1. What is the function of chromosomes?
2. How are proteins used in living organisms?
3a. What is the function of enzymes?
3b. Why are enzymes important?
4. How many different types of chemical reactions can be influenced by one enzyme?
5. Why can an enzyme influence only one type of chemical reaction?
6. What types of molecules are present in a chromosome?
7. Which part of a chromosome contains information for constructing proteins?
8. How many chromosomes are present in a human cell?
9. What is a gamete? Name two examples of gametes.
10. How many chromosomes are present in a human gamete?
11. How many different kinds of chromosomes are present in a human gamete?
12. A human gamete contains how many copies of each type of chromosome?
13. What does the term “haploid” mean?
14. Name two examples of human cells that are haploid.
15. Name another term that is used to describe a haploid cell.
16. How many chromosomes are present in a human cell that is described as being haploid or 1N ?
17. What does the term “diploid” mean?
18. How many chromosomes are present in a human cell that is diploid?
19. Which cells in the human body are diploid?
20. Name the type of cell division that results in the production of gametes.
21. How does the number of chromosomes in daughter nuclei produced by meiosis compare to the number of chromosomes in the parent cell nucleus? (i.e., what is the effect of meiosis on chromosome numbers?)
22. A germ cell is diploid. Which of the two copies of chromosome No. 1 will be present in a gamete produced from this germ cell?
23. What term is used to describe two chromosomes that control or influence the same characteristics?
24. What name is used to describe the part of a chromosome that controls one characteristic?
25. How many genes are present on one chromosome?
26. Approximately how many human traits (= characteristics) are thought to be genetically controlled or influenced?
27. If humans have 75,000 genetically controlled traits, how many genes does a human have?
28. What is the major goal of the Human Genome Project?
29. What chemical molecule stores information in a gene?
30. What is a chromosome?
31. What name is used to describe genes that control the same characteristic?
32. A diploid human cell possesses how many genes for each trait?
33. If two genes influence the same trail, does this mean that the genes are identical?
34. Should a gene for eye color and a gene for hair color be called alleles?
35. What does the term “homozygous” mean?
36. What does the term “heterozygous” mean?
37. What does the term “dominant allele” mean?
38. What does the term “recessive allele” mean?
39. Can a gamete be described as “homozygous” or “heterozygous” for a particular trait?
WITCHES
Witches have either 4 or 5 fingers on each hand. This trait (= number of fingers) is controlled by a single pair of genes. The dominant gene (represented by the upper case letter F) results in five fingers while the recessive gene (represented by the lower case f) has information that results in four fingers.
Witches have either green eyes or orange eyes. The allele for green (represented by the upper case letter G) dominates the allele for orange (represented by the lower case letter g) when both alleles are present in the same individual.
40. What is the genotype of a witch who is described as being “homozygous recessive” for the trait “number of fingers”?
41. What is the genotype of a witch who is heterozygous for both traits?
42. Why did the genotype given in answer No. 40 have only two letters while the genotype given in answer No. 41 have four letters?
43. What is the genotype of a witch who is homozygous dominant for both traits?
44. What is the phenotype of witch who is described as being “homozygous recessive” for the trait “number of fingers” ?
45. What is the genotype and phenotype of a witch who is heterozygous for both traits?
46. List all of the types of gametes produced by a witch who is homozygous dominant for eye color?
47. List all of the types of gametes produced by a witch who is heterozygous for eye color.
48. List all of the types of gametes produced by a witch who is homozygous recessive for number of fingers.
49. What percentage of the offspring from two orange eyed witches will have orange eyes?
50. What percentage of the offspring from a male who is homozygous dominant for number of fingers and a female who is heterozygous for this trait will have 5 fingers?
END OF WITCH QUESTIONS
51. What is incomplete dominance?
52. Incomplete dominance in short horned cattle: What percent of the offspring from a red male and red female will have red hair?
53. Incomplete dominance in short horned cattle: What percent of the offspring from two roan individuals will have red hair?
54. Incomplete dominance in short horned cattle: Can two alleles produce three phenotypes?
BLOOD TYPING
The human population contains three alleles for blood types: allele A, allele B, and allele O. Each individual possesses only one (AA or BB or OO) or two (AO or AB or BO) of these alleles. No individual has all three alleles: there is no genotype ABO. Allele A and allele B each dominate allele O, but allele A and allele B show incomplete dominance.
55. What is the genotype of a person with Type O blood?
56. What is the genotype of a person with Type A blood?
57. What is the genotype of a person with Type AB blood?
58. Can a female with Type O blood and a male with Type AB blood have any offspring with Type O blood?
59. Can two persons with Type A blood have any offspring with Type O blood?
SEX CHROMOSOMES AND SEX LINKED TRAITS
60. What are sex chromosomes?
61. What is the name of the two types of sex chromosomes?
62. What sex chromosomes does a human female possess?
63. What sex chromosomes does a human male possess?
64. Which sex contributes the chromosome that determines the sex of the offspring?
65. What is the sex of the offspring if the male contributes a Y chromosome to the offspring?
66. What is the sex of the offspring if the male contributes an X chromosome to the offspring?
67. Which sex chromosome is longer?
68. Which sex chromosome has more genes present?
69. What is a sex-linked trait?
70. Name two examples of sex-linked traits.
HEMOPHILIA: AN EXAMPLE OF A SEX-LINKED TRAIT
Hemophilia is a sex-linked trait. The dominant allele (N) results in the ability to form blood clots at the normal rate. The recessive allele (n) prevents the normal formation of blood clots and results in the condition known as hemophilia.
71. Is the gene for blood clot formation located on the X chromosome or on the Y chromosome?
72. Since the gene for blood clot formation is located on the X chromosome, does this mean that this gene has an influence on sex determination?
73. What is the genotype of a male with hemophilia?
74. What is the genotype of a female with hemophilia?
75. How many recessive alleles must a male possess in order to have hemophilia?
76. How many recessive alleles must a female possess in order to have hemophilia?
77. Why must a female have two recessive alleles in order to have hemophilia while the male with only one recessive allele has hemophilia?
78. What is the genotype of a female who is described as being normal (with respect to forming blood clots) but who is a carrier?
79. A carrier female and a male with hemophilia have several offspring.
80. Can a normal female and a normal male have any male offspring with hemophilia?
ANSWERS
1. Chromosomes contain information for building proteins.
2. Proteins are used several ways in living cells. Some protein molecules are used in the construction of cells and tissues. Most enzymes are composed of proteins. Other proteins have special functions; for example, hemoglobin is a protein whose primary function is to transport oxygen in the bloodstream.
3a. Enzymes control the rates at which most chemical reactions occur in living cells. Generally, a reaction will not occur (or will not occur fast enough to produce a significant amount of the product) if an enzyme is not present to increase the speed of the reaction.
3b. Enzymes determine which chemical reactions occur in a cell. These reactions, in turn, determine the size, shape, and activities of each cell. In effect, enzymes determine each of our characteristics. “We are what we are because of our enzymes.”
4. Each enzyme can usually influence only one specific chemical reaction (but an enzyme may be used millions of times for this reaction).
5. Each enzyme has areas (called active sites) where the reacting molecules (which are called substrates) attach. Since the active sites have specific shapes, only certain molecules can attach and be involved in the reaction regulated by that enzyme.
6. Chromosomes primarily contain deoxyribonucleic acid (DNA) molecules and protein molecules.
7. The information for constructing proteins is stored in the deoxyribonucleic acid (DNA) molecules.
8. The number of chromosomes present in a cell will depend on whether the cell is a somatic cell (= body cell) or a gamete (= egg or sperm).
9. A gamete is a type of sex cell that is used for sexual reproduction. A gamete contains only one of each kind of chromosome, unlike body cells (e.g., liver cells, skin cells, muscle cells) which contain two of each kind of chromosome. Two types of gametes are egg and sperm.
10. Each human gamete (i.e., egg or sperm) will normally contain 23 chromosomes.
11. A human gamete will normally have 23 different kinds of chromosomes.
12. A human gamete will normally have one of each kind of chromosome.
13. The term “haploid” is used to describe a cell that has one of each kind of chromosome.
14. Two examples of human cells that are haploid are egg and sperm.
15. Another term used to describe a haploid cell is “1N” or “N.”
16. A 1N (= haploid) human cell would contain 23 chromosomes.
17. The term “diploid” refers to a cell that has two of each kind of chromosome that is normally found in that species. This cell can also be described as being “2N.”
18. A diploid human cell contains a total of 46 chromosomes, two of each of the 23 different kinds.
19. All of the cells in a human except the eggs and sperm are normally diploid.
20. The process of meiosis is involved in the production of gametes.
21. The process of meiosis reduces chromosome numbers by 1/2. Normally, the cell which enters meiosis is a germ cell which is 2N. This chromosome number is reduced to 1N (= N) in each of the gametes that is produced.
22. Since the germ cell is diploid, it has two copies of chromosome No. 1. Let's refer to them as 1' and 1”. Each gamete is going to have one copy of chromosome No. 1. Either copy of chromosome No. 1 could be in a gamete. Pure chance determines which chromosome goes into a particular gamete. That is, about 50% of the gametes would have chromosome 1' and the other 50% of the gametes would have chromosome 1”.
23. The term “homologous” is used to describe two chromosomes that control the same characteristics.
24. The term “gene” is used to describe the part of a chromosome that controls or influences one characteristic.
25. The number of genes present on a chromosome depends, in part, on the length of the chromosome and the length of each gene. Chromosomes, like books, differ in length. The number of genes present on a chromosome, like the number of pages in a book, may be small or large.
26. Some 50,000 to 100,00 human traits are thought to be genetically influenced.
27. At least one gene influences each trait; therefore, at least 75,000 genes would be present. However, some traits, such as height, are influenced by several sets of genes so the total could be well over 75,000.
28. The goal of the workers in the Human Genome Project is to prepare a “map” of each of the 23 different kinds of human chromosomes. This map would show the position and size of each gene. Each gene would be analyzed and the sequence of nucleotides in it would be determined.
29. The information storing molecule in a gene is deoxyribonucleic acid (DNA).
30. A chromosome is a cell organelle that contains genetic information in a molecule called deoxyribonucleic acid (DNA); the sections of DNA that control specific traits are called genes.
31. The term “alleles” is used to describe genes that control or influence the same characteristic.
32. A diploid human cell would possess at least two genes for each trait. For example, if Chromosome No. 4' has a gene that influences eye color, then its homologue, Chromosome No. 4”, would also have a similar gene at the same location. (Remember, since this is a diploid cell, two of each kind of chromosome would be present in the cell.) Since these two genes influence the same trait, they would be alleles.
33. No. Two genes that influence the same trait would be called alleles but they do NOT have to be identical. For example, one gene might have information that produces brown eyes, while another gene might have information that produces blue eyes. These two genes could still be called alleles even though they are not identical.
34. No. Since they influence different traits they are not alleles. All of the genes influencing eye color could be called alleles, and all of the genes influencing hair color could be called alleles. But describing the gene for eye color and the gene for hair color as alleles would be as incorrect as saying that two females with different mothers and different fathers are sisters.
35. The term “homozygous” means that two alleles (= genes controlling the same characteristic) in a diploid cell are identical. For example, in a diploid cell that is described as being homozygous the two genes for eye color may both have information for blue eyes, or they may both have information for brown eyes.
36. The term “heterozygous” means that a diploid cell has two slightly differing alleles for a specific trait. For example, one of the alleles for eye color might have information for brown eyes and the other allele might have information for blue eyes.
37. In a diploid cell that is heterozygous for a particular trait, one of the two alleles will be used (= “expressed”) while the other allele is not used. For example, if the cell has one allele for brown eyes and one allele for blue eyes, the allele for brown will be used and the allele for blue will not be used. Therefore, the allele for brown is dominant over the allele for blue.
38. In a diploid cell that is heterozygous for a particular trait, the allele that is not used is said to be recessive. For example, in question 37 the allele for brown eye color is used but the allele for blue eye color is not used. The allele for brown is “dominant,” and the allele for blue is “recessive.”
39. No, a gamete cannot be correctly described as being “homozygous” or as being “heterozygous.” Remember that a gamete (= egg or sperm) has only one of each kind of chromosome and therefore has only one gene for each trait (unless the trait is controlled by several sets of genes, a special situation we have not dealt with in this review.)
40. The term “genotype” refers to the actual types of genes that are present in an organism. Since we normally use letters, either upper case or lower case, to represent genes, the genotypes will virtually always be some combination of pairs of letters. Unless told otherwise, we assume that the individual is diploid (= two of each type of chromosome per cell) and would therefore have two genes in each somatic cell for this trait. In this problem, we are told that the individual is “homozygous”' this means that the two genes are identical. We are also told that the individual is homozygous for the “recessive” allele. Since the recessive allele is represented by the lower case f, the genotype is “ff.”
41. The term “heterozygous” means that the two alleles controlling a trait in a specified individual are different. Since we are told that the individual in this problem is heterozygous for both traits, the genotype is “FfGg.”
42. In question No. 40, the alleles describing only one trait were given; in question No. 41, the alleles describing two traits were given.
43. The genotype of a witch who is homozygous dominant for both traits is “FFGG.”
44. The term “phenotype” is used to describe the consequences of having specific genes. When describing hair color, eye color, height, blood type, number of fingers, etc. we are giving the phenotype of the individual. We determined in question No. 40 that the genotype of this individual is ff. Since this individual has no allele for five fingers, the phenotype will be “four fingers.” Remember, the phenotype is a written description of a trait.
45. The term “heterozygous” means that each diploid cell possesses two differing alleles for a trait. The genotype would be “FfGg.” The phenotype would be “five fingers” (because F dominates f) and “green eyes” (because G dominates g).
46. When asked to describe the gametes we are being asked to determine which alleles will be present in the gametes. We must list all possible combinations in which the alleles might appear. In order to determine the types of gametes produced we need to know the genotype of the individual. In this case, we are told that the witch is “homozygous dominant.” This means that both of the alleles are alike and both are of the dominant form. Therefore, the genotype is “GG.” Remember that gametes (= eggs and sperm) are haploid and contain only one of each type of chromosome and, consequently, only one of each type of gene. In this case each gamete will contain “G.”
47. Since she is heterozygous, her genotype is Gg. Even though G dominates g when they are together in a cell, this does not mean that G is inherited more frequently than g. In fact, about 50% of her eggs (= gametes) will have G and about 50% of her eggs will have g.
48. Since she is homozygous for the recessive allele, her genotype is ff. Each of her gametes will contain “f.”
49. Step 1. Determine the genotypes of the parents:
The only genotype that will result in orange eyes is gg. (If the genotype were Gg or GG, the eye color would be green.) The genotype of each parent is gg.
Step 2. List all of the types of gametes produced by each parent:
All of the sperm would contain g, and all of the eggs would contain g.
Step 3. Combine the gametes in all possible combinations.
Since all of the sperm contain g and all of the eggs contain g, the only possible combination for offspring is gg. This means that all (100%) of the offspring will have orange eyes.
50. Step 1: genotype of the parents: male = FF female = Ff
Step 2: types of gametes: male has sperm with F; female has 50% eggs with F and 50% eggs with f
Step 3: types of offspring: FF and Ff (The sperm with F could fertilize an egg with F or, equally likely, an egg with f.)
Since all of the offspring have at least one dominant allele, all (=100%) of the offspring will have 5 fingers.
51. Incomplete dominance occurs when a cell has two alleles that differ from each other but neither dominates the other. In this situation, a heterozygous genotype will result in a phenotype that is intermediate to the two homozygous genotypes. For example, in short horned cattle hair color is determined by alleles for red, which are represented by R, and alleles for white, which are represented by R'. An individual with the genotype RR will have red hair; an individual with the genotype R'R' will have white hair. An individual with the genotype RR' will have neither red hair nor white hair but will have an intermediate color known as roan.
52. Genotypes of the parents: RR X RR
53. Genotypes of the parents: RR' X RR'
Gametes produced: R and R' from each parent
Genotypes of the offspring:
| R | R' | |
| R | RR | RR' |
| R' | RR' | R'R' |
ANSWER: one (RR) out of four offspring types has red hair, therefore 25% of the offspring will probably have red hair.
54. Yes. In the case of incomplete dominance, the two alleles can produce three phenotypes: RR = red; RR' = roan; and, R'R' = white.
55. A person with Type O blood has the genotype OO. If either allele A or allele B were present, the person would not have Type O blood. (Genotype AO gives phenotype Type A; genotype BO gives phenotype Type B.)
56. The genotype of a person with Type A blood could be either AO or AA.
57. The genotype of a person with Type AB blood is AB.
58. Genotypes of the parents: OO X AB
Gametes: eggs: O sperm: A and B
Offspring: AO and BO
Answer: No. A female with Type O blood and a male with Type AB blood could not have any children with Type O blood. Approximately half of the offspring would have Type A blood and the other half would have Type B blood.
59. Yes. Two persons with Type A blood could have a child with Type O blood if both of the parents have the genotype AO. If both parents are AA or if one is AO and the other is AA, all offspring would be Type A.
About 75% of their offspring would have Type A blood and about 25% of their offspring would have Type O blood.
| A | O | |
| A | AA | AO |
| O | AO | OO |
60. Sex chromosomes possess genes that determine whether the individual will be a male or a female. Each body cell (all of which are normally diploid) will contain two sex chromosomes; each gamete (egg or sperm) will normally contain one sex chromosome.
61. The two types of sex chromosomes are “X” and “Y”.
62. A human female possesses two X chromosomes in each body cell. Each of her eggs will contain one X chromosome.
63. A human male possesses an X and a Y sex chromosome in each of his body cells. Each of his sperm will contain either an X or a Y chromosome.
64. The human male parent contributes the chromosome that will determine the sex of the offspring. A sperm may contain either an X or a Y chromosome while each egg should contain one X chromosome.
65. If the male parent (= father) contributes a Y chromosome, the offspring will be a male. The mother will contribute an X chromosome. The combination XY results in a male.
66. If the father contributes a sperm that contains an X chromosome, the offspring will be a female. The mother will contribute an X chromosome in her egg. The combination XX results in a female.
67. The X chromosome is longer than the Y chromosome.
68. The X chromosome apparently contains more genes than the Y chromosome.
69. A sex-linked trait is controlled by a gene (or genes) that occurs on the X chromosome but not on the Y chromosome. The genes for sex-linked traits normally have no influence on sex determination.
70. Two examples of sex-linked traits are (1) ability to form blood clots, and (2) red-green color vision.
71. A gene for blood clot formation is located on the X chromosome. When a trait is described as being sex-linked, this means that the gene controlling the trait will be on the X chromosome but not on the Y chromosome.
72. No. The genes for blood clot formation do not influence sex determination. While the X chromosome does possess genes that are involved in determining the sex of the individual, sex-linked chromosomes are located on the part of the X chromosome that has nothing to do with sex determination.
73. The genotype of a human male with hemophilia is XnY.
74. The genotype of a human female with hemophilia is XnXn.
75. A human male would have to possess only one recessive allele in order to have hemophilia. (Why not two recessives? Because the Y chromosome does not have any genes that influence blood clot formation. Whatever is on the X chromosome of the male will be expressed.)
76. A human female must possess two recessive alleles in order to have hemophilia.
77. The female has two X chromosomes. If either of the X chromosomes has the dominant allele (for normal ability to form blood clots) she will not have hemophilia. Since the male has only one X chromosome (and no gene for blood clot formation on the Y chromosome), this single gene (whether dominant or recessive) on the X chromosome will be expressed.
78. A human female who has normal ability to form blood clots but who is described as being a carrier who have the genotype XNXn. She is heterozygous.
79. Genotypes of the parents: female = XNXn male = XnY
Gametes produced: female: XN, Xn male: Xn, Y
Offspring:
| Xn | Y | |
| XN | XNXn | XNY |
| Xn | XnXn | XnY |
80. The genotype of a normal female could be either XNXN or XNXn. Since we do not know which of these she is (the problem only tells us that she is normal), we must consider both possibilities. The genotype of the male is XNY.
If the female is XNXN :
She would produce eggs with XN; his sperm would contain either XN or Y.
The offspring would be either XNXN (normal female) or XNY (normal male); neither of these combinations results in offspring with hemophilia.
If the female is XNXn :
She would produce eggs with either XN or Xn; his sperm would contain either XN or Y.
The offspring would be:
| XN | Y | |
| XN | XNXN | XNY |
| Xn | XNXn | XnY |
One group (XnY) out of four would have hemophilia; therefore, we would predict that about 25% of the offspring would have hemophilia. None of the female offspring would have hemophilia, but about 50% of the male offspring would probably have hemophilia.
Answer: Yes. A normal female and a normal male can have male offspring with hemophilia, but only if the female is heterozygous (i.e., she is a carrier).