Chapter 13

Meiosis And Sexual Life Cycle

An Introduction to Heredity

Heredity: the transition of traits from one generation to the next

Along w/ similarities, there is also variation- offspring differ somewhat in appearance from parents to offspring

Genetics: the study of heredity and hereditary variation

An Introduction to Heredity

Parents pass information to offspring through coded hereditary units called genes.- 30 to 40 thousand genes in humans- genes are segments of DNA

Concept 13.1

Two types of reproduction:Asexual reproduction: single

parent passes copies of all its genes to offspring; “like begets like.”

Sexual reproduction: two parents give rise to offspring; results in greater variation

Concept 13.2

Life cycle: generation-to-generation sequence of stages in the reproductive history of an organism

The human life cycle- each somatic cell has 46

chromosomes (23 pair); 2 copies of each pair are called homologous chromosomes

Concept 13.2

- chromosomes can be displayed through a karyotype

- pairs 1-22 are called autosomes- pair 23 are called sex

chromosomes- XX = female; XY = male

- we inherit one chromosome of each pair from each parent

Concept 13.2

Concept 13.2

Concept 13.2

- gametes, or sex cells, contain 22 autosomes and 1 sex chromosome; a cell with a single chromosome set is called a haploid cell.- haploid cells are abbreviated 1n (n = 23)

- the combining of gametes forms a zygote; becomes a diploid cell (2n)

Concept 13.2

- The only cells of the human body not produced by mitosis are gametes- this process is a form of cellular division called meiosis; meiosis reduces the chromosome number in half

Concept 13.2

The variety of sexual life cyclesAnimal cells

- after meiosis, the gametes undergoes no division before fertilization

Fungi- after meiosis, mitosis occurs and produces a 1n multicellular organism; forms 1n gametes that then fertilize

Concept 13.2

Plants (alternation of generations)- has both diploid (sporophyte) and haploid (gametophyte) multicellular stages;

Concept 13.2

Concept 13.3

Meiosis, like mitosis, is preceded by the replication of chromosomes; however, the single replication is followed by 2 consecutive divisions (meiosis I and meiosis II)

Concept 13.3

Concept 13.3

Meiosis I- during prophase I, chromosomes pair up in synapsis; 4 chromatids form a tetrad- during metaphase I, homologous pairs line up on equator- during anaphase I, chromosomes, not chromatids, separate to poles

Concept 13.3

Meiosis II- goes through the same steps as meiosis I, but does not replicate DNA

Meiosis outcome is 4 1n gametes from a single cell

Concept 13.4

3 mechanisms contribute to the genetic variation arising from sexual reproduction

Independent assortment of chromosomes-each homologous pair of chromosomes is positioned independently of the others; variation is 223 or about 8 million

Concept 13.4

Concept 13.4

Crossing over- when chromosomes line up along the equator, parts of chromatids can combine genes from parents

Random fertilization- even w/out considering crossing over, any two parents will produce a zygote with any of 70 trillion combination (223 x 223)

Concept 13.4