fetching new traits, it's a good idea to find the birds in a stud, where the breeder has been purebreeding the traits that your birds are week in. In most cases you will see a growth in size and vitality in your birds, if you make a cross with such birds.

This phenomenon is called heterosis and can be explained by the dominance theory mentioned earlier.

If two individuals from two unrelated inbred lines (homozygotes) are paired together, you'll get maximum heterosis effekt, if the parents has the following genome for the traits. You can see, that depression and heterosis are two sides of the same coin.
P generation  AbcDeFGh aBCdEfgH F1 generation AbcDeFGh
  AbcDeFGh aBCdEfgH   aBCdEf gH
Number of dominant gene pairs 
Number of dominant gene pairs 
Effect per individual 
Effect per individual 

We can see that in this ideal case the effect is doubled. Even if the genes from the two lines are not fitting as well as in this example, you wil still get some heterosis effect. In farming breeding work they are aiming towards a constant heterosis effect.

It's however of paramount importance that you are keeping track with the development of your stud when you are putting inbreeding into practise. Remember, you can breed good traits out of the stud and purebreed bad traits!


Something to think about.

Now we'll look at some of the possibilities.
  1. If a wanted trait is dominant.
  2. If a wanted trait is recessive.
  3. If an unwanted trait is dominant.
  4. If an unwanted trait is recessive.
  5. If a wanted trait is sexlinked.
  6. If an unwanted trait is sexlinked.
  7. If wanted traits are linked with unwanted traits.
I'll give the following suggestions to think about:
  1. If a wanted trait is dominant.

  2. The trait is purebred through contol pairings. As an example you can imagine that you would only keep green budgerigars. You have to ensure that none of the green birds are split for blue. With 50 - 100 birds it would be a rather difficult task.
  3. If a wanted trait is recessive.

  4. The trait is purebred by inbreeding an selection. As an example you can imagine that you would only keep blue budgerigars and your material to start with was green birds split for blue. All blue chicks are to be kept and the green ones are to be culled. When all your birds are blue, this trait is purebred.
  5. If an unwanted trait is dominant.

  6. Culling and the problem is solved.
  7. If an unwanted trait is recessive.

  8. The trait are expressed through inbreeding and culled with.
  9. If a wanted trait is sexlinked.

  10. The trait is purebred in the same way you would do if you only want to keep e.g. opaline budgerigars. The hens express' the trait - if they got it - and can be selected. The cock birds, which can be split opaline, are paired to opaline hens. If none of the chicks express the trait, the cock is not split for opaline, and he plus the offspring are not kept. If however both hens and cocks among the chicks are expressing opaline, the cock is split opaline. Keep him if he is an outstanding cock plus the progeny that expresses opaline.
  11. If an unwanted trait is sexlinked.

  12. The trait is purebred in the same way you would do if you don't want to keep e.g. opaline budgerigars. Again it's the cock birds that makes it difficulty. If the hen does not express opaline, she don't have it. If you want to be sure if a normal cock is split opaline, you have to pair it to an opaline hen to see if any of the chicks produced are opaline.
  13. If wanted traits are linked with unwanted traits.

  14. The far apart the genes for these traits are located the greater is the chance that you can "swap" the unwanted traits with wanted traits through crossingover. But be aware that the "swapping" also can go the other way! So watch for these unfavourable crossingovers.

    [a cura di Alberto Masi]