Genetics Notes

Gregor Mendel and Genetic Principles

• Gregor Mendel, a 19th-century monk, is known as the father of genetics for his pioneering work on inheritance patterns, despite not knowing about chromosomes.
• Mendel discovered that traits are inherited through units (genes) and proposed that individuals have two copies (alleles) for each gene.
• Historical significance: Mendel’s work was not fully appreciated until decades later, highlighting the importance of his observations on patterns of inheritance.

Modern Genetic Framework

• The focus is on understanding modern biology rather than historical anecdotes and plants studied by Mendel (e.g., pea plants).
• Important terms include:
  • Diploid organisms: Organisms with two sets of chromosomes (e.g., humans).
  • Haploid: Refers to gametes (sperm and egg), which contain one set of chromosomes.

Chromosomes in Humans

• Humans have a total of 46 chromosomes, consisting of 23 pairs:
  • 22 pairs are called autosomal chromosomes (non-sex chromosomes) and do not determine sex.
  • 1 pair (23rd) consists of sex chromosomes: XX in females and XY in males.

Chromosomes and their Functions

• Autosomes: Involves characteristics unrelated to sex, includes genes contributing to various bodily features (e.g., height, eye color).
• Sex Chromosomes: In females (XX), both chromosomes are homologous, while in males (XY), the sex chromosomes are not homologous (Y does not match X).
  • Genes on the X chromosome are expressed in both sexes; however, males express whatever is on their single X since they don’t have another to mask it.
  • The Y chromosome is less important for survival and mainly influences male sex determination.

Inheritance Patterns and Probabilities

• Genetic concepts on sex-linked traits differ for males and females:
  • In females, traits are governed by two alleles, while males express the trait on their single X chromosome directly.
• Examples of sex-linked traits include color blindness and hemophilia as recessive conditions linked to the X chromosome.
• Punnett squares are useful for predicting genetic outcomes based on parental genotypes, with sex-linked traits providing unique challenges.

Example Scenarios

• Cross Analysis: Understanding how to set up crosses based on parental genotypes is critical.
  • Example 1: A male (XY) who is not color blind and a female (XX) who is a carrier for color blindness could have a child with varying probabilities of being color blind depending on the alleles contributed.
• In predictive scenarios, one must account for which parent contributes the X or Y chromosomal alleles and then apply understanding of dominant and recessive traits accordingly.

Epistasis in Genetic Expression

• Epistasis: Interaction between genes where the expression of one gene can mask or modify the effects of another gene.
• Example with mice colors: Two different genes affect pigmentation in fur, where a color-producing gene (B) and a pigment application gene (C) interact—if the second gene is non-functioning, it can result in a white phenotype regardless of the first gene's expression.

Conclusion

• Understanding genetics requires both knowledge of chromosomal behavior and application of probability in predicting genetic outcomes.
• The role of environmental factors, alongside genetic blueprints, affects observable traits, leading to a rich spectrum of appearances across individuals.