Chapter 13 Meiosis and Sexual Life Cycles
Lecture Outline
Overview: Hereditary Similarity and Variation
Living organisms are distinguished by their ability to reproduce their own kind.
Offspring resemble their parents more than they do less closely related individuals of the same species.
The transmission of traits from one generation to the next is called heredity or inheritance.
However, offspring differ somewhat from parents and siblings, demonstrating variation.
Farmers have bred plants and animals for desired traits for thousands of years, but the mechanisms of heredity and variation eluded biologists until the development of genetics in the 20th century.
Genetics is the scientific study of heredity and variation.

Concept 13.1 Offspring acquire genes from parents by inheriting chromosomes
Parents endow their offspring with coded information in the form of genes.
Your genome is comprised of the tens of thousands of genes that you inherited from your mother and your father.
Genes program specific traits that emerge as we develop from fertilized eggs into adults.
Genes are segments of DNA. Genetic information is transmitted as specific sequences of the four deoxyribonucleotides in DNA.
This is analogous to the symbolic information of language in which words and sentences are translated into mental images.
Cells translate genetic “sentences” into freckles and other features with no resemblance to genes.
Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.
The transmission of hereditary traits has its molecular basis in the precise replication of DNA.
This produces copies of genes that can be passed from parents to offspring.
In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.
After fertilization (fusion of a sperm cell and an ovum), genes from both parents are present in the nucleus of the fertilized egg, or zygote.
Almost all the DNA in a eukaryotic cell is subdivided into chromosomes in the nucleus.
Tiny amounts of DNA are also found in mitochondria and chloroplasts.
Every living species has a characteristic number of chromosomes.
Humans have 46 chromosomes in almost all of their cells.
Each chromosome consists of a single DNA molecule associated with various proteins.
Each chromosome has hundreds or thousands of genes, each at a specific location, its locus. Like begets like, more or less: a comparison of asexual and sexual reproduction.
Only organisms that reproduce asexually can produce offspring that are exact copies of themselves.
In asexual reproduction, a single individual is the sole parent to donate genes to its offspring.
Single-celled eukaryotes can reproduce asexually by mitotic cell division to produce two genetically identical daughter cells.
Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a mass of cells by mitosis.
An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals.
Members of a clone may be genetically different as a result of mutation.
In sexual reproduction, two parents produce offspring that have unique combinations of genes inherited from the two parents.
Unlike a clone, offspring produced by sexual reproduction vary genetically from their siblings and their parents.

Concept 13.2 Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.
It starts at the conception of an organism and continues until the organism produces its own offspring. Human cells contain sets of chromosomes.
In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
Each chromosome can be distinguished by size, position of the centromere, and pattern of staining with certain dyes.
Images of the 46 human chromosomes can be