Rucete ✏ AP Biology In a Nutshell
11. Mendelian Genetics and Probability
This chapter reviews Mendel’s laws of inheritance and how they relate to probability. It explains how traits are passed through generations using terms like genotype, phenotype, and how to solve genetic problems using Punnett squares and probability rules.
Mendelian Genetics Overview
• Genetic information is passed from generation to generation via genes (segments of DNA).
• Mendel’s experiments with pea plants led to two foundational laws:
– Law of Segregation: • Each organism carries two alleles for a trait (one from each parent). • These alleles separate during meiosis (anaphase I), and each gamete receives only one.
– Law of Independent Assortment: • Alleles for different traits are inherited independently, as long as genes are on different chromosomes. • This occurs during metaphase I of meiosis.
Linked Genes
• Mendel’s laws apply only to genes on different chromosomes. • Genes close together on the same chromosome are “linked” and tend to be inherited together.
Pedigree Analysis
• Pedigrees show inheritance of traits over multiple generations. – Circles = females; squares = males. – Shaded = individual has the trait. – Horizontal line = mating; vertical line = offspring.
• Dominant traits appear in every generation. • Recessive traits may skip generations. • Sex-linked traits often appear more in males.
Key Genetic Terms
• Genotype: genetic makeup (e.g., AA, Aa, aa)
• Phenotype: physical trait expression (e.g., purple or white flowers)
• Homozygous: two identical alleles (AA or aa)
• Heterozygous: two different alleles (Aa)
• Dominant: one copy needed for expression
• Recessive: two copies needed for expression
Monohybrid Crosses
• Involve one gene with two alleles.
• Typical genotypic ratio: 1 AA : 2 Aa : 1 aa
• Typical phenotypic ratio (dominant vs. recessive): 3 : 1
Dihybrid Crosses
• Involve two genes, each with two alleles.
• Assumes genes are unlinked (on different chromosomes).
• Typical phenotypic ratio: 9 : 3 : 3 : 1
• Shows the Law of Independent Assortment in action.
Test Cross
• Used to determine the genotype of an individual with a dominant phenotype.
• Cross the individual with a homozygous recessive.
• If any offspring show the recessive phenotype → unknown parent is heterozygous.
Probability Rules
• Use to predict outcomes of genetic crosses without full Punnett squares.
– Multiplication rule: • Probability of two independent events both occurring = product of their probabilities. • Example: chance of Aa × Aa producing aa = ½ × ½ = ¼
– Addition rule: • Probability of either of two mutually exclusive events = sum of their probabilities. • Example: chance of getting Aa OR aA = ¼ + ¼ = ½
Extensions to Mendelian Genetics
• Incomplete dominance: heterozygote shows intermediate phenotype (e.g., red × white → pink)
• Codominance: both alleles expressed equally (e.g., AB blood type)
• Multiple alleles: more than two alleles in population (e.g., ABO blood types: Iᴬ, Iᴮ, i)
• Pleiotropy: one gene affects multiple traits (e.g., sickle cell gene affects blood shape and oxygen transport)
• Epistasis: one gene affects expression of another (e.g., coat color in Labrador retrievers)
• Polygenic inheritance: traits controlled by many genes (e.g., height, skin color)
In a Nutshell
Mendelian genetics explains how traits are inherited through dominant and recessive alleles. Punnett squares and probability rules help predict outcomes of crosses. Extensions like codominance and polygenic inheritance reveal that genetics can be more complex than simple dominant/recessive traits.