Topic 4.3 Review Book
Theoretical Genetics
Name ___________________
4.3 Theoretical Genetics
1. In genetics, genotype refers to the alleles of an organism and is usually represented by letters.
2. Phenotype refers to the characteristics of an organism. It is the result of a particular genotype.
3. Every organism normally has two alleles for each trait. If the alleles for the trait are the same, the
organism is said to be homozygous. If the alleles for the trait are different, the organism is said to be
heterozygous.
4. Dominant alleles have the same effect whether they are homozygous or heterozygous.
5. Recessive alleles will only have an effect if they exist in the homozygous state.
6. Codominant alleles are pairs of alleles that both have an effect on the phenotype of the organism.
7. Locus refers to a particular position on a chromosome.
8. A carrier is heterozygous for a particular trait. The recessive allele that is present in a carrier causes
a genetic disease, but does not in the carrier because the dominant or non-disease allele is also
present.
9. Test crosses involve crossing an organism with the dominant phenotype for a particular trait with an
organism with the recessive phenotype to determine whether the dominant showing organism is
heterozygous or homozygous.
10. A monohybrid cross involves only one trait and is represented by a Punnett grid.
11. Genes that have more than two alleles are said to involve multiple alleles as in blood genetics in
humans.
12. A dihybrid cross involves two traits.
13. The sex chromosomes in humans are referred to as X and Y, and they control gender.
14. The X sex chromosome contains some genes that are not present on the shorter Y sex chromosome.
15. Sex linkage is the association of a characteristic with gender. This is because the gene controlling the
characteristic is on a sex chromosome.
16. Sex linked genes are usually always on the X chromosome.
17. Because males have only one X chromosome they may have only one sex-linked gene. The result of
this is that in males a recessive allele may show itself when it is by itself. Examples of this are
hemophilia and red-green color blindness.
18. A human female may be heterozygous or homozygous with respect to sex-linked genes. Therefore,
females show sex-linked conditions less often.
19. A pedigree is a representation using symbols of the occurrence of a trait within several generations
of a family.
20. Pedigrees may be used to deduce whether a character is caused by a dominant or recessive allele
and whether it is sex linked or not.
21. In a pedigree, squares represent males and circles represent females. If the square or circle is
shaded it means the organism has the condition. Unshaded symbols represent unaffected
individuals.
22. Genotypic and phenotypic ratios after specific crosses may be used to determine specific patterns of
inheritance.
Complete the following.
32. Using appropriate symbols represent the genotype of a plant showing the tall trait.____________
33. In human blood genetics what does the phenotype refer to?
Topic 4.3 Review Book
Theoretical Genetics
Name ___________________
34. What is the source of the two alleles we have for most traits?
35. Explain two genetic conditions when a dominant allele would show?
36. Explain two genetic conditions when a recessive allele would show?
37. Use the term heterozygous to explain what a carrier is?
38. Why would a person be called a carrier if they were Aa for the albino condition?
39. When is a test cross done?
40. Show the two Punnett grids possible when a test cross is performed?
Topic 4.3 Review Book
Theoretical Genetics
Name ___________________
41. What phenotypic ratio would result when two heterozygous plants showing the tall condition were
crossed?
42. What genotypic ratio would result when two heterozygous plants showing the tall condition were
crossed?
43. Why is human blood type said to be determined by multiple alleles?
44. Complete this table:
Phenotype Genotype(s)
A
B
AB
O
45. Could a man with blood type O be the father of a baby with blood type B if the mother was type AB?
Show a Punnett grid to verify your answer.
46. Explain sex determination in humans.
47. What are genes called that are carried on the sex chromosomes?_________________
48. Why are sex linked traits most commonly seen in males?
Topic 4.3 Review Book
Theoretical Genetics
Name ___________________
49. What symbols are used in IB for the alleles possible for colorblindness?
50. What symbols are used in IB for the alleles possible for hemophilia?
51. What is the probability that two color blind parents will have a colorblind child? Include a Punnett
grid.
52. Of what value is a pedigree?
53. What do the symbols used in a pedigree represent?
54. If codominance is the condition, what is the probability of a white flowered plant when two pink
flowered plants are crossed? Show the Punnett grid.