1. In monohybrid cross why did only about one fourth of Mendel's F₂ plants exhibit the recessive trait.
Answer: In a monohybrid cross, Mendel crossed true-breeding dominant (TT) and recessive (tt) plants. The F₁ generation was all heterozygous (Tt) and exhibited the dominant trait. When the F₁ plants were self-pollinated, the F₂ generation consisted of a 3:1 phenotypic ratio. This is because the heterozygous F₁ plants produce two types of gametes (T and t) in equal proportions. A Punnett square of the F₂ generation shows that only one out of four possible combinations is homozygous recessive (tt), which exhibits the recessive trait. Therefore, approximately one-fourth of the F₂ plants showed
the recessive phenotype.
2. Hypothesize that in a dihybrid inheritance pattern of colour and texture of pea seed, the two traits are not interdependent.
Answer: If the two traits (seed color and seed texture) are not interdependent, they will assort independently during gamete formation, as stated by Mendel's Law of Independent Assortment. This means that the inheritance of seed color does not influence the inheritance of seed texture. A dihybrid cross between two heterozygous individuals (RrYy x RrYy) would yield a 9:3:3:1 phenotypic ratio in the F₂ generation. This ratio includes all possible combinations: 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green. This independent assortment occurs because the genes for these traits are located on different chromosomes (or far apart on the same chromosome).
3. What are the exceptions to the Mendel's laws of inheritance?
Answer: Several exceptions to Mendel's laws have been discovered:
1. Incomplete Dominance: The heterozygous phenotype is an intermediate blend (e.g., pink flowers in snapdragons).
2. Codominance: Both alleles are expressed equally in the heterozygote (e.g., AB blood type).
3. Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood groups).
4. Polygenic Inheritance: Traits controlled by multiple genes (e.g., human height, skin color).
5. Epistasis: One gene masks the expression of another gene.
6. Linkage: Genes located close together on the same chromosome are inherited together, violating independent assortment.
7. Sex-Linked Inheritance: Genes located on sex chromosomes show different inheritance patterns.
4. What is probability? How probability does relate to genetics?
Answer: Probability is the mathematical likelihood that a particular event will occur. It is expressed as a ratio of the number of favorable outcomes to the total number of possible outcomes. In genetics, probability is used to predict the outcomes of genetic crosses. For example, a Punnett square uses probability to determine the likelihood of offspring having a particular genotype or phenotype. The probability of an event is calculated by dividing the number of ways the event can occur by the total number of possible outcomes. In a monohybrid cross, the probability of getting a recessive phenotype is 14 or 25%. Probability is essential for understanding and predicting inheritance patterns.
5. Write the differences between incomplete dominance and co-dominance.
Answer:
| Feature | Incomplete Dominance | Co-dominance |
| Heterozygous
Phenotype | Intermediate blend of the two homozygous
phenotypes. | Both homozygous phenotypes are expressed
simultaneously. |
| Example | Pink flowers in snapdragons (RR = red, rr =
white, Rr = pink). | AB blood type in humans (I^A I^B expresses both
A and B antigens). |
| Allele Interaction | Neither allele is completely dominant. | Both alleles are equally dominant. |
| | |
Phenotypic Ratio 1:2:1 (in F₂ generation). 1:2:1 (in F₂ generation).
6. Describe the multiple alleles and state the alleles responsible for the traits of ABO blood groups.
Answer: Multiple alleles refer to the existence of more than two alternative forms of a gene in a population. For the ABO blood group system, there are three alleles: I^A, I^B, and i (or I^O). The I^A allele codes for the A antigen, the I^B allele codes for the B antigen, and the i allele codes for no antigen (O). I^A and I^B are codominant, meaning both are expressed if present together (AB blood type). Both I^A and I^B are dominant over i. This results in four possible blood types: A (I^A I^A or I^A i), B (I^B I^B or I^B i), AB (I^A I^B), and O (ii).
7. Why can multiple alleles provide many different phenotypes for a trait?
Answer: Multiple alleles can provide many different phenotypes because they increase the number of possible combinations of alleles in a population. With multiple alleles, there can be many different heterozygous combinations, each potentially producing a different phenotype. For example, in the ABO blood group system, three alleles produce four different phenotypes (A, B, AB, and O). Additionally, multiple alleles can exhibit different dominance relationships (complete dominance, codominance, incomplete dominance), further increasing phenotypic diversity. This genetic variation is important for adaptation and evolution.
8. Name various human blood group systems.
Answer: Various human blood group systems include:
1. ABO System: The most important blood group system, with four types (A, B, AB, O).
2. Rh System: Named after the Rhesus monkey, with Rh-positive and Rh-negative types.
3. MN System: With M, N, and MN types.
4. Kell System: With K and k antigens.
5. Duffy System: With Fy^a and Fy^b antigens.
6. Kidd System: With Jk^a and Jk^b antigens.
7. Lewis System: With Le^a and Le^b antigens. These systems are important for blood transfusions and organ transplants.
9. Describe the antigens of ABO blood group system.
Answer: The ABO blood group system is based on the presence or absence of two antigens, A and B, on the surface of red blood cells.
Type A: Has A antigens on the red blood cells. Type B: Has B antigens on the red blood cells. Type AB: Has both A and B antigens. Type O: Has neither A nor B antigens. These antigens are carbohydrates attached to lipids or proteins on the cell membrane. The presence of these antigens is determined by the alleles inherited from the parents. The antigens trigger an immune response if incompatible blood is transfused.
10. Describe the antibodies of ABO blood group system.
Answer: The ABO blood group system also involves antibodies present in the plasma.
Type A: Has anti-B antibodies in the plasma. Type B: Has anti-A antibodies in the plasma. Type AB: Has no antibodies (universal recipient). Type O: Has both anti-A and anti-B antibodies (universal donor). These antibodies are naturally occurring (IgM type) and are directed against the antigens not present on the individual's own red blood cells. If incompatible blood is transfused, the antibodies cause agglutination (clumping) and hemolysis (destruction) of the red blood cells.
11. Why Rh-incompatibility could be a danger to the developing foetus and mothers?
Answer: Rh-incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus. During pregnancy or childbirth, fetal Rh-positive red blood cells may enter the mother's bloodstream. The mother's immune system recognizes the Rh antigen as foreign and produces anti-Rh antibodies. In a subsequent pregnancy with an Rh-positive fetus, these antibodies can cross the placenta and attack the fetal red blood cells, causing hemolytic disease of the newborn (erythroblastosis fetalis). This can lead to severe anemia, jaundice, brain damage, or even death of the fetus. The mother is not usually harmed, but she becomes sensitized.
12. Investigate the reasons for O (negative) individuals as the Universal donor and AB+ (positive) as the Universal recipient.
Answer:
O negative as Universal Donor: O negative blood lacks A, B, and Rh antigens on the red blood cells. Therefore, it does not trigger an immune response in recipients of any blood type. This makes it safe for transfusion to
anyone, especially in emergencies. AB positive as Universal Recipient: AB positive blood has A, B, and Rh antigens. Individuals with this blood type do not produce antibodies against A, B, or Rh. Therefore, they can receive blood from any donor without experiencing an immune reaction. These concepts are crucial in blood transfusion medicine.
13. Why a recessive group allele "i" is more frequent in population.
Answer: The recessive allele "i" (for blood type O) is more frequent in some populations due to genetic drift, founder effect, or selective advantage. For example, in some populations, the O blood type may provide a selective advantage against certain diseases, such as malaria. Additionally, the "i" allele is recessive, so it can be carried by individuals with type A or B blood without being expressed. This allows the allele to persist in the population without being subject to strong selection. The frequency of the "i" allele varies among different populations.
14. List at least five polygenic traits discovered in humans.
Answer: Five polygenic traits in humans are:
1. Height: Determined by multiple genes.
2. Skin Color: Controlled by multiple genes.
3. Eye Color: Influenced by multiple genes.
4. Body Weight: Affected by multiple genes and environmental factors.
5. Intelligence: A complex trait influenced by many genes. These traits show continuous variation and are often influenced by environmental factors as well.
15. Write the differences between dominance and epistasis.
Answer:
| Feature | Dominance | Epistasis |
| Definition | Interaction between alleles of the same
gene. | Interaction between different genes. |
| Effect | One allele masks the expression of
another allele at the same locus. | One gene masks or modifies the expression of another gene
at a different locus. |
| Example | In pea plants, tall (T) is dominant over
short (t). | In Labrador retrievers, the gene for black/brown pigment is
epistatic to the gene for pigment deposition. |
| Phenotypic
Ratio | 3:1 in F₂ for monohybrid cross. | 9:3:4 or 12:3:1 in F₂ for dihybrid cross. |
16. What is gene linkage?
Answer: Gene linkage is the tendency of genes that are located close together on the same chromosome to be inherited together during meiosis. Linked genes do not assort independently, violating Mendel's Law of Independent Assortment.
The closer two genes are on a chromosome, the more likely they are to be inherited together. Linkage can be complete (if genes are very close) or incomplete (if crossing over occurs between them). Linkage is important for understanding genetic mapping and inheritance patterns.
17. Explain crossing over.
Answer: Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. It occurs between non-sister chromatids at points called chiasmata. Crossing over breaks the linkage between genes, creating new combinations of alleles on a chromosome. This process is a major source of genetic variation in sexually reproducing organisms. The frequency of crossing over between two genes is proportional to the distance between them, which is used to create genetic maps.
18. Describe XX-XY type pattern of sex determination.
Answer: The XX-XY type of sex determination is found in humans and many other mammals. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The sex of the offspring is determined by the father, as he can produce two types of sperm: X-bearing and Y-bearing. If an X-bearing sperm fertilizes the egg, the offspring is female (XX). If a Y-bearing sperm fertilizes the egg, the offspring is male (XY). The Y chromosome carries the SRY gene, which triggers male development. This system ensures a 1:1 ratio of males to females.
19. Why is human male referred as heterogametic?
Answer: Human males are referred to as heterogametic because they produce two different types of gametes (sperm)
with respect to sex chromosomes: half carry the X chromosome and half carry the Y chromosome. In contrast, females are homogametic because they produce only one type of gamete (egg) with respect to sex chromosomes, all carrying the X chromosome. The heterogametic nature of males determines the sex of the offspring. This is a characteristic of the XX- XY sex determination system.
20. What is sex linkage?
Answer: Sex linkage is the phenomenon where genes are located on the sex chromosomes (X or Y) and their inheritance patterns are related to the sex of the individual. In humans, most sex-linked genes are located on the X chromosome (X- linked). Examples include hemophilia, color blindness, and Duchenne muscular dystrophy. X-linked recessive traits are more common in males because males have only one X chromosome, so a single recessive allele is expressed. Females must inherit two copies of the recessive allele to express the trait. Y-linked traits are passed only from father to son.
21. Describe Y-linked inheritance in man.
Answer: Y-linked inheritance (also called holandric inheritance) involves genes located on the Y chromosome. These traits are passed exclusively from father to son, as only males have a Y chromosome. There is no transmission to daughters. Examples of Y-linked traits in humans are rare but include the SRY gene (which triggers male development) and genes related to spermatogenesis. Y-linked traits are not expressed in females because they lack a Y chromosome. The inheritance pattern is straightforward: an affected father will pass the trait to all his sons, and none of his daughters will be affected or carriers.
22. Name some of the sex-linked disorders of man and Drosophila.
Answer:
In Humans: Hemophilia A and B, Red-green color blindness, Duchenne muscular dystrophy, Fragile X syndrome, and X-linked agammaglobulinemia. In Drosophila: White eye color, miniature wings, and bar eye shape. These disorders are caused by mutations in genes located on the X chromosome. They show characteristic inheritance patterns, such as being more common in males.
23. How gene linkage encounters independent assortment?
Answer: Gene linkage occurs when genes are located close together on the same chromosome. These genes tend to be inherited together, which contradicts Mendel's Law of Independent Assortment. Independent assortment occurs only for genes on different chromosomes or genes that are far apart on the same chromosome. Linked genes do not assort independently because they are physically connected on the chromosome. However, crossing over can separate linked genes, creating new combinations. The closer the genes, the lower the chance of crossing over between them, and the stronger the linkage.
24. Under what circumstances is it possible for father and son to suffer from haemophilia?
Answer: Hemophilia is an X-linked recessive disorder. For a father and son to both suffer from hemophilia, the father must be affected (X^h Y) and the mother must be at least a carrier (X^H X^h). If the mother is a carrier, there is a 50% chance that a son will inherit the affected X chromosome from her and be affected. If the mother is affected (X^h X^h), all sons will be affected. However, the father passes his Y chromosome to his son, not his X chromosome. So, the son inherits the affected X from his mother. Therefore, it is possible if the mother is a carrier or affected.
25. Explain dichromacy.
Answer: Dichromacy is a type of color blindness in which one of the three cone pigments (red, green, or blue) is missing or defective. This results in the ability to perceive only two primary colors instead of three. There are three types of dichromacy:
Protanopia: Red-blind (missing red cones). Deuteranopia: Green-blind (missing green cones). Tritanopia: Blue-blind (missing blue cones). Dichromacy is usually inherited as an X-linked recessive trait. It is more common in males. People with dichromacy have difficulty distinguishing certain colors, such as red and green.
26. Explain monochromacy.
Answer: Monochromacy is a rare form of color blindness in which an individual has only one type of cone pigment or no cone function at all. This results in the ability to perceive only one color or shades of gray. There are two types:
Rod Monochromacy (Achromatopsia): Complete color blindness; individuals see only black, white, and shades of gray. They also have poor vision and severe photophobia. Cone Monochromacy: Individuals have only one type of cone, so they can perceive only one color (e.g., blue) and have poor color discrimination. Monochromacy is usually inherited as an autosomal recessive trait.
27. What is a sex limited trait?
Answer: A sex-limited trait is a trait that is expressed in only one sex, even though the gene responsible for the trait may be present in both sexes. The expression of the trait is limited by the sex of the individual, often due to hormonal differences. Examples include milk production in female mammals, antler development in male deer, and beard growth in human males. The genes for these traits are autosomal, but their expression is influenced by sex hormones.
28. What is sex influenced trait?
Answer: A sex-influenced trait is a trait whose expression is influenced by the sex of the individual, but the gene is present in both sexes. The trait may be dominant in one sex and recessive in the other. A classic example is pattern baldness in humans. The allele for baldness is dominant in males but recessive in females. This means that a heterozygous male will be bald, while a heterozygous female will not be bald (but may have thinning hair). The difference is due to the influence of sex hormones (androgens).
29. How do multiple alleles and polygenic traits differ?
Answer:
| Feature | Multiple Alleles | Polygenic Traits |
| Definition | More than two alleles for a single gene in a
population. | A trait controlled by two or more genes. |
| | |
| Effect | Different alleles produce different phenotypes. | The combined effect of multiple genes produces
continuous variation. |
| Example | ABO blood groups (I^A, I^B, i). | Human height, skin color. |
| Inheritance | Follows Mendelian patterns (dominance,
codominance). | Shows quantitative inheritance; no clear Mendelian ratios. |
30. Solve the following:
(a) Problem: Show how the "test cross" works, using round-seeded pea of unknown generation.
Answer: A test cross is used to determine the genotype of an individual showing the dominant phenotype (round seeds). The unknown genotype could be homozygous dominant (RR) or heterozygous (Rr). To determine this, the individual is crossed with a homozygous recessive (wrinkled seed, rr).
If the unknown is RR: All offspring will be round (Rr). If the unknown is Rr: 50% of offspring will be round (Rr) and 50% will be wrinkled (rr). By observing the phenotypes of the offspring, we can deduce the genotype of the unknown parent.
(b) Problem: If a four-o'clock plant having red flowers is crossed with a white-flowered four o'clock, the F₁ plants are all pink flowered. From this evidence alone, what ratios would you expect in these crosses: red crossed with red; red X pink; pink X white; white X white?
Answer: This is a case of incomplete dominance. Red (RR) x White (WW) -> Pink (RW).
Red x Red (RR x RR) -> All Red (RR). Red x Pink (RR x RW) -> 1 Red (RR) : 1 Pink (RW). Pink x White (RW x WW) -> 1 Pink (RW) : 1 White (WW). White x White (WW x WW) -> All White (WW).
(c) Problem: Suppose a man with blood type A marries a woman who has type AB blood. What blood types would you expect to find among their children? In this problem we have no way of knowing whether the man is homozygous or heterozygous for I^A allele. What would tell you which of the genotype he has?
Answer: The woman is AB (I^A I^B). The man is A, which could be I^A I^A or I^A i.
If the man is I^A I^A: Children can be A (I^A I^A) or AB (I^A I^B). (50% A, 50% AB) If the man is I^A i: Children can be A (I^A I^A or I^A i), B (I^B i), or AB (I^A I^B). (50% A, 25% B, 25% AB) To determine the man's genotype, we would need to look at his parents or his other children. If he has a child with type B blood, he must be I^A i. If all his children are A or AB, he could be either.
(d) Problem: Two white sheep produce a black offspring. What is the parent's genotype for the colour? What is the probability that their next offspring will be black?
Answer: Since two white sheep produce a black offspring, white must be dominant (W) and black recessive (w). The parents must both be heterozygous (Ww).
Parent's genotype: Ww x Ww. Probability of next offspring being black (ww): 14 or 25%.
(e) Problem: Albinism (lack of pigment) in man is caused by a recessive gene. If normal parents have an albino child, what is the probability that their next child will be normal for colour?
Answer: Since normal parents have an albino child, both parents must be carriers (heterozygous, Aa).
Parents: Aa x Aa. Probability of next child being normal (AA or Aa): 34 or 75%.
31. Write the differences between:
(a) genes and alleles
Answer: A gene is a segment of DNA that codes for a trait. Alleles are different versions of a gene.
(b) alleles and multiple alleles
Answer: Alleles are alternative forms of a gene. Multiple alleles refer to the existence of more than two alleles for a gene in a population.
(c) monohybrid and dihybrid cross
Answer: A monohybrid cross involves one trait; a dihybrid cross involves two traits.
(d) first and second filial generation
Answer: F₁ is the first generation of offspring from a parental cross; F₂ is the generation from crossing F₁ individuals. (e) genotype and phenotype
Answer: Genotype is the genetic makeup; phenotype is the physical appearance. (f) homozygous and heterozygous
Answer: Homozygous has two identical alleles; heterozygous has two different alleles. (g) homozygous and hemizygous
Answer: Homozygous has two identical alleles; hemizygous has only one allele (e.g., X-linked genes in males). (h) incomplete dominance and co-dominance
Answer: Incomplete dominance produces an intermediate phenotype; codominance expresses both phenotypes simultaneously.
(i) dominance and epistasis
Answer: Dominance is interaction between alleles of the same gene; epistasis is interaction between different genes. (j) sex determination in Drosophila and human
Answer: In Drosophila, sex is determined by the ratio of X chromosomes to autosomes. In humans, sex is determined by
the presence of the Y chromosome (XX = female, XY = male). (k) X-linked and Y-linked inheritance
Answer: X-linked genes are on the X chromosome; Y-linked genes are on the Y chromosome.
(l) sex limited traits and sex influenced traits
Answer: Sex-limited traits are expressed in only one sex; sex-influenced traits are expressed in both sexes but differently.
(m) pattern of baldness in human male and female.
Answer: Baldness is dominant in males and recessive in females.
