Mendelian Genetics
MENDELIAN GENETICS - AP BIOLOGY UNIT 5
GREGOR MENDEL (1822-1884)
Mendel was a monk who worked as a gardener, teacher, and researcher.
He did groundbreaking work on hereditary theories using pea pod plants.
Studied seven basic characteristics of pea pod plants:
Shape, color, and coat of the pea seed.
Shape and color of the pea pod.
Flower position.
Height of the plant.
Discovered three basic laws governing the passage of traits, known as Mendelian Genetics.
MENDEL’S LAWS
Law of Segregation:
Offspring inherit two different genes for each trait; only one trait can come from each parent.
Genes separate so each gamete (sex cell) receives only one copy of each gene.
Law of Independent Assortment:
Characteristics are passed down independently from one another (basis for recessive and dominant gene composition).
Principle of Dominance:
Each inherited allele is determined by two hereditary factors (one from each parent), deciding whether a gene is dominant or recessive.
Example: If a seed gene is recessive, it will remain undetected in the plant; however, the dominant trait will be expressed.
MENDEL’S DISCOVERIES
Led to:
Discovery of particulate inheritance: This concept explains how traits are inherited through discrete units, or genes, passed from parents to offspring, thus providing a foundation for the study of genetics.
Understanding of dominant and recessive traits.
Concepts such as genotype (genetic makeup) and phenotype (physical appearance).
Heterozygosity (two different alleles for a trait) and homozygosity (two identical alleles for a trait).
Mendel is recognized as the father of genetics, with actual proof of gene existence.
His work was underappreciated until the early 1900s but remains significant.
His experiments on peas have gained renewed interest over time.
MENDEL’S EXPERIMENTAL, QUANTITATIVE APPROACH
Advantages of Pea Plants for Genetic Study:
Diverse varieties with distinct heritable features.
Controlled mating possible.
Each flower has both stamens (sperm-producing) and carpels (egg-producing).
Cross-pollination can be achieved by dusting one plant with pollen from another.
Plants develop faster than animals.
Experimental Techniques:
Parental Generation (P): True-breeding varieties used for hybridization.
First Filial Generation (F1): Hybrid offspring created from P generation.
Second Filial Generation (F2): When F1 plants self-pollinate or cross-pollinate.
Mendel focused on characters that occurred in two distinct forms and used true-breeding plants.
MENDEL’S MODEL
Four Related Concepts:
Variation through Alleles:
Alternative gene versions account for inherited character variations (e.g., gene for flower color exists in versions for purple and white flowers).
These alternate forms are called alleles.
Each gene is located at a specific locus on a specific chromosome.
Inheritance of Alleles:
Every organism inherits two alleles for each character, one from each parent.
Alleles at a locus may be identical (true-breeding plants) or differ (F1 hybrids).
Dominance of Alleles:
If two alleles are present and differ, the dominant allele determines appearance.
Example: In F1 plants with purple flowers, the purple allele is dominant over the white.
Law of Segregation:
During gamete formation, alleles separate (segregate) so each gamete carries only one allele of each gene.
This corresponds to homologous chromosome distribution in meiosis.
USEFUL GENETIC VOCABULARY
Traits: Specific characteristics.
Genes: Segments composed of DNA determining traits (e.g., hair color).
Alleles: Different forms of a gene (represented by letters; e.g., Bb = brunette).
Hybrids: Offspring from crosses with distinct traits (e.g., Labradoodle).
P1: Parental group (grandparents).
F1: First generation offspring (e.g., parent traits).
F2: Second generation offspring (resulting from F1 crosses).
Phenotype: Physical characteristics of an organism.
Genotype: Genetic composition of an organism.
Dominant: Traits expressed when present, shown in capital letters (DD or Dd).
Recessive: Traits expressed only when dominant traits are absent, shown in lowercase letters (dd).
Homozygous: Organism with two identical alleles (e.g., DD or dd).
Heterozygous: Organism with two different alleles (e.g., Dd).
SINGLE/DOUBLE TRAIT CROSSES
Punnett Squares: Used to determine offspring genotypes and phenotypes.
Principle of Dominance: Dominant traits expressed if present, recessive traits shown only when no dominant traits are present.
Law of Segregation: Alleles separate into gametes during meiosis; each gamete carries one gene copy.
Law of Independent Assortment: Genes for different traits are inherited independently.
PHENOTYPIC AND GENOTYPIC RATIOS
Single Trait Cross Ratios:
Phenotypic Ratio: 3:1 (dominant:recessive).
Genotypic Ratio: 1:2:1 (Homozygous Dominant: Heterozygous: Homozygous Recessive).
When 2 Parents are both heterozygus.. their genotype is 9:3:3:1
PUNNETT SQUARE EXAMPLES
Example of a Single Trait Cross:
P Generation: Purple flowers x White flowers.
F1 Generation: 100% Purple flowers.
F2 Generation: 705 purple flowers: 224 white flowers (dominant:recessive ratio 3:1).
THE TESTCROSS
A method to determine the genotype of an individual with a dominant phenotype (could be homozygous dominant or heterozygous).
The testcross is done by breeding the individual with a homozygous recessive partner.
Results of Predictions:
If the purple-flowered parent is homozygous dominant: all offspring will display purple flowers.
If the purple-flowered parent is heterozygous: 50% purple and 50% white offspring.
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In this scenario, the recessive phenotype only manifests when the individual is homozygous recessive, demonstrating the clear distinction in gene expression between dominant and recessive alleles.
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties; for instance, when a red flowered plant is crossed with a white flowered plant, the resulting offspring may exhibit a pink phenotype, illustrating a blending of traits rather than a clear dominance.
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Frequency of Dominant Alleles
– Dominant alleles are not necessarily more common in populations than
recessive alleles
– For example, 1 baby out of 400 in the United States is born with extra
fingers or toes