Mendel's Law

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Mendel's Law
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1. Who was Gregor Mendel?  
Mendel is considered the father of Genetics. He was a monk, biologist and botanist born in Austria in 1822 and who died in 1884. During the years 1853 to 1863 he cultivated pea plants in the gardens of his monastery to be used in his research. His experiments consisted of crossing pea plants of distinct characteristics (size, color of the seeds, etc.), cataloging the results and interpreting them. The experiments led him to enunciate his laws, results published in 1886 with no scientific repercussion at that time. Only at the beginning of the 20th century, in 1902, 18 years after his death, were his merits broadly recognized.



2. What in Genetics is hybridization?  
 Hybridization in Genetics is the crossing of individuals from “pure” and different lineages in relation to a given trait, i.e., the crossing of different homozygous for the studied trait.  In Mendel’s experiments with peas, for example, a plant from a green pea lineage obtained from self fecundation of its ascendants through several generations was crossed (cross fecundation) with another plant from a yellow lineage also obtained by self fecundation of ascendants. (The self fecundation through several generations of ascendants and the exclusive obtainment of individuals with the desired characteristics ensured that the individuals of the parental generation were “pure”, i.e., homozygous for that characteristic.)


3. What is monohybridism?  
 Monohybridism is the study of only one characteristic in the crossing of two pure individuals (hybridization) for that characteristic. 


4. Considering hybridization in a trait like the color of the flowers of a given plant species (red dominant/ yellow recessive) conditioned by a pair of different alleles, what are the phenotypical results of the first generation (F1) and the phenotypical results of the second generation (F2, formed by crossing among F1 genotypes)? What are the phenotypical proportions in F1 and F2?   
In relation to genotypes and phenotypes the hybridization comprises of: parental generation (P): RR (read), yy (yellow). F1 generation (RR x yy): Ry (red). F2 generation (Ry x Ry): RR (red), Ry (red), Ry (red) and yy (yellow).  In the F1 generation the proportion of red flowers is 100%. In the F2 generation, the phenotypical proportion is three red (75%) to one yellow (25%).
 

5. Considering hybridization in a trait like the color of the flowers of a given plant species (red/yellow) conditioned by a pair of different alleles in relation to complete dominance (red dominant/ yellow recessive), why in the F1 generation is one of the colors missing? 
  In this monohybridism one of the colors does not appear in the F1 generation because their parental generators are pure, i.e., homozygous, and in F1 all descendants are heterozygous (each parental individual forms only one type of gamete). Since only heterozygous genotypes appear and red is dominant over yellow the individuals of the F1 generation will present only red flowers.
 

6. Considering hybridization in a given trait like the color of the hair of a mammalian species (white/black) conditioned by a pair of different alleles under complete dominance (black dominant, B/ white recessive, w), how can the phenotypical proportion obtained in the F2 generation be explained? What is this proportion? 
  In the monohybridism conditioned by two different alleles the F1 generation presents only heterozygous individuals (Bw). In F2 there is one individual BB, two individuals Bw and one individual ww. In relation to the phenotype there are in F2 two black individuals and one white individual, since black is the dominant color. So the proportion is 3:1, three black-haired to one white-haired.
 

7. What is meant by saying that in relation to a given trait conditioned by a gene with two different alleles the gametes are always “pure”?  
 To say that gametes are pure means that they always carry only one allele of the referred trait. Gametes are always “pure” because in them the chromosomes are not homologous, they contain only one chromosome of each type. 
 

8. What is the Mendel’s first law?  
 The Mendel’s first law postulates that a characteristic (trait) of an individual is always determined by two factors, one inherited from the father and the other from the mother and the direct offspring of the individual receives from it only one of these factors (aleatory). In other words, each trait is determined by two factors that segregate during gamete formation.  The Mendel’s first law is also known as the law of purity of gametes. Mendel deduced the way genes and alleles were transmitted and traits were conditioned without even knowing of the existence of these elements.
 

9. Which is the type of gamete (for a given trait) produced by a dominant homozygous individual? What is the genotypical proportion of these gametes? What about a recessive homozygous individual?  
 If an individual is dominant homozygous, for example, AA, it will produce only gametes having the allele A. The proportion thus is 100% of AA gametes.  If an individual is recessive homozygous, for example, aa, it will produce only gametes having the allele a, also in a 100% proportion. 
 

10. Which is the type of gamete produced by a heterozygous individual? What is the genotypical proportion of these gametes?  
 Heterozygous individuals, for example, AA, produce two different types of gametes: one containing the allele A and another type containing the allele a. The proportion is 1:1. 

  

11. In the F2 generation of a hybridization for a given trait conditioned by a pair of alleles T and t, according to Mendel’s first law what are the genotypes of each phenotypical form? How many respectively are the genotypical and phenotypical forms?   In the mentioned hybridization the genotypical forms in F2 will be TT, tt and Tt. Therefore there will be three different genotypical forms and two different phenotypical forms (considering T dominant over t).
 

12. Why can the crossing of an individual that manifests dominant phenotype with another that manifests recessive phenotype (for the same trait) determine whether the dominant individual is homozygous or heterozygous?  
 From the crossing of an individual having recessive phenotype with another having dominant phenotype (for the same trait) it is possible to determine whether the dominant individual is homozygous or heterozygous. This is true because the genotype of the recessive individual is obligatorily homozygous, for example, aa. If the other individual is also homozygous, AA, the F1 offspring will be only heterozygous (aa x AA = only Aa). If the other individual is heterozygous there will be two different genotypical forms, Aa and aa in the 1:1 proportion. So if a recessive phenotype appears in the direct offspring the parental individual that manifests dominant phenotype is certainly heterozygous.
 

13. What is a genetic family tree?   
Genetic family tree is a schematic family tree that shows the biological inheritance of some trait through successive generations.  Genetic family trees are useful because it is practically impossible and ethically unacceptable to make experimental crossings for genetic testing between human beings. With the help of family tress the study is made by analysis of marriages (and crossings) that have already occurred in the past. From the analysis of family trees, for example, information on probabilities of the emergence of some phenotype and genotypes (including genetic diseases) in the offspring of a couple can be obtained.


14. What are the main conventional symbols and signs used in genetic family trees? 
  In genetic family trees the male sex is usually represented by a square and the female by a circle. Crossings are indicated by horizontal lines that connect squares to circles and their direct offspring are listed below and connected to that line. The presence of the studied phenotypical form is indicated by a complete hachure (shading) of the circle or the square correspondent to the affected individual. It is useful to enumerate the individuals from left to right and from top to bottom for easy reference.


15. What are the three main steps for a good study of a genetic family tree?  
 Step 1: to determine whether the studied phenotypical form has a dominant or recessive pattern. Step 2: to identify recessive homozygous individuals. Step 3: to identify the remaining genotypes.
 

16. What is Mendel’s second law?  
 Mendel’s second law postulates that two or more different traits are also conditioned by two or more pair of different factors and that each inherited pair separates independently from the others. In other words, gametes are formed always with an aleatory representative of each pair of the factors that determine phenotypical characteristics.  Mendel’s second law is also known as the law of independent segregation of factors, or law of independent assortment.
 

17. What is the condition for Mendel’s second law to be valid?  
 Mendel’s second law is only valid for genes located in different chromosomes. For genes situated in the same chromosome, i.e., linked genes (genes in linkage) the law is not valid since the segregation of these genes is not independent. 


18. According to Mendel’s second law, in the crossing between homozygous individuals concerning two pairs of nonlinked alleles, AABB x aaBB, what are the genotypical and phenotypical proportions in F1 and F2? 
  Parental genotypes: AABB, aaBB. Gametes from the parental generation: Ab and aB. Thus F1 will present 100% AaBb gametes (and the phenotypical correspondent form).  As F1 are AaBb individuals the gametes from their crossing can be: AB, Ab, aB and ab. The casual combination of these gametes forms the following genotypical forms: one AABB, two AABb, two AaBb, four AaBB, one Aabb, one Aabb, one aaBB, two aaBb and two aabb. The phenotypical proportion then would be: nine A_B_ (double dominant); three A_bb (dominant for the first pair, recessive for the second); three aaB_ (recessive for the first pair, dominant for the second); one aabb (double recessive).
 

19. Considering independent segregation of all factors, how many types of gametes does a VvXXWwYyzz individual produce? What is the formula to determinate such number?   
The mentioned individual will produce eight different types of gametes (attention, gametes and not zygotes).  To determine the number of different gametes produced by a given multiple genotype the number of heterezygous pairs is counted (in the mentioned case, three) and the result is placed as an exponent of two (in the example, 2.2.2 = 8).
 

20. How is it possible to obtain the probability of emergence of a given genotype formed of more than one pair of different alleles with independent segregation from the knowledge of the parental genotypes?  
 Taking as example the crossing of AaBbCc with aaBBCc, for each considered pair of allele it is possible to verify which genotypes it can form (as in an independent analysis) and in which proportion. AA x aa: Aa, aa (1:1). Bb x BB: BB, Bb (1:1). Cc x Cc: CC, Cc, cc (1:2:1). The genotype to which the probability is to be determined is for example aaBbcc. For each pair of this genotype the formation probability is determined: to aa, 0.5; to Bb, 0.5; to cc, 0.25. The final result is obtained by multiplication of these partial probabilities, 0.5 x 0.5 x, 0.5, resulting 0.0625.

 
05-19-2014 08:46 PM
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