Genetics Chapter: Inheritance and Mendel's Experiment

Inheritance

• The passing of characteristics from one generation to the next is called inheritance.
• A pure tall plant crossed with another pure tall plant will produce a pure tall offspring.
• If this is repeated across generations, the offspring will continue to be pure tall.
• Pure breeding lines are essential for genetic experiments.
• To create a pure breeding line, the offspring must be repeatedly crossed with plants that share its same traits.

Mendel's Experiment

• Mendel studied seven characteristics of pea plants by creating pure breeding lines for seven traits.
• He spent years creating these pure lines to ensure the results of his experiments were accurate.
• Each characteristic has multiple possible expressions called traits.
• For example, seed shape is a characteristic, with traits being round or wrinkled.
• A dominant trait will always express itself if present, even if a recessive trait is also present.
• A recessive trait will only express itself if both copies of the trait are recessive.
• Traits that are expressed in the heterozygous condition are dominant traits, and traits that are only expressed when the allele is homozygous are recessive traits.
• Mendel's experiment on pea plants yielded a 3:1 phenotypic ratio for the offspring.
• Mendel's experiment yielded a 1:2:1 genotypic ratio for the offspring.

Mendel's Laws

• Mendel's Law of Dominance states that in a heterozygous condition, the dominant allele will express itself.
• Mendel's Law of Segregation describes how traits are inherited. Each parent gives one allele for each trait to their offspring. This law also supports the idea that alleles are passed down from one generation to the next without being blended together.
• Mendel's Law of Independent Assortment states that the inheritance of one trait does not affect the inheritance of another.

Other Types of Inheritance

• Incomplete Dominance is when the resulting phenotype differs from both parental phenotypes. This occurs when multiple copies of a gene must be present in order for a specific trait to be expressed.
• Co-dominance occurs when both alleles are expressed equally in the offspring.
• Polygenic Inheritance is when multiple genes are responsible for the inheritance of a trait.

Chromosomal Theory of Inheritance

• Chromosomal Theory of Inheritance states that chromosomes are responsible for inheriting traits from one generation to the next.
• Chromosomes are the vehicles which carry genes from the parents to the offspring.
• The chromosomal theory of inheritance describes the mechanism by which genes are passed from one generation to the next.

Mendel's Work Rediscovered

• In 1900, three scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak, independently rediscovered Mendel's work.
• This re-discovery led to widespread acceptance of Mendel's laws and their important role in inheritance.

Polygenic Inheritance

• Controlled by multiple genes
• Example: Human skin color, height
• Multiple genes contribute to a wide range of phenotypic expressions

Pleiotropy

• A single gene controls multiple phenotypic traits
• Example: Starch synthesis in pea seeds
• A gene can control both shape (round) and size of the pea seed

Linkage and Recombination

• Linked genes present close to each other on a chromosome tend to stay together during gamete formation, resulting in less recombination.
• Non-linked genes exhibit more recombination as they are located farther apart on the chromosome.
• Inversely proportional relationship: Strong linkage leads to weak recombination and vice versa.

Morgan's Experiments with Fruit Flies

• Demonstrated the concept of linkage and recombination using Drosophila melanogaster (fruit fly).
• Traits studied:
  • Body color: Brown (dominant), yellow (recessive)
  • Eye color: Red (dominant), white (recessive)
  • Wing Size: Large (dominant), miniature (recessive)
• Findings:
  • Body color and eye color are linked genes, showing high linkage and low recombination (98.7% parental combinations).
  • Eye color and wing size are non-linked genes, exhibiting higher recombination rates (37% recombinant combinations).

Genetic Mapping

• Developed by Alfred H. Sturtevant.
• Unit of measurement: Centimorgan (cM)
• Demonstrates the arrangement of genes on a chromosome and their relative distances from each other.
• Genes closer together exhibit stronger linkage and are thus more likely to be inherited together.

Sex Determination

• Refers to the mechanism by which an individual's sex is determined.

Types of Sex Determination:

• XX-XY type: Found in humans and Drosophila.
  • Females have two X chromosomes (XX).
  • Males have one X and one Y chromosome (XY).
• XX-XO type: Observed in many insects.
  • Females have two X chromosomes (XX).
  • Males have only one X chromosome (XO).
• ZZ-ZW type: Found in birds and some reptiles.
  • Females have two different sex chromosomes (ZW).
  • Males have two identical sex chromosomes (ZZ).
• Haplo-Diploidy: Seen in honeybees.
  • Males (drones) develop from unfertilized eggs and are haploid (16 chromosomes).
  • Females (queens and workers) develop from fertilized eggs and are diploid (32 chromosomes).

Mutation

• Changes in the nucleotide sequence of DNA or RNA.
• Can be caused by physical agents (radiation) or chemical mutagens.
• Mutations can be spontaneous or induced.

Types of Mutations:

• Point mutation: A single nucleotide change in the DNA sequence.
  • Example: Sickle Cell Anemia – a point mutation in the beta globin gene of hemoglobin causing a change in the amino acid sequence.

Genetic Disorders

• Caused by alterations in genetic material.

Categories of Genetic Disorders:

• Autosomal Disorders: Disorders caused by mutations in genes located on autosomes (non-sex chromosomes).
• Sex-linked Disorders: Disorders caused by mutations in genes located on sex chromosomes (X or Y chromosomes).

Autosomal Disorders:

• Dominant Autosomal Disorders:
  • Require only one copy of the mutant allele for the disorder to manifest.
• Recessive Autosomal Disorders:
  • Require two copies of the mutant allele for the disorder to manifest.

Sex-linked Disorders

• X-linked Disorders:
  • Mutations in genes present on the X chromosome.
  • Can be dominant or recessive.
• Y-linked Disorders:
  • Mutations in genes present on the Y chromosome.
  • Only affect males.

Common Genetic Disorders:

• Color Blindness:
  • X-linked recessive disorder affecting mainly males.
  • Difficulty distinguishing between red and green colors.
  • Types: Deuteranopia (green blindness), Protanopia (red blindness).
• Hemophilia:
  • X-linked recessive disorder.
  • Difficulty in blood clotting, resulting in excessive bleeding.
• Sickle Cell Anemia:
  • Autosomal recessive disorder.
  • Point mutation in the beta globin gene leading to abnormal hemoglobin structure and sickle-shaped red blood cells.
• Phenylketonuria (PKU):
  • Autosomal recessive disorder.
  • Deficiency of the enzyme phenylalanine hydroxylase leading to a buildup of phenylalanine, causing mental retardation and skin pigmentation abnormalities.
• Thalassemia:
  • Autosomal recessive disorder.
  • Reduced production of alpha or beta globin chains of hemoglobin, affecting red blood cell production.
  • Types: Alpha thalassemia (chromosome 16), Beta thalassemia (chromosome 11).
• Myotonic Dystrophy:
  • Autosomal dominant disorder.
  • Affects muscle function, causing progressive muscle weakness, fatigue, and stiffness.

Chromosomal Disorders

• Down Syndrome:
  • Trisomy 21 (extra copy of chromosome 21).
  • Characterized by mental retardation, distinctive physical features, and health problems.
• Klinefelter Syndrome:
  • XXY condition (extra copy of X chromosome in males).
  • Characterized by tall stature, small testes, and reduced fertility.
• Turner Syndrome:
  • XO condition (missing one X chromosome in females).
  • Characterized by short stature, webbed neck, and infertility.

Pedigree Analysis

• A diagrammatic representation of a family's genetic history used to track the inheritance of a particular trait or disorder.

Symbols used in Pedigree:

• Square: Male
• Circle: Female
• Filled shape: Affected individual
• Unfilled shape: Unaffected individual
• Diamond: Sex unspecified
• Single horizontal line: Marriage
• Double horizontal line: Consanguineous marriage (marriage between close relatives)
• Vertical lines: Offspring

Interpreting Pedigrees:

• Dominant: The trait appears in every generation.

• Recessive: The trait may skip generations.

• X-linked: The trait appears primarily in males, and daughters may be carriers.

• Key points:

  • When interpreting pedigrees, it is important to consider the pattern of inheritance and the frequency of the trait in the family.
  • The presence or absence of the trait in different generations can provide clues about whether it is dominant or recessive.
  • The distribution of the trait between males and females can indicate whether it is autosomal or sex-linked.

Explore the concepts of inheritance and the foundational experiments conducted by Gregor Mendel. This quiz covers the significance of pure breeding lines, characteristics, and traits in pea plants. Test your understanding of dominant and recessive traits and their expressions in plant genetics.