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16.05 Genes, Alleles, Genotype and Phenotype

Posted14/11/2024
Updated29/12/2024
ByBMF
Views15

1. What is Genetics?

  • Definition:
    • Genetics is the branch of biology that studies how traits are determined by genes and how these traits are passed from parents to offspring.
  • Key Components:
    • Genes: Fundamental units of heredity.
    • DNA (Deoxyribonucleic Acid): The molecule that carries genetic information.

2. Fundamental Genetic Concepts

a. Gene

  • Definition:
    • A gene is a specific segment of DNA that codes for a polypeptide (a chain of amino acids forming proteins).
  • Structure:
    • DNA Structure: Consists of a double helix formed by nucleotide sequences.
  • Function:
    • Directs the synthesis of proteins, which perform various functions within the cell.

b. Nucleotide Sequence

  • Definition:
    • The order of nucleotides (adenine [A], thymine [T], cytosine [C], guanine [G]) in DNA determines the genetic code.
  • Importance:
    • The specific sequence of bases encodes instructions for building proteins.

c. Triplet Code (Codons)

  • Definition:
    • A triplet code refers to sequences of three nucleotide bases that correspond to a single amino acid in a polypeptide.
  • Example:
    • Codon AUG codes for the amino acid methionine and also serves as the start signal for protein synthesis.
  • Process:
    • Occurs during translation in ribosomes where mRNA is read in sets of three bases to assemble proteins.

d. Chromosome

  • Definition:
    • A chromosome is a long DNA molecule containing many genes, regulatory elements, and other nucleotide sequences.
  • Structure:
    • Humans have 23 pairs of chromosomes, each containing thousands of genes.
  • Function:
    • Ensures DNA is accurately replicated and distributed during cell division.

3. Genes and Alleles

a. Gene Locus

  • Definition:
    • The specific physical location of a gene on a chromosome.
  • Consistency:
    • The locus is the same across individuals of the same species, ensuring uniformity in gene placement.

b. Alleles

  • Definition:
    • Alleles are different forms of the same gene that arise by mutation and are located at the same locus on homologous chromosomes.
  • Genetic Diversity:
    • Variation in alleles contributes to the diversity of traits within a population.
  • Example: Coat Color in Rabbits
    • B Allele: Codes for an enzyme that produces brown pigment → Brown Coat
    • b Allele: Codes for a non-functional enzyme → White Coat
  • Allele Symbols:
    • Dominant Allele: Represented by a capital letter (e.g., B)
    • Recessive Allele: Represented by a lowercase letter (e.g., b)

4. Genotype and Phenotype

a. Genotype

  • Definition:
    • The genetic makeup of an organism; the combination of alleles an individual possesses.
  • Types:
    • Homozygous Genotype: Two identical alleles (e.g., BB or bb)
    • Heterozygous Genotype: Two different alleles (e.g., Bb)

b. Phenotype

  • Definition:
    • The observable physical or biochemical characteristics of an organism, determined by both genotype and environmental factors.
  • Example:
    • In rabbits:
      • BB: Brown coat
      • Bb: Brown coat
      • bb: White coat

c. Genotype-Phenotype Relationship in Rabbits

GenotypePhenotype
BBBrown coat
BbBrown coat
bbWhite coat

5. Dominance and Recessiveness

  • Dominant Allele (B):
    • Always expressed in the phenotype if present.
    • Masks the presence of a recessive allele.
  • Recessive Allele (b):
    • Only expressed in the phenotype if two copies are present (bb).
    • Not visible in the presence of a dominant allele (B).
  • Illustration:
    • Bb (heterozygous) will display the dominant phenotype (brown coat) despite carrying a recessive allele.

6. Codominance and Multiple Alleles

a. Codominance

  • Definition:
    • A form of inheritance where both alleles in a heterozygous genotype are fully expressed in the phenotype.
  • Example:
    • AB Blood Type in humans where both IA and IB alleles are expressed.

b. Multiple Alleles

  • Definition:
    • More than two allele forms exist for a gene within a population.
  • Human Blood Groups:
    • Alleles: IA, IB, and Io
    • Symbols:
      • IA: Blood group A (dominant)
      • IB: Blood group B (dominant)
      • Io: Blood group O (recessive to both IA and IB)

c. Blood Group Genotype-Phenotype Relationships

GenotypePhenotype
IAIABlood group A
IAIBBlood group AB (codominant)
IAIoBlood group A
IBIBBlood group B
IBIoBlood group B
IoIoBlood group O
  • Notation for Codominance:
    • Use a capital letter to represent the gene (I for blood group) with superscripts for different alleles (A, B, o).
    • Example: I<sup>A</sup>I<sup>B</sup> represents the AB blood type.

7. Solving Genetics Problems

a. Tips for Success

  1. Understand the Basics:
    • Master the definitions of key terms: gene, allele, genotype, phenotype, dominance, etc.
  2. Use Punnett Squares:
    • Visualize genetic crosses by arranging parental alleles to predict offspring genotypes and phenotypes.
  3. Organize Information:
    • Clearly list possible genotypes and their corresponding phenotypes.
    • Systematically consider all allele combinations.
  4. Apply Principles:
    • Recognize patterns of inheritance, including dominance, recessiveness, codominance, and multiple alleles.
  5. Practice Problem-Solving:
    • Regularly work through genetics problems to reinforce understanding and improve accuracy.

b. Example Problem

Question: If two heterozygous rabbits (Bb) are crossed, what is the probability of obtaining a white-coated offspring?

Solution:

  1. Genotypes of Parents: Bb × Bb
  2. Punnett Square:
Bb
BBBBb
bBbbb
  1. Possible Offspring Genotypes:
    • BB: Brown coat
    • Bb: Brown coat
    • Bb: Brown coat
    • bb: White coat
  2. Probability of White Coat (bb): 1 out of 4 → 25%

8. Key Terms

  • Allele: Different forms of the same gene.
  • Genotype: The genetic makeup of an organism.
  • Phenotype: The observable characteristics of an organism.
  • Homozygous: Having two identical alleles for a trait.
  • Heterozygous: Having two different alleles for a trait.
  • Dominant: An allele that expresses its phenotype even in the presence of a different allele.
  • Recessive: An allele that only expresses its phenotype when paired with an identical allele.
  • Codominance: A situation where both alleles are fully expressed in the phenotype.
  • Multiple Alleles: More than two allele forms exist for a gene within a population.

9. Figures & Diagrams

  • Figure 1:Structure of a Gene
    • Illustrates the location of a gene on a chromosome, highlighting exons and introns.

  • Figure 2:Punnett Square Example
    • Demonstrates a monohybrid cross between two heterozygous individuals.

  • Figure 3:Human Blood Group Inheritance
    • Shows how IA, IB, and Io alleles combine to form different blood types.

Practice Questions

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