10.1 Mendel's Laws and Probability

Mendel's laws revolutionized our understanding of inheritance. By studying pea plants, he discovered that traits are passed down through discrete units called genes. His work laid the foundation for modern genetics and our grasp of heredity.

Mendel's experiments revealed the principles of segregation and independent assortment. These laws explain how traits are inherited and why certain characteristics appear in offspring. Understanding Mendel's work is crucial for grasping the basics of genetic inheritance.

Mendel's Laws

Gregor Mendel's Experiments

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• Gregor Mendel, an Austrian monk, conducted experiments on pea plants in the mid-19th century
• Studied seven distinct traits in pea plants, such as flower color (purple or white) and seed shape (round or wrinkled)
• Carefully tracked the inheritance patterns of these traits over multiple generations
• Mendel's meticulous experiments laid the foundation for modern genetics

Law of Segregation

• Mendel's Law of Segregation states that each individual possesses two alleles for a given trait, one inherited from each parent
• During gamete formation (meiosis), these alleles segregate, or separate, so that each gamete carries only one allele for each trait
• When fertilization occurs, the offspring receives one allele from each parent, resulting in a new combination of alleles

Law of Independent Assortment and Crosses

• Mendel's Law of Independent Assortment states that the inheritance of one trait is independent of the inheritance of other traits
• During meiosis, alleles for different traits are sorted independently into gametes
• Monohybrid cross involves breeding individuals that differ in a single trait (e.g., crossing a purple-flowered pea plant with a white-flowered pea plant)
• Dihybrid cross involves breeding individuals that differ in two traits (e.g., crossing a round, yellow pea with a wrinkled, green pea)
• These crosses demonstrate the independent assortment of alleles and help predict the probability of offspring inheriting specific traits

Alleles and Genotypes

Dominant and Recessive Alleles

• Alleles are alternative forms of a gene that can result in different phenotypes
• Dominant allele is an allele that is expressed in the phenotype when present in either one (heterozygous) or two copies (homozygous)
• Recessive allele is an allele that is only expressed in the phenotype when present in two copies (homozygous recessive)
• In pea plants, purple flower color is dominant over white flower color, and round seed shape is dominant over wrinkled seed shape

Genotypes and Zygosity

• Genotype refers to the specific alleles an individual possesses for a given trait
• Homozygous individuals possess two identical alleles for a specific trait (e.g., two dominant alleles or two recessive alleles)
• Heterozygous individuals possess two different alleles for a specific trait (e.g., one dominant allele and one recessive allele)
• The genotype determines which alleles can be passed on to offspring during reproduction

Phenotypes and Punnett Squares

Phenotypes and Genetic Expression

• Phenotype is the observable physical or biochemical characteristics of an organism, resulting from the interaction between its genotype and the environment
• In Mendel's pea plant experiments, phenotypes included flower color (purple or white) and seed shape (round or wrinkled)
• The expression of dominant or recessive alleles in the genotype determines the phenotype of an individual

Punnett Squares and Probability

• Punnett square is a diagram used to predict the probability of offspring having a particular genotype or phenotype
• Constructed by drawing a grid with the possible alleles from one parent listed across the top and the possible alleles from the other parent listed down the left side
• Each cell in the Punnett square represents a potential offspring and contains the alleles it would inherit from each parent
• Punnett squares help determine the probability of offspring exhibiting specific traits based on the genotypes of the parents (e.g., a cross between two heterozygous individuals for a dominant trait will result in a 3:1 ratio of offspring with the dominant phenotype to offspring with the recessive phenotype)