Biyo-Tek Günlüğüm…

Monohybrid Cross

A cross in which only one characteristic is tracked is a monohybrid cross.A cross between a pea plant that is true-breeding for producing purple flowers and one that is true-breeding for producing white flowers is an example of a monohybrid cross. Biologists use a diagram called a Punnett square, to aid them in predicting the probable distribution of inherited traits in the offspring.

Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box.

Monohybrid Cross: Homozygous Plants

The predicted genotype is Pp in every case. Thus, there is a 100 percent probability that the offspring will have the genotype Pp and thus the phenotype purple flower color.

Monohybrid Cross: Heterozygous Plants

The ratio of the genotypes that appear in offspring is called the genotypic ratio. The probable genotypic ratio of the monohybrid cross represented in above is 1 : 2 : 1. The ratio of the offspring’s phenotypes is called the phenotypic ratio. The probable phenotypic ratio of the cross represented in above is 3 : 1.

Determining Unknown Genotypes: Test Cross

Animal breeders, horticulturists, and others involved in breeding organisms often need to know whether an organism with a dominant phenotype is heterozygous or homozygous for a trait. In a test cross, an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual.

Questions:

The diagram below shows the expected results of a cross between two pea plants. T and t represent the alleles for the tall and dwarf traits, respectively. (Tallness is dominant) Use the figure below to answer questions 1–3.

1. What are the genotypes of the plants that were crossed?

2. What genotypic ratio is expected in the offspring of this cross?

3. If this cross produced 240 offspring, how many of the offspring would be expected to have the dwarf trait?

4. A genetic cross between two F1-hybrid pea plants for spherical seeds will yield what percent spherical-seeded plants in the F2 generation? (Recall, spherical-shaped seeds are dominant over dented seeds.)

5. A genetic cross between two F1-hybrid pea plants having yellow seeds will yield what percent green-seeded plants in the F2 generation? Yellow seeds are dominant to green.

6. To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.

7. Two short haired guinea pigs are mated several times. Out of 100 offspring, 25 of them have long hair. What are the probable genotypes of the parents?

8. If 355 short-winged fruit flies and 1065 long-winged fruit flies are obtained by mating two type of fruit flies, what are the genotypes of each fruit fly? ( Long wing allele is dominant to short wing allele)

9. A brown coat is dominant to a white coat in cattle. A farmer has a brown bull. How would you determine whether the bull is a heterozygote for the dominant allele?

10. In dogs, wiry hair (W) is dominant to smooth hair (w). If a homozygous wire-haired dog is bred with a smooth-haired dog, what sort of hair will their offspring have?