Basic Genetics
Gregor Mendel
-used pea plants
-examined simple characteristics with a "yes" or "no" answer
Gene = unit of inheritance
e.g. flower color
Allele = particular version of a gene
e.g. red or white
| Organism | Number of Genes |
| virus | 3-300 |
| bacteria | 500-4000 |
| plants/animals | 5000-50,000 (2 copies) |
If a red colored flower is the wild type then that plant has a gene that encodes an enzyme to work on a precursor molecule and express the red color. The mutant has a mutant version of this enzyme gene in which no enzyme is made and therefore the flower is white.
1. Wild type allele
-as found in nature
-functional
2. Mutant allele
-defective version
Beadle and Tatum
"one gene - one enzyme" - exceptions do exist but this usually is true
Two-step Pathway
gene I gene II || || || || \/ \/ enzyme I enzyme II || || || || \/ \/ A + B =PIGMENT
If gene I is defective = no pigment
If gene II is defective = no pigment
If both are defective = no pigment
phenotype = description of observable characteristics
genotype = description of the genes
Chromosome = giant molecule of DNA carrying many genes
Haploid = has one copy of each gene
Diploid = has two copies of each gene
Homozygous = both alleles are identical
e.g. RR = red (homozygous dominant) R is dominant
rr = white (homozygous recessive) r is recessive
Heterozygous = one copy of each allele
The wild type is usually dominant, but this is not always the case
The nucleus of a somatic cell is diploid containing chromosome pairs. The cell undergoes meiosis and this "chooses" a single set of chromosomes at random. This haploid cell is a germ cell (egg or sperm).
Genetic Cross
Parents: Red Flowers (homozygous RR) and
White Flowers (homozygous rr)
Following meiosis, the gametes are:
RR --> R
rr --> r
F1 or First Filial Generation = Rr
This cross will yield flowers that are heterozygous and red.
Rr and Rr Cross of F1 generation
The F2 generation will have a 3:1 red:white phenotypic ratio.
The genotypes will be 1/4 RR, 1/4 Rr, 1/4 rR and 1/4 rr.
Checkerboard Cross of Rr and Rr
Parent #1 is horizontal across the top
Parent #2 is vertical down the left side
R |
r | |
R |
RR |
Rr |
r |
Rr |
rr |
1:2:1 = Genotypic ratio
3:1 = Phenotypic ratio
Sex Determination
The 23rd pair of chromosomes in humans are the X and Y
Female = XX
Male = XY
Sex-linked defects are only in males because have only one copy of the X chromosome.
X |
X | |
X |
XX |
XX |
Y |
XY |
XY |
There is a 1:1 ratio of producing a male (XY) or female (XX) child
Partial Dominance
RR = two doses of red pigment
Rr = one dose of red pigment
rr = no doses of red pigment
With dominance one copy of active gene is sufficient so Rr = Red
Less common situation -
Rr = pink = partial dominance, where one good copy is not enough
Codominance
R = makes red pigment
r = makes no pigment
B = makes altered, blue pigment
RR and Rr = red
BB and Br = blue
rr = white, no pigment
BR = PURPLE, a mixture of red and blue pigments because R and B are both functional alleles of this gene
Linkage
below are 2 pairs of chromosomes (DNA)
A B C r
-----------------------------------------
a b c R
-----------------------------------------
D E
---------------------
d e
----------------------
A, B, C, and r are linked
D is not linked to A, B, C, r linkage group
D is linked to E
gametes = ABCr, abcR
genes on the same molecule of DNA stay together
Recombination
The further apart genes are the more frequent the recombination.
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SIUC / College of Science / Microbiology / courses / MICR302
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Last updated: 4-Feb-99 / laa