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Genetics Problems

1. Every organism has number of chromosomes for each trait. However, they can only pass on chromosome for each trait to their offspring (kids).

Since this is true, organisms must have a way to pass on just ½ of their genetic material (genes). This is accomplished by MEIOSIS. During MEIOSIS, chromosomes are separated and are prepared to be passed on. After they are separated, we now can considered each a separate part. This is called the law of independent assortment. It is also the basis of how to do genetics problems.

How do we solve genetics problems?

DOMINANT/RECESSIVE TYPE INHERITANCE

In dominant/recessive type inheritance, there is a purely dominant gene and a purely recessive gene. We start to solve problems by assigning LETTERS (symbols) to each gene. The dominant genes get to be CAPITAL letters, the recessive genes get to be lower case letters. Then we figure out the genotypes of all the individuals involved. Then we do a punnett square to finish the problem.

For each of the problems below, you need to complete the following pieces. A.) Assign the letters to the genes, B.) Write out the genotypes of the parents that are to be crossed/mated, C.) Do the punnett square, D.) Write out all of the possible genotypes and the chances that each of the genotypes might happen, E.) Write out all of the possible phenotypes and the chances that each of the phenotypes might happen.

Example:

Black fur is dominant over having white fur in polar bears. If two heterozygous black polar bears mate, what are their chances of having a white fur cub?

A. B = Black fur C. D. __ __ = ¼ or 25%

b = white fur __ __ = 2/4 or 50%

__ __ = ¼ or 25%

B. X E. = ¾ or 75%

= ¼ or 25%

Problems

1. In a certain species of rat, black color dominates white. Cross a homozygous black rat with a white rat.

A. ______C. D. ______

______

B. ______

E. ______

______

2. Using the traits in problem #1, cross two heterozygous black rats.

A. ______C. D. ______

______

B. ______

E. ______

______

3. In garden peas, tall plants dominate short plants. Cross a heterozygous tall with a short.

A. ______C. D. ______

______

B. ______

E. ______

______

4. Using the traits in #3, cross a heterozygous plant with a homozygous tall one.

A. ______C. D. ______

______

B. ______

E. ______

______

5. In wild flowers, red dominates white wild flowers. Cross a white flower with heterozygous red flower.

A. ______C. D. ______

______

B. ______

E. ______

______

6. Using the traits in #5, cross homozygous red with heterozygous red.

A. ______C. D. ______

______

B. ______

E. ______

______

7. Tall is dominant over dwarf. A homozygous plant for tall is crossed with one homozygous for dwarf to create the first generation of offspring (F1). Show as much work as you can.

a.  What will be the genotypic and phenotypic proportions of the F1?

b.  Cross one offspring from F1 with another from F1. What will be the genotypic & phenotypic ratios for this F2?

c.  Cross an F1 with its tall parent. What will be the genotypic & phenotypic ratios for this F2?

d.  Cross an F1 and its dwarf parent. What will be the genotypic & phenotypic ratios for this F2?

8. A tall plant crossed with a dwarf one produces offspring of which about 1/2 are tall and 1/2 are dwarf. What are the genotypes of the parents?

9. Lack of pigment in the human body is an abnormal recessive trait called "albinism". Using "N" and "n" to represent the dominant (normal) allele and the recessive (albino) allele respectively, three genotypes and two phenotypes are possible, list them.

10. An extra finger in humans (polydactylism) is rare but is due to a dominant gene. When one parent is normal and the other parent has an extra finger but is heterozygous for the trait, what is the probability that their first child will be normal? Their second child? Their third child? Explain.

11. A dominant gene "G" is responsible for the gray body color of fruit flies: its recessive allele "g" produces black body color. A cross of a gray female with a black male produces 52 black and 58 gray in the F1. If the gray F1 females are crossed to their black F1 brothers, what genotypic and phenotypic ratios would be expected in the F2?