Mendelian
Genetics |
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Summary Outline of Crosses & Squares: A. Punnett Square 1. Purpose 2. Example B. Monohybrid Cross 1. Definition 2. Pea Plant Example 3. Alternative Method 4. Genotypic Outcome 5. Phenotypic Outcome C. Test Cross 1. Purpose 2. Pea Plant Example 3. Results D. Dihydrid Cross 1. Definition 2. Product Rule 3. Law of Independent Assortment 4. Pea Plant Example 5. Phenotypic Outcome E. Incomplete Dominance 1. Definition 2. Example 3. Clue F. Codominance 1. Definition 2. Example 3. Clue G. Linkage Groups 1. Definition 2. Clue H. Crossing-over 1. Definition 2. Recombinant Chromosomes |
A. Punnett
Square
1. Purpose
- To use a diagramatic method for predicting genotypes and phenotypes
2. Example:
R | R | |
R | RR | RR |
r | Rr | Rr |
Key to the Punnett
Square:
Gray Boxes - Parental Alleles
White Boxes - Possible Offspring Genotypes
B. Monohybrid Cross
1. Definition
- a genetic cross in which only one trait of the parents is of interest
2. Pea Plant
Example:
a. Purple Flower (PP) * White Flower (pp) - P1Cross-Pollination
P | P | |
p | Pp | Pp |
p | Pp | Pp |
F1 Genotypic
Outcome = All Pp
F1 Phenotypic Outcome = All Purple Flowers
b. Purple Flower (Pp)
* Purple Flower (Pp) - F1Self-Pollination
P | p | |
P | PP | Pp |
p | Pp | pp |
F2 Genotypic Outcome = 1PP : 2Pp : 1pp
F2 Phenotypic Outcome = 3 Purple : 1 White
3.
Alternative Method - Employs a mathematical calculation for allelic
possibilities in the male and female gametes, rather than a Punnett Square
a. Purple Flower (Pp) * Purple Flower (Pp) - F1Self-Pollination
b. (1/2P + 1/2p)(1/2P + 1/2p) = 1/4PP + 1/2Pp +
1/4pp
c. F2 Genotypic Outcome = 1PP : 2Pp : 1pp
F2 Phenotypic Outcome = 3 Purple : 1 White
4. Genotypic
Outcome:
** Monohybrid Cross = 1 : 2 : 1
5. Phenotypic
Outcome:
** Monohybrid
Cross = 3 : 1
C. Test Cross
1. Purpose
- To determine an organism's genotype when the dominant phenotype is
expressed. The organism is crossed with an individual that is homozygous
recessive for the trait in question.
2. Pea Plant
Example - What is the genotype of a Purple Flowered Second Filial
Plant? It could be Pp or PP.
Option #1 - Heterozygous
P | p | |
p | Pp | pp |
p | Pp | pp |
Option #2 - Homozygous Dominant
P | P | |
p | Pp | Pp |
p | Pp | Pp |
3. Results:
a. If there are any offspring from the test cross that show the recessive
phenotype (white flowers here), then the tested individual must be heterozygous.
b. If all of the offspring from the test cross show the dominant
phenotype (purple flowers here), then the tested individual must be homozygous
dominant.
D. Dihydrid Cross
1. Definition -
A genetic cross in which two traits of the parents are of interest.
2. Product Rule
- The probability of independent events occurring at the same time is the
product of their individual probabilities. Therefore, a dihybrid cross is
basically the product of two monohybrid crosses.
Example - The probability of tossing two coins and getting two tails
is.....(1/2)*(1/2) = 1/4
3. Law of Independent
Assortment - The alleles for one trait may be distributed to the gametes
independently of the alleles for other traits.
4. Pea Plant
Example
a.
Smooth, Yellow (SSYY) * Wrinkled, Green (ssyy)- P1True-breeding
Cross-Pollination
SY | |
sy | SsYy |
F1 Genotypic
Outcome = All SsYy
F1 Phenotypic Outcome = All Smooth, Yellow
b. SsYy
* SsYy - F1Self-Pollination
SY | Sy | sY | sy | |
SY | SSYY | SSYy | SsYY | SsYy |
Sy | SSYy | SSyy | SsYy | Ssyy |
sY | SsYY | SsYy | ssYY | ssYy |
sy | SsYy | Ssyy | ssYy | ssyy |
F2 Genotypic and Phenotypic Outcome:
9 S_Y_ Smooth Yellow
3 S_yy Smooth Green
3 ssY_ Wrinkled Yellow
1 ssyy Wrinkled Green
5. Phenotypic
Outcome:
** Dihybrid
Cross = 9 : 3 : 3 : 1
E. Incomplete
Dominance
1. Definition -
A genetic situation where neither allele masks the presence of the other;
therefore, the heterozygote has a different genotype and phenotype from both the
dominant homozygote and the recessive homozygote.
2. Example -
Flower Color in Snapdragons
a. Alleles Involved:
R = Red Flower
r = White Flower
b. P1 - RR (red) * rr (white)
c. F1 - Rr (pink)
d. F2 - 1 RR (red) : 2 Rr (pink) : 1 rr (white)
3. Clue -
Heterozygote has a phenotype that is intermediate
between a homozygous dominant and homozygous recessive phenotype.
F. Codominance
1. Definition -
A genetic situation in which both dominant and recessive phenotypes are
expressed independently in the heterozygote, rather than producing an
intermediate phenotype as incomplete dominance does.
2. Example -
Sickle Cell Anemia in Humans
a. Alleles Involved:
N = Normal Red Blood Cell (circular in shape)
n = Sickled Red Blood Cell (crescent moon shape)
b. Phenotypes Involved:
NN = Normal Red Blood Cells
nn = Sickle Cell Anemia (Sickled Red Blood Cells cannot carry enough oxygen
resulting in this condition)
Nn = Normal AND Sickled Red Blood Cells
3. Clue -
Heterozygote has a phenotype that
shows both the
recessive and dominant phenotypes
G. Linkage Groups
1. Definition -
Genes that are inherited as a pair or group because they are located close
together on the same chromosome.
2. Clue -
Genotypic and phenotypic ratios from a cross involving linked genes will differ
from those normally expected from monohybrid and dihybrid crosses.
H. Crossing-over
1. Definition -
The exchanging of DNA segments between homologous chromosomes during Prophase I
of Meiosis I.
2. Recombinant
Chromosomes - chromsomes with new combinations of alleles which result from
crossing over
a. The farther apart two genes are on a chromosome, the greater their
chances of crossing over.
b. The closer together two genes are on a chromosome, the more often they
are linked.
c. Physical locations for genes can be determined simply by knowing the
crossing over percentages.
Example:
Crossing Over Percentage:
1) AB = 90%
2) BC = 7%
3) AC = 12%
Physical Location on Chromosome:
A
C
B
To test your knowledge about Genetic Crosses and Squares, click on the Cross Questions Link at the top of this page. After you answer the questions, be sure to check your responses by clicking on the Cross Answers Link.
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