EXERCISES

EXERCISE 1. READ AND ANSWER THE QUESTIONS MENDELIAN GENETICS

Mendel explained how a dominant allele can mask the presence of a recessive allele.

There are many different breeds of dogs, such as Labrador retrievers, dachshunds, German shepherds, and poodles. You might like a certain breed of dog because of its height, coat clor, an genral appearance. These traits are passed from generation to generation. The work of an Austrian monk led to a greater understanding of how genetic traits are passed on to the next generation.

How Genetics Began

In 1866, Gregor Mendel, an Austrian monk and a plant breeder, published his findings on the method and the mathematics of inheritance in garden pea plants. The passing of traits to the next generation is called inheritance, or heredity. Mendel was successful in sorting out the mystery of inheritance because of the organism he chose for his study – the pea plant. Pea plants are easy to grow and many are true-breeding, meaning that they consistently produce offspring with only one form of a trait.

Pea plants usually reproduce by self-fertilization. A common occurrence in many flowering plants, self-fertilization occurs when a male gamete within a flower combines with a female gamete in the same flower. Mendel also discovered that pea plants could easily be cross-pollinated by hand. Mendel performed cross-pollination by transferring a male gamete from the flower of one pea plant to the female reproductive organ in a flower of another pea plant.

Mendel rigorously followed various traits in the pea plants he bred. He analyzed the results of his experiments and formed hypotheses concerning how the traits were inherited. The study of genetics, which is the science of heredity, began with Mendel, who is regarded as the father of genetics.

The Inheritance of Traits

Mendel noticed that certain varieties of garden pea plants produced specific forms of a trait, generation after generation. For instance, he noticed that some varieties always produced green seeds and others always produced yellow seeds. In order to understand how these traits are inherited, Mendel performed cross pollination by transferring male gametes from the flower of a true-breeding green-seed plant to the female organ of a flower from a true-breeding yellow-seed plant. To prevent self-fertilization, Mendel removed the male organ from the flower of the yellow-seed plant. Mendel called the green-seed plant and the yellow-seed plant the parent generation -also known as the P generation. Figure 1.

F₁ and F₂ generations. When Mendel grew the seeds from the cross between the green-seed and yellow-seed plants, all of the resulting offspring had yellow seeds. The offspring of this P cross are called the first filial (F₁) generation. The green-seed trait seemed to have disappeared in the F₁ generation, and Mendel decided to investigate whether the trait was no longer present or whether it was hidden, or masked.

Mendel planted the F₁ generation of yellow seeds, allowed the plants to grow and self-fertilize, and then examined the seeds from this cross. The results of the second filial (F₂) generation – the offspring from the F₁ cross-Figure1. Of the seeds Mendel collected, 6022 were yellow and 2001 were green, which almost is a perfect 3:1 ratio of yellow to green seeds.

Mendel studied seven different traits – seed or pea color, flower color, seed pod color, seed shape or texture, seed pod shape, stem length, and flower position – and found that the F₁ generation plants from these crosses also showed a 3:1 ratio.

Genes in pairs Mendel concluded that there must be two forms of the seed trait in the pea plants – yellow-seed and green-seed – and that each was controlled by a factor, which now is called an allele. An allele is defined as an alternative form of a single gene passed from generation to generation. Therefore, the gene for yellow seeds and the gene for green seeds are each different forms of a single gene.

Mendel concluded that the 3:1 ratio observed during his experiments could be explained if the alleles were paired in each of the plants. He called form of the trait that appeared in the F₁ generation dominant and the form of the trait that was masked in the F₁ generation recessive. In the cross between yellow-seed plants and green-seed plants, the yellow seed was the dominant form of the trait and the green seed was the recessive form of the trait.

Dominance. When he allowed the F₁ generation to self-fertilize, Mendel showed that the recessive allele for green seeds had not disappeared but was masked. Mendel concluded that the green-seed form of the trait did not show up in the F₁ generation because the yellow-seed form of the trait is dominant and masks the allele for the green-seed form of the trait.

Because the yellow-seed form of the trait is dominant, the allele for the yellow-seed form of the trait is represented by a capital Y. The allele for the green-seed form of the trait is represented by a lowercase y because it is recessive. An organism with two of the same alleles for a particular trait is homozygous for the trait. Homozygous, yellow-seed plants are YY and green-seed plants are yy. An organism with two different alleles for a particular trait is heterozygous for the trait, in this case Yy. When alleles are present in the heterozygous state, the dominant trait will be observed.

Genotype and phenotype. A yellow-seed plant could be homozygous or heterozygous for the trait form. The outward appearance of an organism does not always indicate which pair of alleles is present. The organism’s allele pairs are called its genotype. In the case of plants with yellow seeds, their genotypes could be YY or Yy. The observable characteristic or outward expression of an allele pair is called the phenotype. The phenotype of pea plants with the genotype yy will be green seeds.

Mendel’s law of segregation. Mendel used homozygous yellow-seed and green-seed plant in his P cross.

In Figure 2, the first drawing shows that each gamete from the yellow-seed plant contains one Y. Recall that the chromosome number is divided in half during meiosis. The resulting gametes contain only one of the pair of seed-color alleles. The second drawing shows that each gamete from the green-seed plant contains one y allele. Mendel’s law of segregation states that the two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite. The third drawing the alleles uniting to produce genotype Yy during fertilization. All resulting F₁ generation plants will have the genotype Yy and will have yellow seeds because yellow is dominant to green. These heterozygous organism are called hybrids.

Monohybrid cross. Mendel continued his experiments by allowing the Yy plants to self-fertilize. A cross such as this one that involves hybrids for a single trait is called a monohybrid cross. The Yy plants produce two types of gametes – male and female – each with either the Y or y allele. The combining of these gametes is random event. This random fertilization of male and female gametes results in the following genotypes – YY, Yy. Yy or yy, as shown Figure 3. Notice that the dominant Y allele is written first, whether it came from the male or female gamete. In Mendel’s F₁ cross, there are three possible genotypes: YY, Yy, and yy; and the genotypic ratio is 1:2:1. The phenotyoic ratio is 3:1 -yellow seeds to green seeds.

Dihybrid cross. Once Mendel established inheritance patterns of a single trait, he began to examine simultaneous inheritance of two or more traits in the same plant. In garden peas, round seeds ® are dominant to wrinkled seeds (r ), and yellow seeds (Y) are dominant to green seeds (y). If Mendel crossed homozygous yellow, round-seed pea plants, the P cross could be represented by YYRR x yyrr. The F₁ generation genotype would be YyRr – yellow, round-seed plants. These F₁ generation plants are called dihybrids because they are heterozygous for both traits.

Law of independent assortment. Mendel allowed F₁ pea plants with the genotype YyRr to self-fertilize in dihybrid cross. Mendel calculated the genotypic and phenotypic ratios of the offspring in both the F₁ and F₂ generations. From these results, he developed the law of independent assortment, which states that a random distribution of alleles occurs during gamete formation. Genes on separate chromosomes sort independently during meiosis.

As shown in Figure 3, the random assortment of alleles results in four possible gametes: YR, Yr, yR or yr, each of which is equally likely to occur. When a plant self-fetilizes, any of the four allele combinations could be present in the male female gamete. The results of Mendel’s dihybrid cross included nine different genotypes: YYRR, YYRr, YYrr, YyRR, YyRr, Yyrr, yyRR, yyRr and yyrr. He counted and recorded four different phenotypes: 315 yellow round, 108 green round, 101 yellow wrinkled and 32 green wrinkled. These results represent a phenotypic ratio of approximately 9:3:3:1.

QUESTIONS:

1) Explain the differences between the vocabulary terms in the following sets:

a) Dominant/recessive

b) Genotype/phenotype

2) If a black guinea pig (Bb) were crossed with a white guinea pig (bb) what would be the resulting phenotypic ratio?

a) 0:1 black to white

b) 1:0 black to white

c) 1:1 black to white

d) 3:1 black to white

3) Describe Mendel’s laws.

4) In garden peas, purple flowers (P) are dominant to white (p) flowers, and tall plants (T) are dominant to short plants (t). If a purple tall plant (PpTt) is crossed with a white short plant (pptt), what is the resulting phenotypic ratio?

a) 1:1:1:1 purple tall to purple short to white tall to white short

b) 3:2 purple tall to purple short

c) 9:3:3:1 purple tall to purple short to white tall to white short

d) All purple tall

5) Write the term or phrase that best completes each statement. Use these choices:

cross-pollination

dominant

gametes

inherited

recessive

self-fertilization

trait

1. Mendel was the first person to succeed in predicting how traits are ­­­­­­ from generation to generation.

2. In peas, both male and female sex cells, which are called ­­­ , are in the same flower.

3. ­­­­­­­­­­­­­­­­­­­­­­­­­­ occurs when a male gamete fuses with a female gamete in the same flower.

4. Mendel used the technique called to breed one plant with another.

5. Mendel studied only one at a time and analyzed his data mathematically.

6. In individuals with a heterozygous genotype, the allele of a trait is hidden by the expression of the other phenotype.

7. In individuals with a heterozygous genotype, the allele of a trait is visible in the phenotype.

6) Explain why sexual reproduction is better in a frequently changing environment.

7) Use each of the terms below only once to complete the passage:

dihybrid -gene-genotypes-monohybrid-phenotypic ratio

A cross between plants that involves one characteristic is called a cross. Mendel also performed crosses, which involve two pairs, with pea plants. When he crossed two pea plants that were heterozygous for both seed shape (Rr) and for seed color (Yy), he observed a 9:3:3:1 among the seeds of the offspring. A Punnett square shows the possible phenotypes and of the offspring.

EXERCISE 2: GENETICS ACTIVITIES

1- Els individus que manifesten un caràcter recessiu. Són homozigots o heterozigots pel caràcter? Per qué?

2- L’acondroplàsia és una forma d’enanisme degut a un creixement anormalment petit dels ossos llargs, que s’hereda per un únic gen. Dos nans acondroplàsics es van casar i van tenir un fill acondroplàsic i després un fill normal.

a) És la acondroplàsia un caràcter dominant o recessiu? ¿Per qué?

b) Quins són els genotips dels pares?

3- La llana negra dels xais es deguda a un al·lel recessiu, n, i la llana blanca al seu al·lel dominant, N. Al creuar un xai blanc amb una ovella negra, en la descendència va apareixer un xai negre.

Quins són els genotips dels parentals?

4- La bella de nit(Mirabilis jalapa), el color vermell de les flors ho determina l’al·lel C^R, dominant incomplet sobre el color blanc produit per l’al·lel C^B, siguent roses les flors de les plantes heterozigòtiques. Si una planta amb flors vermelles es creua amb una altra de flors blanques:

a) Quin serà el fenotip de les flors de l’F₁ i de l’F₂ resultant de creuar entre sí dues plantes qualsevols de l’F₁ ?

b) Quin será el fenotip de la descendència obtinguda d’un creuament de les F₁ amb el seu progenitor vermell, i amb el progenitor blanc?

5- Un granger ha creuat dues linia pures de gallines, unes de plomatge marró (M) i cresta senzilla (s) i altres de plomatge blanc (m) i cresta en roseta (S). Si els caràcters marró i cresta roseta són dominants.

Quines proporcions fenotípiques s’obtindran a l’F₂?