Chromosome maps... so confused!

I'm doing higher bio and at the moment we're doing genetics and looking at chromosomes maps and dihybrid crossed.
I'm having such a hard time getting to grips with the idea of genes and alleles.

On the chromosome map for a fruit fly it shows 4 chromosomes. On chromo 1 it shows the gene location for miniature wings and on chromo 2 there are curved wings, held-out wings, dumpy wings, vestigial wings and so on.

Is there a gene in each location or does it only have a gene if it has that chracteristic.. because if it has a gene in each location the fly can't have all those types of wings...curved wings, dumpy wings, held-out wings.
And since there are 2 of each chromosome (a pair) does it mean there are 2 genes for each characteristic?

So for curved wings it will have a dominant allele and a recessive allele and the same for held-out wings, etc.

I hope that made sense.

Thanks!

I'm so confused!

Reply 1

Revenged

15

There are two alleles for a gene... not two genes for every characteristic...

Reply 2

Lou_

OP

15

See... I am so confused lol!

Okay I have a simpler example;
in a fruit fly -
on chromosome 2 there is a gene for black body
on chromosome 3 there is a gene for ebony body

if the fly is black then it will have no alleles present on the ebony body gene location on either of its chromosome 3s (because there are 2 of every chromosome) as this is the gene for ebony body and the fly isn't ebony.

Also, in a dihybrid cross where the characteristics are on different chromosomes and you get the phenotypic ratio 9:3:3:1... recombination is brought about by independent assortment and not crossing over?

Reply 3

Revenged

15

Lou_

Also, in a dihybrid cross where the characteristics are on different chromosomes and you get the phenotypic ratio 9:3:3:1... recombination is brought about by independent assortment and not crossing over?

Crossing over IS recombination!...

Crossing over (recombination) and independent assortment are mechanisms to ensure more variation during meiosis...

Both occur during meiosis... It's just that if chromosomes are on not linked (i.e. are on different chromosomes) then they are not going to be affected by recombination...

As for your example... You aren't really given me enough information to go on...

Reply 4

Lou_

OP

15

Thank you!

Okay on chromosome 1 there are gene locations for sable body and yellow body, chromosome 2 there is black body, chromsome 3 there is ebony body.

On my gene map these are the only genes that are related to body colour.

Reply 5

Laura Lou

13

Lou_

See... I am so confused lol!

Okay I have a simpler example;
in a fruit fly -
on chromosome 2 there is a gene for black body
on chromosome 3 there is a gene for ebony body

if the fly is black then it will have no alleles present on the ebony body gene location on either of its chromosome 3s (because there are 2 of every chromosome) as this is the gene for ebony body and the fly isn't ebony.

I think you are still a bit confused with what an allele is...
Every gene has a number of alleles depending on the DNA code - a gene cannot have NO alleles.
Using your example, if the fly is black then the gene coding for an ebony body on (both chromosome 3s) is inactive, whilst the gene coding for the black body is active.

Reply 6

Lou_

OP

15

Thank you so much... I understand it now!
Our teacher never explains stuff like that to us.

Chromosome maps... so confused!

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