2. What is hybridization in genetics?
In genetics, hybridization is the cross-breeding of individuals from different “pure” lineages in relation to a given trait, that is, the cross-breeding of different homozygous individuals for the studied trait.
In Mendel’s experiments with peas, for example, a plant from a green pea lineage obtained from the self-fertilization of its ancestors over several generations, was crossed (cross-fertilization) with another plant from a yellow lineage also obtained by self-fertilization of its ancestors. (The self-fertilization over several generations of ancestors and the exclusive obtainment of individuals with the desired characteristics ensured that the individuals of the parental generation were “pure”, or rather, homozygous for that characteristic.)
3. What is monohybridism?
Monohybridism is the study of only one characteristic in the cross-breeding of two pure individuals (hybridization) for that characteristic.
4. Considering hybridization in a trait such as 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 between F1 genotypes)? What are the proportions of phenotypes in F1 and F2?
In relation to genotypes and phenotypes, hybridization conists of: the parental generation (P): RR (red), 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 such as 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 is one of the colors missing in the F1 generation?
In monohybridism, one of the colors does not appear in the F1 generation because its parents are pure, or rather, homozygous, and, therefore, all individuals in F1 are heterozygous (each parent 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 such as the hair color 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 monohybridism conditioned by two different alleles, the F1 generation presents only heterozygous individuals (Bw). In F2, there is one BB individual, two Bw individuals and one ww individual. In relation to the phenotype, F2 contains three black individuals and one white individual, since black is the dominant color. Therefore, the proportion is 3:1: three black-haired to one white-haired.
7. What is meant by saying that the gametes are always “pure” when referring to a given trait conditioned by a gene with two different alleles?
To say that gametes are pure means that they always carry only one allele of the specified trait. Gametes are always “pure” because, in them, the chromosomes are not homologous; they contain only one chromosome of each type.
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Mendel's First Law
8. What is Mendel’s First Law?
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 that the direct offspring of the individual receives only one of these factors (random) from it. In other words, each trait is determined by two factors that separate during gamete formation.
Mendel’s First Law is also known as the law of purity of gametes. Mendel deduced the way genes and alleles were transmitted and how traits were conditioned without even knowing of the existence of these elements.
9. What type of gamete (for a given trait) is 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 with the allele A. The proportion is therefore 100% AA gametes.
If an individual is recessive homozygous, for example, aa, it will produce only gametes with the allele a, also a 100% proportion.
10. Which type of gamete is produced by a heterozygous individual? What is the genotypic 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 phenotypic form? How many genotypic and phenotypic forms are there, respectively?
In the mentioned hybridization, the genotypic forms in F2 will be TT, tt and Tt. Therefore, there will be three different genotypic forms and two different phenotypic forms (considering that T is dominant over t).
12. Why can the cross-breeding of an individual that presents a dominant phenotype with another that presents a recessive phenotype (for the same trait) determine whether the dominant individual is homozygous or heterozygous?
From the cross-breeding of an individual with a recessive phenotype with another with a 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 necessarily 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 genotypic forms, Aa and aa in a 1:1 proportion. Therefore, if a recessive phenotype appears in the direct offspring, the parent that presents the dominant phenotype is necessarily heterozygous.
Genetic Family Trees
13. What is a genetic family tree?
A genetic family tree is a schematic family tree that shows the biological inheritance of a certain trait through successive generations.
Genetic family trees are useful because it is practically impossible and ethically unacceptable to cross-breed human beings for genetic testing. With the help of family trees, the study is carried out through the analysis of marriages (and cross-breeding) that have already occurred in the past. From the analysis of family trees, for example, information on the probability of the emergence of a certain phenotype or genotype (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, males are usually represented by a square and females 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 corresponding to the affected individual. It is useful to number the individuals from left to right and from top to bottom for easy reference.
15. What are the three main steps to properly study a genetic family tree?
Step 1: determine whether the studied phenotypic form has a dominant or recessive pattern. Step 2: identify recessive homozygous individuals. Step 3: identify the remaining genotypes.
Mendel's Second Law
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 pairs of different factors, and that each inherited pair separates independently from the others. In other words, gametes are always formed with a random representative of each pair of the factors that determine phenotypical characteristics.
Mendel’s Second Law is also known as the 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 located in the same chromosome, such as linked genes (genes in linkage), the law is not valid since the assortment of these genes is not independent.
18. According to Mendel’s Second Law, in the cross-breeding of homozygous individuals concerning two pairs of unlinked alleles, AABB x aaBB, what are the genotypic and phenotypic proportions in F1 and F2?
Parental genotypes: AABB, aaBB. Gametes from the parental generation: Ab and aB. Therefore, F1 will present 100% AaBb gametes (and the corresponding phenotypical form).
As F1 consists of 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 would then 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); and one aabb (double recessive).
19. Considering the independent assortment of all factors, how many types of gametes does a VvXXWwYyzz individual produce? What is the formula to determinate that number?
That individual will produce eight different types of gametes (take not, gametes, not zygotes).
To determine the number of different gametes produced by a given multiple genotype, the number of heterozygous pairs is counted (in the case mentioned, three) and the result is calculated as an exponent of two (in the example, 2 x 2 x 2 = 8).
20. How is it possible to obtain the probability of the emergence of a given genotype formed of more than one pair of different alleles with independent assortment based on knowledge of the parental genotypes?
Taking as an example the crossing of AaBbCc with aaBBCc, for each allele pair considered, it is possible to verify which genotypes it can form (like 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 for which the probability is to be determined is for example aaBbcc. For each pair of this genotype, the formation probability is determined: for aa, 0.5; for Bb, 0.5; for cc, 0.25. The final result is obtained by the multiplication of these partial probabilities, 0.5 x 0.5 x, 0.5, resulting in 0.0625.