Dihybrid Problems

(Two traits being followed from one generation to the next.)

You will still use the Punnett square, only each egg or sperm will have alleles for two different genes and the square will have sixteen boxes instead of four.

4. In horses, black color (B) dominates chestnut color (b). The trotting gait (T) dominates the pacing gait (t). A cross is made between a horse homozygous for black color and the pacing gait, and a horse which is homozygous for both chestnut color and trotting gait. List the probable genotypes and phenotypes of their offspring.

5. What possible offspring could result from crossing two of the offspring from problem number four?

6. Humans may have Rh+ blood or Rh- blood. Rh+ is dominant. Normal insulin production is dominant over abnormal insulin production (diabetes mellitus).

If both parents are heterozygous for the Rh factor and for insulin production, what is the phenotype and genotype of the parents? What will be the phenotype(s) and genotype(s) of their children?

7. If the father is homozygous for Rh+ blood and has diabetes mellitus and the mother has Rh- blood and is homozygous for insulin production, what phenotypes could they produce in their offspring?

8. In humans, there is a type of blindness due to a dominant allele; normal vision is the result of a recessive allele in this case. Migraine headaches are due to a dominant allele and normal (no headaches) is recessive.

A man who is blind and does not suffer from headaches marries a woman who has normal vision and suffers from migraines. Could they produce a child with normal vision who does not suffer from headaches? If yes, can the probability of such a child be determined?

Sex-linked Genes

There are genes located on the X chromosome which have no corresponding allele on the Y chromosome. (The Y chromosome is shorter than its homolog.) Females carry two X chromosomes; males carry one X and one Y. Thus males have only one allele of some genes carried by the X chromosome.

9. In humans, the condition for normal blood clotting dominates the condition for non-clotting (hemophilia). Both alleles are linked to the X chromosome.

A male hemophiliac marries a woman who is a carrier (i.e. heterozygous). What are the chances, if they have a son, that he will be hemophiliac?

10. A male who has normal blood clotting marries a woman who is a carrier for this condition. What are the chances, that if they have a son, he will be normal for blood clotting?

11. In humans, the condition for normal vision dominates color blindness. Both alleles are linked to the X chromosome.

A color blind male marries a color blind female. If they have a daughter, what are the chances that she will have normal vision?

12. A normal visioned man marries a color blind woman. She gives birth to a color blind daughter. The husband claims that the child is not his. The wife claims the child is his. Can you support the argument of either parent? If yes, which one? Why?

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