dominant and recessive. doesn’t work like this recessive dominant

Dominant and Recessive

Doesn’t work like this

recessive dominant

Homologous chromosomes

Same genes, but possibly different versions

Allele different versions of a gene

Let’s look at a plant. On one of the chromosomes, there is a gene that determines flower color. Allele 1 has the following sequence: and codes for a red protein in the petals. Allele 2 has the following sequence: and produces a white protein.

Our plant chromosome

Red protein

Red protein

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Red flower

White protein

White protein

+

White flower

Red protein

White protein

+

Pink flower

A different plant chromosome

Auxin: a plant cell growth hormone.

As long as a plant cell has some auxin, it will grow to its full height. Mo’ auxin doesn’t mean mo’ height.

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+

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Dominant

Only need to have one copy of a dominant allele in order to see its effects in the cell

(plant with one auxin gene is just as tall as the plant with two)

Recessive

Will not see the effects of a recessive allele if there are any dominant alleles of that gene on the other chromosome

(DOESN’T mean you need two copies to see its effects)

Dominant alleles . . .

. . . are NOT stronger

. . . are NOT more common

. . . are NOT “better”

. . . are dominant because you’ll see the effect of their protein product no matter what else happens

Recessive alleles . . .

. . . are NOT weaker

. . . are NOT always rare

. . . are NOT always harmful

. . . are NOT bowing down to dominant alleles in any way

. . . are recessive because the effect of their protein product can be masked by something else

More plant traits

What about our red/white alleles?

Red protein

White protein

+

Pink flower

Alleles are codominant – you can see the effect of both proteins

Usually more than two alleles for each gene

2 of each chromosome(well, only for half of us . . . )

X chromosome:Lots of genes!

Y chromosome:Almost nothing on

it; “genetic wasteland”

Red-green colorblindness

Gene for protein that lets your brain tell the difference between red and green is on the X chromosome

Two alleles: one that makes the protein properly, and one that doesn’t

Most colorblind people are male

Two chances to get a good copy

Only one chance

Are there . . .

Two identical alleles?

The organism is homozygous for that

trait.

Two different alleles?

The organism is heterozygous for that

trait.