The post Mendelian Inheritance: Part III appeared first on Plantlet.
]]>If you have read the first and second part you may have understood that this is our third and final part. In this part we are going to discuss
KEY CONCEPT
Genes encode proteins that produce a diverse range of traits.
Traits, Genes and Alleles
The same gene can have many versions called alleles.
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Genes influence the development of traits.
All of an organism’s genetic material is called the genome.
A recessive allele is expressed as a phenotype only when two copies are present. (rr = recessive; therefore the pea is wrinkled) 
These are few questions that might come in your mind:
Monohybrid Cross
Reciprocal Cross
Traits Present in the Progeny of the Hybrids
Mendel was the first to show that the characteristics of the progeny produced by a cross do not depend on which parent is the male and which the female. In this example, the seeds of the hybrid offspring are round whether the egg came from the round variety and the pollen from
the wrinkled variety (A) or the other way around (B).
The trait expressed in the hybrids Mendel called the dominant trait; the trait not expressed in the
hybrids he called recessive.
The principal observations from the experiments were:
Mendel’s Genetic Hypothesis and Its Experimental Tests
Two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half of the gametes carry one allele and the other half carry the other allele.
Principle of Segregation
Confirmation of Mendel’s First Law Hypothesis
True Breeding Plant
A truebreeding plant is one that, when selffertilized, only produces offspring with the same traits. The alleles for these type of plants are homozygous. Examples: The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A truebreeding round seed plant would be (RR) for that trait and a truebreeding wrinkled seed plant would be (rr). 
Dihybrid Cross
The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs.
Meiosis
The results of this experiment led Mendel to formulate his second law
A dihybrid cross tracks two traits. Both parents are heterozygous, and one allele for each trait exhibits complete dominance . This means that both parents have recessive alleles, but exhibit the dominant phenotype. The phenotype ratio predicted for dihybrid cross is 9:3:3:1.
The Principle of Independent Assortment:
Segregation of the members of any pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells.
Or
Members of one gene pair segregate independently from other gene pairs during gamete formation
Confirmation of Mendel’s second Law Hypothesis
Mendel confirmed the results of his second law by performing a backcross – F1 dihybrid x recessive parent.
Let’s use the example of the yellow, round seeded F1.
The Backcross YyRr X yyrr
⇓ ⇓
Gametes YR Yr yR yr yr
The phenotypic ratio of the test cross is:
The ratio is 1:1:1:1
A Trihybrid Cross Example Using Mendel’s Sweet Peas
A trihybrid cross is between two individuals that are heterozygous for three different traits.
Our trihybrid cross example:
RrYyCc x RrYyCc is a trihybrid cross.
The shape of the pea is controlled by one set of alleles, where round is completely dominant to wrinkled: RR = round Rr = round rr = wrinkled 
The second set of alleles in this example controls the color of the peas. Green is
dominant to yellow: YY = green Yy = green yy = yellow 
The third set of alleles in this example controls the shape of the pea pod. Smooth is completely dominant to constricted: CC = smooth Cc = smooth cc = constricted 
The gametes for each parent in a trihybrid cross would be RYC, RYc, RyC, Ryc, rYC, rYc, ryC, ryc, with oneeighth of a chance for any of them.
27:9:9:9:3:3:3:1 ratio: a trihybrid cross yields a phenotypic ratio of 27:9:9:9:3:3:3:1. This reflects the phenotypes generated by the 64 genotypic combinations resulting from 8 different male gametes fertilizing 8 different female gametes.
Punnett square
A diagram that predicts the expected outcome of a genetic cross by considering all combinations of gametes in the cross.
F1:
F2:
Phenotypic ratio: 1:2:1 (3:1)
In case of dihybrid cross:
YYRR X yyrr Parent generation
⇓ ⇓
YR yr Gametes
⇓
YyRr F1
Gamete Formation: Trihybrid cross
What size of Punnett square needed for analysis?
Trihybrid Cross – Phenotypes Forkedline Method
Probability in Mendelian inheritance
A working knowledge of the rules of probability for predicting the outcome of chance events is basic to understanding the transmission of hereditary characteristics.
Mendelian inheritance reflects rule of probability
Laws of probability help explain genetic events
Genetic ratios are most properly expressed as probabilities:
Ex. 3/4 tall: 1/4 dwarf
The probability of each zygote having the genetic potential for becoming tall is 3/4, etc.
How do we calculate the probability of 2 or more events happening at the same time?
Rule of multiplication (AND)
Rule of Multiplication
Probability that 2 coins tossed at the same time will land heads up
Chance of tossing 1 with first coin = 1/2
Chance of tossing 1 with second coin = 1/2
Chance of rolling two 1’s = 1/2 X 1/2 = ¼
Similarly, the probability that a heterogyzous pea plant (Pp) will produce a whiteflowered offspring (pp) depends on an ovum with a white allele mating with a sperm with a white allele.
This probability is 1/2 x 1/2 = 1/4.
The rule of multiplication also applies to dihybrid crosses.
Rule of Addition
The ChiSquare Test
An important question to answer in any genetic experiment is how can we decide if our data fits any of the Mendelian ratios we have discussed. A statistical test that can test out ratios is the ChiSquare or Goodness of Fit test.
ChiSquare Formula:
Degrees of freedom (df) = n1 where n is the number of classes
Let’s test the following data to determine if it fits a 9:3:3:1 ratio.
Number of classes (n) = 4
df = n1
= 41 = 3
Chisquare value = 0.47
By statistical convention, we use the 0.05 probability level as our critical value. If the calculated chisquare value is less than the 0 .05 value, we accept the hypothesis. If the value is greater than the value, we reject the hypothesis. Therefore, because the calculated chisquare value is greater than the we accept the hypothesis that the data fits a 9:3:3:1 ratio.
We have finished our first topic of 2nd year Fundamental Genetics syllabus. Stay with us for the next topics.
The post Mendelian Inheritance: Part III appeared first on Plantlet.
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