BIO 311C Chapter Notes - Chapter 13: Gamete, Heredity, Mendelian Inheritance
Chapter 13: MEIOSIS AND SEXUAL LIFE CYCLES
●Heredity - the transmission of traits from one generation to the next, aka inheritance
● Along with inherited similarity, there is also variation.
● Genetics - the study of heredity and inherited variation
● The process of meiosis and fertilization maintain a species’ chromosome count during the
sexual life cycle.
13.1
● Inheritance of Genes
○ Parents endow their offspring with coded information in the form of hereditary units
called genes.
■ Account for family resemblances
■ Our genes program specific traits that emerge as we develop from fertilized
eggs into adults.
■ Most genes program cells to synthesize specific enzymes and other
proteins, whose cumulative action produces an organism’s inherited traits.
○ The genetic program is written in the language of DNA, the polymer of four
different nucleotides.
○ The transmission of hereditary traits has its molecular basis in the replication of
DNA.
○ Reproductive cells called gametes are the vehicles that transmit genes from one
generation to the next.
■ During fertilization, male and female gametes (sperm and eggs) unite,
passing on genes of both parents to their offspring.
○ Humans have 46 chromosomes in their somatic cells--all cells of the body except
the gametes and their precursors.
○ Locus - a gene’s specific location along the length of a chromosome
● Comparison of Asexual and Sexual Reproduction
○ Only organisms that reproduce asexually have offspring that are exact genetic
copies of themselves.
○ In asexual reproduction, a single individual is the sole parent and passes copies
of all its genes to its offspring without the fusion of gametes.
■ Ex. single-celled eukaryotes can reproduce asexually by mitotic cell
division, in which DNA is copied and allocated equally to two daughter cells.
■ Clone - a group of genetically identical individuals (arise from asexual
reproduction)
● ** Genetic differences occasionally arise in arise in asexually
reproducing organisms as a result of changes in the DNA called
mutations.
○ In sexual reproduction, two parents give rise to offspring that have unique
combinations of genes inherited from the two parents.
■ In contrast to a clone, offspring of sexual reproduction vary genetically from
their siblings and both parents.
13.2
●life cycle - the generation-to-generation sequence of stages in the reproductive history of
an organism, from conception to production of its own offspring
● Sets of Chromosomes in Human Cells
○ Careful examination of a micrograph of the 46 human chromosomes from a single
cell in mitosis reveals that there are two chromosomes each of 23 types.
■ This becomes clear when images of the chromosomes are arranged in
pairs, starting with the longest chromosomes. The resulting order display is
called a karyotype.
○ The two chromosomes of a pair have the same length, centromere position, and
staining pattern. There are called homologous chromosomes (or homologs).
■ X and Y chromosomes are an important exception to the general pattern of
homologous chromosomes in human somatic cells.
○ Typically, human females have a homologous pair of X chromosomes (XX), while
males have one X and one Y chromosome (XY)
■ Most of the genes carried on the X chromosome do not have counterparts
on the tiny Y, and the Y chromosome has genes lacking on the X.
○ The X and Y chromosomes are called sex chromosomes, due to their role in sex
determination.
■ The other chromosomes are called autosomes.
○ ** We inherit one chromosome of a pair from each parent.
■ Thus, the 46 chromosomes in our somatic cells are actually two sets of 23
chromosomes
○ The number of chromosomes in a single set is represented by n
.
■ Any cells with two chromosome cells is called a diploid cell and has a
diploid number of chromosomes, abbreviated 2n
.
● For humans, the diploid number is 46 (2n
=46)
● Even though chromosomes are duplicated, we still say the cell is
diploid, or 2n
. This is because it has only two sets of informations
regardless of the number of chromatids, which are merely copies of
the information in one set.
■ Unlike somatic cells, gametes contain a single set of chromosomes. Such
cells are called haploid cells, and each has a haploid number of
chromosomes (n
).
● For humans, the haploid number is 23 (n
=23)
● The set of 23 consists of 22 autosomes plus a single sex
chromosome.
○ An unfertilized egg contains an X chromosome; a sperms
contains either an X or a Y chromosome.
■ The chromosome number generally does not correlate with the size or
complexity of a species’ genome; it simply reflects how many linear pieces
of DNA make up the genomes.
● Behavior of Chromosome Sets in the Human Life Cycle
○ In each generation, the number of chromosomes is halved during meiosis but
doubles at fertilization.
○ Fertilization - union of gametes, culminating in fusion of their nuclei.
■ The resulting fertilized egg, or zygote, is diploid because it contains two
haploid sets of chromosomes (bearing maternal and paternal genes).
■ As a human develops into a sexually mature adult, mitosis of the zygote
and its descendant cells generates all the somatic cells of the body.
■ The only cells of the human body not produced by mitosis are the gametes,
which develop from specialized cells called germ cells
in the
gonads--ovaries in females and testes in males.
● IF human gametes were made by mitosis: They would be diploid like
the somatic cells. At the next round of fertilization, when the two
gametes fused, the normal chromosome number of 96 would double
to 92, and each subsequent generation would double the number of
chromosomes yet again.
○ BUT THIS DOESN’T HAPPEN BC OF MEIOSIS
■ Gamete formation involves a type of cell division called meiosis.
● This type of cell division reduces the number of chromosomes from
two to one in the gametes, counterbalancing the doubling that
occurs in fertilization.
● As a result of meiosis, each human sperm and egg is haploid
(n
=23).
● The Variety of Sexual Life
○ The timing of fertilization and meiosis in the life cycle varies, depending on the
species. However, they share a fundamental result: genetic variation among
offspring.
○ In the type that occurs in humans, gametes are the only haploid cells.
○ Plants and some species of algae exhibit a second type of life cycle called
alternation of generations.
■ This type includes both diploid and haploid stages that are multicellular.
The multicellular diploid stage is called the sporophyte
.
● Meiosis in the sporophyte produces haploid cells called spores.
○ Unlike a gamete, a haploid spore doesn’t fuse with another
cell but divides mitotically, generating a multicellular haploid
stage called the gametophyte
.
■ Cell of the gametophyte give rise to gametes by
mitosis.
Document Summary
Heredity - the transmission of traits from one generation to the next, aka inheritance. Along with inherited similarity, there is also variation . Genetics - the study of heredity and inherited variation. The process of meiosis and fertilization maintain a species" chromosome count during the sexual life cycle. Parents endow their offspring with coded information in the form of hereditary units called genes . Ex. single-celled eukaryotes can reproduce asexually by mitotic cell. Clone - a group of genetically identical individuals (arise from asexual. ** genetic differences occasionally arise in arise in asexually division , in which dna is copied and allocated equally to two daughter cells. reproduction) reproducing organisms as a result of changes in the dna called mutations . In sexual reproduction , two parents give rise to offspring that have unique combinations of genes inherited from the two parents. Our genes program specific traits that emerge as we develop from fertilized eggs into adults.
