alleles.html: 14_04Alleles_L.jpg
For each observable trait (phenotype), an organism inherits 2 alleles,
one from each parent. These alleles make up its genotype.
If the 2 alleles at a locus (the region on a chromosome where a gene is found)
differ, the organism is heterozygous, otherwise it is homozygous.
assortment.html: 14_08IndependentAssort.jpg
Mendel's law of independent assortment.
The P plants are true-breeding: one with
yellow-round seeds and the other with green-wrinkled seeds.
The F1 dihybrids are heterozygous for both characters.
Self-pollination of the F1 yields
a phenotypic ratio of 9:3:3:1
in the F2,
NOT the 3:1 typical of a monohybrid cross.
Thus the 2 traits assort independently of each
other.
One way to ensure you have all the combination of gametes for a dihybrid cross is the
FOIL
method.
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| Genotype | Antigens on RBCs | Antibodies in Serum | Blood Group |
|---|---|---|---|
| IAIA or IAi | A | Anti-B | A |
| IBIB or IBi | B | Anti-A | B |
| IAIB | A and B | neither | AB |
| ii | neither | A and B | O |
bloodABO.html: 14_02ABObloodGroup_T.jpg
The A, B, AB, or O phenotypes are affected by 3 different alleles.
IA and IB alleles
produce different antigens on the surface of red blood cells, thus are dominant
to the
i allele which produces no antigen.
IA and IB
are codominant to each
other
because the RBCs bear both antigens.
color-add.html: 14_color-additive-RGB.gif
color.html: 14_color-subtractive-CMY.gif
epistasis.html: 14_11Epistasis.jpg
A gene at one locus may affect phenotypic expression of a gene at another locus by epistasis.
The C/c gene, which is epistatic to the B/b gene,
controls whether or not pigment of any color will be deposited in the hair.
A homozygous recessive cc mouse has no hair pigment and will be albino regardless of its B genotype.
genes.html: ../ch17/17_04GeneInfo.jpg
An observble character, or trait (phenotype), such as flower color,
is inherited in units called genes.
Alternative versions of a gene are called alleles.
An organism's total genes
is its genome.
genotype-phenotype.html: 14_06PhenotypeVsGenotype_L.jpg
Phenotype versus genotype.
A monohybrid cross yields a
3:1
phenotypic ratio in the F2,
assuming purple flower color is dominant and white flower color is recessive.
The genotypic ratio is
1:2:1,
since there are 2 types of purple–flowered plants, PP (homozygous) and Pp (heterozygous).
The true-breeding P generation must have identical alleles for that gene and are homozygous.
incomplete_dominance.html: 14_10IncompleteDominance.jpg
Incomplete dominance in snapdragon color.
When red snapdragons are crossed with white ones,
the F1 hybrids have pink flowers.
Superscripts indicate alleles for flower color: CR for red and CW for white.
The F2 generation produces a 1:2:1 ratio for both genotype and phenotype.
melanin.html: 14_12PolygenicInheritance.jpg
Human skin pigmentation is influenced by multiple genes which
produce different melanin
pigment molecules and shows quantitative variation.
This polygenic inheritance also exhibits incomplete dominance.
monohybrid.html: 14_03PurpleWhiteFlowers.jpg
EXPERIMENT
True-breeding purple-flowered plants and
white-flowered plants were crossed. The F1 hybrids were allowed to self-pollinate.
RESULTS
Both purple-flowered plants and white-flowered plants appeared in the F2 generation,
in a ratio of about 3 purple : 1 white.
Similar ratios were observed many other pea characters.
Mendel called the purple flower trait dominant,
and the white flower color trait recessive.
pea.html: 14_02CrossingPeaPlants.jpg
The garden pea has closed flowers and can self-fertilize, but also allow for manual cross-pollination.
APPLICATION
By crossing (mating) two true-breeding
varieties of the pea,
Mendel studied patterns of inheritance of flower color.
RESULTS
When pollen from a white flower fertilizes eggs of a purple flower,
the first-generation hybrids (F1) all have purple flowers.
The result is the same for the reciprocal cross,
the transfer of pollen from purple flowers to white flowers.
pea_characters.html: 14_01PeaPlantCharacters_T.jpg
pedigree.html: 14_14PedigreeAnalysisA.jpg
Pedigree analysis.
In these family trees, squares represent males and circles represent females.
Shaded squares and circles represent individuals who exhibit the trait.
A dominant trait such as widow's peak cannot skip a generation.
pedigree_recessive.html: 14_15cPedigreeAnalysis-L.jpg
Pedigree analysis.
A recessive trait such as attached earlobe may skip a generation.
A dot may be placed within a symbol to represent known
heterozygotes (carriers who do not exhibit the recessive phenotype).
Identify the carriers.
(
Hint
)
segregate.html: 14_09PunnettSquare.jpg
Segregation of alleles and fertilization as chance events.
When a heterozygote (Rr) forms gametes, segregation of alleles is like the toss of a coin.
We can determine the probability for any genotype among the offspring of 2 heterozygotes by
multiplying the individual probabilities of a gamete having a particular allele (R or r).
segregation.html: 14_05LawOfSegregation_2_L.jpg
Mendel's law of segregation.
Each plant inherits 1 allele for flower color from each parent.
The 2 alleles segregate (separate) and end up in different gametes during
meiosis.
To construct a Punnett
square, list all the possible female gametes along one side
and all the possible male gametes along the other side.
Random fertilization between gametes yield predictable ratios in the offspring.
segregation1.html: 14_05LawOfSegregationA.jpg
segregation2.html: 14_05LawOfSegregationB.jpg
segregation3.html: 14_05LawOfSegregationC.jpg
testcross.html: 14_07Testcross.jpg
An organism that exhibits a dominant trait,
such as purple flowers, can be either homozygous for
the dominant allele or heterozygous.
To determine the organism’s genotype, a testcross is done:
the individual with the dominant phenotype is crossed with a
recessive phenotype (white flowers), since we know the latter's genotype is
homozygous recessive.
By observing the phenotypes of the offspring, we can deduce the genotype of the purple-flowered parent.