Basic Genetics

Posted February 6, 200817 yr

Recently I have been reading up on budgies genetics, as after many months of research and preparation, I think I'm finally ready to begin breeding.

I have studied 'human genetics' for the past 4 years, the last 3 years at high school and my first year of university and am attempting to translate that knowledge to budgie genetics.

But most of the websites I have looked at are quite in depth and would be really confusing to someone who does not have a basic knowledge of genetics.

So I thought I would make an attempt at explaining the very basics of genetics, and how they work.

TERMS TO KNOW:

General Terms:

Allele: A gene, each allele 'carries' certain traits.

Sex Chromosome: Determines sex. If I remember correctly, in birds the sex chromosomes are Z & W and the female is the heterozygote.

So, the male = ZZ and female = ZW

Homozygous/Homozygote: Has two identical alleles at a gene locus.

Heterozygous/Heterozygote: Has two different alleles at a gene locus, apparently known in the budgie world as "Split for".

EXAMPLE:

Normal green budgie not split for blue, has alleles GG – this is homozygous

Normal green budgie split for blue, has alleles Gg – heterozygous or 'split'

Normal blue budgie, has alleles gg – homozygous.

Phenotype: The outward appearance of the budgie.

Genotype: The genes of the budgie.

Mutation: A heritable change in the genetic material, or an 'accident' in the genes, as such it is probably fair to say that all budgies types bar the 'wild type' are 'mutations'.

Types of Inheritance:

Dominant: A particular trait that is visible in the homozygote and the heterozygote. One way to remember it is, dominant genes 'block' recessive gene's so that you can't see them.

Recessive: The trait is only seen in the recessive homozygote (such as in the normal blue budgie, with the genes = gg)

Sexlinked: Can also be either dominant or recessive, however the genes are located on the sex chromosomes (either on the Z or W)

There are other types of inheritance as well, such as Incomplete dominance or

Co-dominance, but, for basic genetics, we do not need to worry about them yet.

PUNNET SQUARES:

These are actually quite fun to do and are useful in helping to predict what your budgies offspring may look like.

Starting very simple, we will do a monohybrid cross, all this means is that it is a cross between two budgies involving one trait.

For the monohybrid crosses I will use 'base colour' as an example.

Green Budgies (yellow-based) are dominant to blue budgies (white-based)

So the genes we will use are:

G = green

g = blue

Cross Number 1:

A normal green cock x A normal green hen split to blue

So the genes will be = GG x Gg

Offspring:

Phenotype = 100% Green Budgies

Genotype = 50% homozygous dominant

50% split for blue

Cross Number 2:

A normal green cock split for blue x A normal green hen split for blue

Gg x Gg

Offspring:

Phenotype = 75% Green

25 % Blue

Genotype = 25% homozygous dominant

50% split for blue

25% homozygous recessive

Now it gets a bit trickier, with the dihybrid cross. This is still a cross between two budgies but for two traits.

For the dihybrid cross I will use 'base colour' and spangle wing pattern.

Green Budgies are dominant to blue budgies and

Spangle is dominant to normal.

So the genes we will use are:

G = green

g = blue

S = spangle

s = normal

Cross Number 3:

A spangle green cock x A normal blue hen

SSGG x ssgg

Okay, this is the trickest part, working out what to cross.

Take the first set of genes, SSGG,

Cross the first S with the first G, then the first S with the second G.

You will end up with SG and SG

Then cross the second S with first G, then the second S with the second G.

Now you have four - SG, SG, SG, SG.

These go across the top of your punnet square.

Repeat the process with the second set of genes (ssgg)

You should end up with four – sg, sg, sg, sg.

These go down the side of your punnet square.

Offspring:

Phenotype:100% Green Spangles

Genotype: 100% split for normal

100% split for blue

Now we will do a sex-linked monohybrid cross.

Using opaline wing pattern (which has to be on the Z chromosome, as it is observed in both males and females.)

So opaline males will be ZO ZO

Opaline females will be ZO W

Cross Number 4:

A opaline cock x A normal hen

ZO ZO x Z W

Offspring:

Phenotype = Males: 100% normal

Females: 100% Opaline

Genotype = Males: 100% split for opaline

Females: 100% opaline

Cross Number 5:

A cock split for opaline x an opaline hen

Z ZO x ZO W

Offspring:

Phenotype = Males: 50% Opaline

50% Normal

Females: 50% Opaline

50% Normal

Genotype = Males: 50% Split for opaline

50% homozygous opaline

Females: 50% normal

50% opaline

Notes:

* Use the same letter, a capital and lower case, for one trait.

Example, G = green & g = blue.

Do not use different letters, such as G = green & b = blue, because you will confuse yourself, especially if you are doing a dyhybrid cross.

* When using a letter where the capital and lower case look almost identical make a mark on one so you can differentiate (I put a little stroke on my capital S in the dyhybrid cross)

* With Sex-linked crosses, make sex chromosome letters larger and the trait letters smaller.

* Remember that each cross represents what one chick may be, i.e. if your punnet square tells you that 75% of your offspring will be green and 25% will be blue, that means that for every chick that hatches there is a 75% chance it will be green and a 25% chance it will be blue.

It does not mean that if you have four chicks hatch, 3 (75%) will be green and 1 (25%) will be blue.

A pair could have 100 chicks and every single one could be green, conversely every single one could also be blue. That's the fun of genetics!

I hope that helped a little bit and didn't make anyone even more confused!

(And if I made any mistakes please let me know.)

Edited February 6, 200817 yr by Bec Sta

February 6, 200817 yr

i found it really interesting reading,i have never understood genetics but that explain monohybrid cross to me well. i am still trying to work out dihybrid cross and you wrote it well but it will take me a while to understand it.