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Questions and Answers
What is the expected phenotypic ratio of F₂ individuals when two MN individuals marry?
What characterizes sickle cell anaemia at the genetic level?
What is meant by pleiotropism in genetics?
What is the role of the genes Hb^ and Hb^ in individuals who are heterozygous for sickle cell disease?
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Which of the following is an effect of the pp gene in phenylketonuria?
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In codominance, how do the alleles express themselves in the F₁ generation?
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What is the expected phenotypic ratio in the F₂ generation for codominant traits?
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Which of these blood groups is an example of codominance?
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Which gene combination produces MN blood group in offspring when M and N groups are crossed?
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What phenotype results from a cross between red coat color cattle (RR) and white coat color cattle (rr)?
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What is a characteristic feature of the L^ gene in the ABO blood group system?
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How many different phenotypes can result from the F₂ generation of a cross between two AB blood group individuals?
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Which statement about codominance is FALSE?
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Study Notes
Codominance
- Both alleles of a character are equally dominant and express themselves in the F₁ generation.
- Characters of the parents are mixed together in the offspring.
- Codominance is an allelic interaction.
- In heterozygotes, both alleles express their characters simultaneously.
- These genes are called codominant genes.
- The monohybrid ratio is 1:2:1.
- Codominant genes are represented by superscripts, for example, L^, L^, etc.
Examples of Codominance
-
Coat colour in short horn cattle:
- Red (RR) x White (rr) parents produce Roan (Rr) offspring.
- F₂ generation has a 1:2:1 ratio of Red (RR): Roan (Rr): White (rr).
-
ABO blood group:
- Four blood groups: A, B, AB, and O.
- A group: L^L^ genotype, produces antigen A.
- B group: L^L^ genotype, produces antigen B.
- AB group: L^L^ genotype, produces both antigen A and B.
- The F₂ generation from an AB x AB cross has a 1A: 2AB: 1B ratio instead of the usual Mendelian 3:1 ratio.
-
MN blood group:
- Three blood groups: M, N, and MN.
- M group: L^L^ genotype, produces antigen M.
- N group: L^L^ genotype, produces antigen N.
- MN group: L^L^ genotype, produces both antigen M and N.
- The F₂ generation from an M x N cross has a 1M: 2MN: 1N ratio instead of the usual Mendelian 3:1 ratio.
-
Sickle cell anaemia:
- A hereditary blood disease characterized by sickle-shaped red blood cells under low oxygen pressure.
- Hb^Hb^ genotype produces defective haemoglobin (Hb^), causing sickling of red blood cells.
- Hb^Hb^ genotype produces normal adult haemoglobin (Hb^).
- Heterozygous individuals (Hb^Hb^) produce both normal and defective haemoglobin, demonstrating codominance.
Pleiotropism
- The production of many characters by a single pair of genes.
- A single pair of genes controls many characters.
- For example, the genes pp for phenylketonuria cause:
- Accumulation of phenylalanine in the blood.
- Mental retardation.
- Widely spaced incisors.
- Pigmented patches on skin.
- Excessive sweating.
- Non-pigmented hairs and eyes.
- Another example is the potato mutant gene, which suppresses the growth of meristematic tissue, axillary shoot, and petals, resulting in an apocarpous pistil and dilatatory anthers.
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Description
Explore the concept of codominance in genetics, where both alleles of a gene express equally in the offspring. Discover classic examples like coat color in short horn cattle and the ABO blood group system, highlighting their unique inheritance patterns. This quiz will deepen your understanding of genetic interactions in heterozygotes.
