Unit 5 Topic 3 Packet
Introduction to Mendelian Genetics
Discusses the concept of common ancestry.
DNA and RNA carry genetic information; the genetic code is universal among all living things.
Gregor Mendel's Contributions
Gregor Mendel: Austrian monk known as the father of genetics.
Studied inheritance using pea plants.
Developed key principles about heredity based on his experiments.
Study of pea plants allowed him to identify distinct traits due to their simple, bimodal characteristics.
Why Pea Plants?
Pea plants have many varieties.
Easy to control mating due to their reproducing methods.
Short generation time allows for quick observation of traits.
Key Traits Observed in Pea Plants
Mendel tracked traits coming in two forms.
Color: purple (dominant) or white (recessive).
Seed shape: round (dominant) or wrinkled (recessive).
Use of true breeding plants (organisms that consistently produce the same variety through self-pollination).
Example: True breeding purple plants => only purple offspring; true breeding white plants => only white offspring.
Generational Terminology
P Generation: Parental generation (true breeding plants).
F1 Generation: First filial generation; hybrids from the cross of P generation parents (purple x white).
F2 Generation: Second filial generation; obtained by self-fertilizing the F1 generation.
Outcome: Purple and white flowers reappear in a 3:1 ratio.
Use of Punnett Squares
Punnett Squares: Diagrams to predict allele combinations in offspring.
Dominant traits represented by uppercase letters; recessive traits by lowercase letters.
Basic vocabulary terms:
Homozygous: identical alleles (dominant: AA, recessive: aa).
Heterozygous: one dominant and one recessive allele (Aa).
Genotype: genetic makeup of the organism (AA, Aa, aa).
Phenotype: observable traits.
Test Cross: Used to determine the genotype of a dominant trait by crossing it with a homozygous recessive.
Principles of Heredity
Mendel's experiments led to two main principles:
Law of Segregation: Two alleles for a trait segregate during gamete formation.
Law of Independent Assortment: Genes for different traits are inherited independently.
Mendel's Findings from Pea Plant Crosses
Crossed true breeding purple and white flowered plants; F1 was all purple.
White trait reappeared in F2, leading to the conclusion of dominant (purple) and recessive (white) traits.
Consistent 3:1 ratio in F2 generation confirmed dominance.
Mendel’s Four Concepts
Alternative versions of genes (alleles) exist and account for variations.
Organisms inherit two alleles for each character (one from each parent).
If the alleles differ, the dominant allele determines appearance; recessive allele is not expressed.
Law of Segregation: Two alleles for the same trait separate during gamete formation.
Alleles and Gametes
Somatic cells are diploid, having two copies of each chromosome.
Alleles are variations of a gene that come from each parent.
Examples of Crosses
Monohybrid Cross: A cross between two parents differing in one trait.
Self-crossing of heterozygous individuals leads to expected phenotypic rations like the 3:1 ratio.
Dihybrid Cross: Involves two traits leading to a 9:3:3:1 phenotypic ratio when heterozygous parents are crossed.
Probability in Genetics
Mendelian genetics applies the rules of probability.
Multiplication Rule: Probability of two independent events occurring together.
Addition Rule: Probability of one or another event occurring (mutually exclusive).
Pedigrees in Genetics
Pedigrees illustrate genetic inheritance patterns across generations.
Squares represent males, circles represent females; shaded indicates affected individuals.
Understanding traits like dominance and X-linkage based on pedigree analysis.