🔬General Biology I Unit 12 – Mendel's Experiments and Heredity

Key Concepts and Terminology

• Alleles are different versions of a gene that can result in variations in the characteristic controlled by that gene
• Genotype refers to the genetic makeup of an organism, while phenotype is the physical expression of those genes
• Dominant alleles mask the expression of recessive alleles when both are present (heterozygous)
• Homozygous describes having two identical alleles for a particular gene (e.g., TT or tt)
• Heterozygous refers to having two different alleles for a particular gene (e.g., Tt)
• Punnett squares are diagrams used to predict the probability of offspring genotypes and phenotypes based on parent genotypes
• Monohybrid crosses involve breeding experiments that examine the inheritance of a single trait
• Dihybrid crosses involve breeding experiments that examine the inheritance of two traits simultaneously

Historical Context of Mendel's Work

• Gregor Mendel, an Austrian monk, conducted experiments on pea plants in the mid-19th century
• Prior to Mendel, the blending theory of inheritance was widely accepted, which stated that offspring traits were a blend of parental traits
• Mendel's work was largely ignored until the early 20th century when it was rediscovered and formed the basis of modern genetics
• Mendel's experiments were groundbreaking because they demonstrated that traits are inherited as discrete units (genes) rather than blending together
• Mendel's meticulous record-keeping and use of mathematics to analyze his results set a new standard for scientific experimentation

Mendel's Experimental Design

• Mendel chose pea plants for his experiments because they had several distinct traits and could be easily bred
• He focused on seven traits: seed shape, seed color, flower color, pod shape, pod color, flower position, and plant height
• Mendel used true-breeding plants, which always produced offspring with the same traits when self-pollinated
• He conducted monohybrid crosses by breeding plants that differed in only one trait and analyzing the offspring
• Mendel also performed dihybrid crosses, breeding plants that differed in two traits to study the inheritance of multiple characteristics
• He used large sample sizes and kept detailed records of his results, allowing for statistical analysis
• Mendel's experimental design allowed him to control variables and make clear conclusions about the inheritance of traits

Mendel's Laws of Inheritance

• The Law of Segregation states that each organism has two alleles for each gene, which segregate (separate) during gamete formation
  • Each parent passes on one allele per gene to their offspring
  • The two alleles in an offspring have an equal chance of being expressed
• The Law of Independent Assortment states that the inheritance of one trait is independent of the inheritance of other traits
  • Alleles for different genes are distributed to gametes independently during meiosis
  • This law applies to genes located on different chromosomes or far apart on the same chromosome
• The Law of Dominance states that when an organism has two different alleles for a gene, one allele (the dominant allele) will mask the expression of the other allele (the recessive allele)
  • The recessive allele will only be expressed when an organism is homozygous for that allele

Punnett Squares and Probability

• Punnett squares are used to predict the probability of offspring genotypes and phenotypes based on the genotypes of the parents
• In a monohybrid cross, a 2x2 Punnett square is used to show the possible combinations of alleles in the offspring
• For a heterozygous cross (Tt x Tt), the probability of each genotype in the offspring is: 25% TT, 50% Tt, and 25% tt
• In a dihybrid cross, a 4x4 Punnett square is used to show the possible combinations of alleles for two genes in the offspring
• The probability of each genotype in a dihybrid cross can be calculated using the product rule of probability
  • For example, in a cross between two heterozygous parents (TtRr x TtRr), the probability of an offspring being homozygous dominant for both traits (TTRR) is 1/16 or 6.25%
• Punnett squares and probability calculations help predict the likelihood of specific genotypes and phenotypes in offspring

Extensions of Mendelian Genetics

• Incomplete dominance occurs when the phenotype of the heterozygous offspring is intermediate between the two homozygous phenotypes (e.g., red and white flowers producing pink flowers)
• Codominance occurs when both alleles in a heterozygous individual are expressed equally (e.g., AB blood type)
• Multiple alleles refer to the existence of more than two alleles for a particular gene (e.g., ABO blood type system)
• Polygenic traits are characteristics controlled by multiple genes, each with a small additive effect (e.g., skin color, height)
• Epistasis occurs when the expression of one gene is influenced by the presence of one or more modifier genes
• Pleiotropy refers to a single gene influencing multiple phenotypic traits (e.g., sickle cell anemia affecting red blood cell shape and oxygen transport)
• Gene linkage occurs when genes located close together on the same chromosome are inherited together more often than expected by chance

Modern Applications and Relevance

• Mendelian genetics forms the foundation for modern genetic research and biotechnology
• Understanding inheritance patterns is crucial for predicting and treating genetic disorders (e.g., cystic fibrosis, Huntington's disease)
• Genetic counseling uses Mendelian principles to advise families about the probability of inheriting genetic conditions
• Plant and animal breeding programs apply Mendelian genetics to select for desired traits (e.g., disease resistance, higher yield)
• Genetically modified organisms (GMOs) are created by inserting genes from one species into another to introduce desired characteristics
• Personalized medicine uses an individual's genetic information to tailor treatments and preventive measures
• Forensic science relies on Mendelian genetics to analyze DNA evidence and establish family relationships
• Evolutionary biology builds upon Mendelian genetics to understand how traits evolve in populations over time

Review Questions and Practice Problems

1. What is the difference between genotype and phenotype?
2. Describe the three laws of Mendelian inheritance and their significance.
3. In pea plants, tall (T) is dominant to short (t). If a heterozygous tall plant is crossed with a short plant, what proportion of the offspring will be tall? What proportion will be short?
4. In humans, widow's peak (W) is dominant to straight hairline (w). If two heterozygous individuals have children, what is the probability that their first child will have a widow's peak?
5. In a dihybrid cross between two pea plants heterozygous for both seed shape (Rr) and seed color (Yy), what proportion of the offspring will have round, yellow seeds (RRYY)?
6. Explain the concept of incomplete dominance and provide an example.
7. How does polygenic inheritance differ from Mendelian inheritance?
8. Describe two modern applications of Mendelian genetics and their importance.