Chapter 14: Mendel and the Gene – Principles of Inheritance

Guida di studio - Note intelligenti

Appunti personalizzati basati sui tuoi materiali, ampliati con definizioni chiave, esempi e contesto.

Introduction to Mendel and the Gene

Historical Foundations of Genetics

The study of genetics began with Gregor Mendel, who established the groundwork for the chromosome theory of inheritance through experiments on garden peas. Later, Sutton and Boveri proposed that genetic information is transmitted from one generation to the next via chromosomes, linking inheritance to meiosis and asserting that genes are located on chromosomes. Genetics is the branch of biology that focuses on inheritance.

14.1 Mendel’s Experimental System

Key Concepts in Heredity

Heredity is the transmission of traits from parents to offspring. A trait is any observable characteristic of an individual. Mendel sought to understand heredity and tested two main hypotheses:

Blending inheritance: Parental traits blend so that offspring have intermediate traits.
Inheritance of acquired characteristics: Parental traits are modified through use and then passed on to offspring.

The Garden Pea as a Model Organism

Why Peas Were Chosen

Mendel chose garden peas for his experiments because they were inexpensive, easy to grow, had a short generation time, produced large numbers of seeds, and allowed control over parental mating. These characteristics made peas an excellent model organism for genetic research.

Species used for research
Practical to work with
Conclusions drawn from peas can be applied to other species
Peas have several polymorphic traits (traits that appear commonly in two or more forms, e.g., purple vs. white flowers)

How Did Mendel Control Matings?

Self-Fertilization and Cross-Fertilization

Peas normally self-fertilize (self-pollinate):

Male organs produce pollen grains (sperm cells)
Female organs produce eggs
Pollen falls on the female organ of the same flower

Mendel could prevent self-pollination by removing male organs from the flower and used pollen from one flower to fertilize another, a process called cross-fertilization or a cross.

Diagram of self-fertilization and cross-fertilization in pea flowers

What Traits Did Mendel Study?

Traits and Phenotypes

Mendel worked with pea varieties that differed in seven easily recognizable traits:

Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower and pod position
Stem length

An individual's observable features comprise its phenotype. Mendel's pea population had two distinct phenotypes for each of the seven traits.

Pure Lines and Hybrids

Mendel worked with pure lines (true-breeding lines), which produced offspring identical to themselves when self-fertilized. He used these plants to create hybrids by mating two different pure lines that differed in one trait.

Summary Table 14.1: Terms Used in Mendelian Genetics

Key Definitions

Term	Definition	Example or Comment
Autosomal inheritance	The patterns of inheritance of any genes not on a sex chromosome.	Mendel studied only autosomal patterns of inheritance.
Gene	A hereditary factor that influences a particular trait.	Modern definition: a region of DNA in a chromosome that codes for a particular protein or RNA.
Allele	A particular form of a gene.	The two alleles in an individual may be the same or different.
Genotype	A listing of the alleles of particular genes in an individual.	In diploids, genotype lists two alleles of each gene; in haploid gametes, genotype lists one allele of each gene.
Phenotype	An individual's observable traits.	Phenotype can be observed at levels from molecules to the whole organism; influenced to different degrees by the genotype.
Homozygous	Having two of the same allele.	Refers to a particular gene.
Heterozygous	Having two different alleles.	Refers to a particular gene.
Dominant allele	An allele that produces its phenotype in heterozygous and homozygous genotypes.	Dominant does not imply high frequency or high fitness.
Recessive allele	An allele that produces its phenotype only in homozygous genotypes.	The recessive phenotype "recedes" or disappears in heterozygous individuals; recessive does not imply low frequency or low fitness.
Pure line	Individuals identical in phenotype that, when crossed, produce offspring that all have the same phenotype.	Pure line individuals are homozygous for the gene in question.
Hybrid	Offspring from crosses between homozygous parents with different genotypes.	Hybrids are heterozygous.