Tasks for monohybrid crossing involve the analysis of the inheritance of traits determined by the alleles of one gene. To successfully solve these problems, it is necessary to know the mechanisms of meiosis, the rules for the purity of gametes, the law of uniformity of hybrids of the first generation, the law of splitting, and the types of interaction of allelic genes. Knowledge of the following concepts is required:

complete dominance – a type of interaction of allelic genes, the result of which is the formation of a trait (or traits) in heterozygotes, determined by one of a pair of allelic genes;

incomplete dominance – a type of interaction of allelic genes, as a result of which heterozygotes have an intermediate manifestation of a trait compared to the traits that are determined by each of the alleles in the homozygous state;

coding – a type of interaction of allelic genes, in which case in the heterozygote each of the genes has an independent phenotypic manifestation;

allelic exclusion – a type of interaction of allelic genes, when one allele dominates in some cells of a heterozygous organism, and another – in others;

overdominance – a stronger manifestation of a trait in a hybrid, compared with both recessive and dominant homozygotes.

Task 1.

What pairs are the most profitable to cross to obtain platinum foxes, if platinum dominates over silver, but in the homozygous state, the platinum gene causes the death of the embryo?

Answer: It is most profitable to cross silver and platinum heterozygous foxes.

Task 2.

When crossing two white pumpkins in the first generation, ¾ of the plants were white and ¼ were yellow. What are the genotypes of the parents if white is dominant over yellow?

Answer: Parent plants are heterozygous.


1. In humans, the farsightedness gene dominates the normal vision gene. In the family, the husband and wife suffer from farsightedness, but the mothers of both spouses saw normally. What are the probabilities of having children with normal vision in this family?

2. The black color gene in cattle dominates the red color gene. What offspring will result from crossing a purebred black bull with red cows? What will be the offspring from crossing F1 hybrids with each other? What calves will be born from a red bull and hybrid cows?

3. Standard minks have brown fur, while Aleutian minks have bluish-gray fur. Brown color is dominant. What offspring will result from crossing representatives of two breeds? What happens when you cross F1 hybrids? What will be the result of crossing an Aleutian father with his hybrid daughter?

4. Immunity to smut in oats dominates susceptibility to this disease. What offspring will be obtained from crossing homozygous immune individuals with homozygous plants affected by smut? What happens when these hybrids are crossed? What will be the result of crossing F1 plants with a parental form that lacks immunity?

5. A yellow guinea pig, when crossed with a white guinea pig, always produces cream-colored offspring. Crossing cream pigs with each other always gives a split: 1 part yellow to 2 cream and 1 white. Why?

6. When crossing ermine rooster and chicken, 46 chickens were obtained. Of these, 24 stoats, 12 blacks and 10 whites. How is ermine coloration inherited? What parents should be taken to get only ermine chickens?

7. Two black female mice were crossed with a brown male. One female in several litters gave birth to 20 black and 17 brown offspring, and the second a total of 33 black. Determine the genotypes of parents and offspring.

8. Galactosemia (inability to absorb milk sugar) is inherited as an autosomal recessive trait. The development of the disease can be prevented by a special diet. What is the probability of having sick children in a family where one of the spouses is homozygous for the galactosemia gene, but the development of metabolic disorders in him is prevented by the diet, and the second is heterozygous for galactosemia? What is the probability of having healthy children in this family?

9. A man with brachydactyly (short fingers), whose father, maternal grandmother and great-grandmother were also carriers of this trait, marries a woman with normal fingers. What gene (dominant or recessive) determines brachydactyly? What is the probability of having a child with brachydactyly in this family?


Task 1.

When purebred white chickens are crossed with each other, the offspring turns out to be white, and when black chickens are crossed, they are black. The offspring from a white and black individual turns out to be motley. What kind of plumage will the descendants of a white rooster and a motley hen have?

Answer: half of the chickens will be white and half variegated

Task 2.

Plants of red-fruited strawberries, when crossed with each other, always give offspring with red berries, and plants of white-fruited strawberries – with white ones. As a result of crossing these varieties with each other, pink berries are obtained. What offspring will arise when hybrids with pink berries are crossed?

Answer: half of the offspring will be with pink berries and 25% with white and red.

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