Mendelian Genetics and Heredity

• Genetics explores heredity and variation in living organisms.
• Heredity is the transmission of traits between generations.
• Variation refers to differences in traits within and among populations.

Mendelian Genetics

• Gregor Mendel's experiments with pea plants laid the groundwork for understanding inheritance.
• Mendel's work, conducted in the 19th century, introduced the concept of genes as discrete units of heredity.
• Mendel's laws include:
  • The Law of Segregation
  • The Law of Independent Assortment
  • The Law of Dominance

The Law of Segregation

• Alleles are different versions of a gene.
• Each individual has two alleles for each trait, one inherited from each parent.
• During gamete formation (meiosis), these alleles separate, so each gamete carries only one allele for each trait.
• During fertilization, the fusion of gametes restores the paired condition.

The Law of Independent Assortment

• Genes for different traits are sorted separately from one another during gamete formation.
• The assortment of alleles for one gene does not affect the assortment of alleles for another gene.
• This law applies when genes are located on different chromosomes or are far apart on the same chromosome.

The Law of Dominance

• One allele masks the effect of another allele.
• The dominant allele expresses its trait even when paired with a recessive allele.
• The recessive allele only expresses its trait when paired with another recessive allele.

Key Concepts in Mendelian Genetics

• Genes are the basic units of heredity, carrying information for specific traits.
• Alleles are alternative forms of a gene.
• Genotype refers to the genetic makeup of an individual (e.g., AA, Aa, aa).
• Phenotype refers to the observable traits of an individual (e.g., tall, short).
• Homozygous individuals have two identical alleles for a gene (e.g., AA or aa).
• Heterozygous individuals have two different alleles for a gene (e.g., Aa).
• A dominant allele masks the expression of a recessive allele in a heterozygous individual.
• Recessive traits are only expressed when an individual is homozygous recessive.

Monohybrid Crosses

• A monohybrid cross involves tracking the inheritance of a single trait.
• It demonstrates the Law of Segregation.
• Typically involves crossing two true-breeding (homozygous) parents with different phenotypes.
• The F1 generation (first filial) consists of heterozygous individuals, all displaying the dominant phenotype.
• The F2 generation (second filial), resulting from self-crossing or crossing two F1 individuals, exhibits a phenotypic ratio of 3:1 (dominant:recessive) and a genotypic ratio of 1:2:1 (homozygous dominant: heterozygous: homozygous recessive).

Dihybrid Crosses

• A dihybrid cross involves tracking the inheritance of two traits simultaneously.
• It demonstrates the Law of Independent Assortment.
• Typically involves crossing two true-breeding parents differing in two traits.
• The F1 generation consists of individuals heterozygous for both traits.
• The F2 generation exhibits a phenotypic ratio of 9:3:3:1, representing all possible combinations of the two traits.

Punnett Squares

• Punnett squares are diagrams used to predict the genotypes and phenotypes of offspring from a genetic cross.
• They visually represent the possible combinations of alleles from the parents.
• They help determine the probabilities of different genotypes and phenotypes in the offspring.

Testcrosses

• A testcross involves crossing an individual with an unknown genotype (but displaying the dominant phenotype) with a homozygous recessive individual.
• The phenotypic ratios of the offspring reveal the genotype of the unknown parent.
• If all offspring display the dominant phenotype, the unknown parent is likely homozygous dominant.
• If the offspring exhibit a 1:1 ratio of dominant and recessive phenotypes, the unknown parent is heterozygous.

Extensions of Mendelian Genetics

• Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes.
• Codominance occurs when both alleles are expressed equally in the heterozygous phenotype.
• Multiple alleles exist when a gene has more than two alleles in the population (e.g., human blood types).
• Pleiotropy occurs when a single gene affects multiple phenotypic traits (e.g., sickle cell anemia).
• Epistasis occurs when the expression of one gene affects the expression of another gene.
• Polygenic inheritance occurs when a trait is controlled by multiple genes, resulting in a continuous range of phenotypes (e.g., height, skin color).

Sex-linked Inheritance

• Genes located on sex chromosomes (X and Y in humans) exhibit sex-linked inheritance.
• The X chromosome is larger and carries more genes than the Y chromosome.
• X-linked traits are more commonly expressed in males because they only have one X chromosome.
• Examples of X-linked traits include hemophilia and color blindness.

Pedigree Analysis

• Pedigree analysis is the study of family history to trace the inheritance of genetic traits.
• Pedigrees are diagrams showing the relationships between individuals and their phenotypes.
• They help determine the mode of inheritance of a trait (dominant, recessive, sex-linked).
• Squares typically represent males, and circles represent females.
• Shaded symbols indicate individuals expressing the trait.

Environmental Influences on Phenotype

• The environment can influence the expression of genes and affect phenotype.
• Examples include:
  • Temperature affecting coat color in Siamese cats.
  • Nutrition influencing height in humans.
  • Light affecting chlorophyll production in plants.