Chapter 12: Meiosis and Sexual Reproduction
Introducing Alleles
In ______________________________, a single individual gives rise to offspring that are
identical to itself
Asexual reproduction produces genetically ______________________________
In ________________________________________, two individuals mix their genetic
material in the production of offspring
______________________________of sexually reproducing eukaryotes contain pairs of
homologous chromosomes: One from the mother and one from the father
______________________________: carry genes (one from the mother and one
from the father) of the same characteristics
Different forms of the same gene are called____________________
____________________: forms of a gene with slightly different DNA sequences;
may encode slightly different version of the gene’s product
Paired genes on homologous chromosomes may ____________________
____________________as alleles arise by mutation and are the basis of differences
in shared traits
Offspring of sexual reproducers inherit ____________________of parental alleles resulting
in new combinations of traits
On the Advantages of Sexual Reproduction
Sex ____________________________________________________________, so offspring
have unique combinations of traits, this diversity offers a better chance of surviving
environmental change than clones
Meiosis Halves the Chromosome Number
Sexual reproduction mixes up alleles from two parents and involves the fusion of two mature
reproductive cells called ____________________
_________________ are specialized cells that are the basis of sexual reproduction
____________________ are derived from ______________________________:
immature reproductive cells
In males, the diploid germ cell develops into ____________________
In females, a diploid germ cell becomes an ____________________
All gametes are ____________________,their chromosome number is half of the
____________________number
Human somatic cell (diploid) ____________________
Human gamete (haploid) ____________________
____________________ is a nuclear division that halves the chromosome number in the
reproductive cells, ensures that offspring have the same number of chromosomes as the
parents
Meiosis involves ________________________________________where a single diploid
cell becomes four haploid cells:
Prior to the cellular divisions ______________________________occurs
First cellular division ____________________): instead of sister chromatids splitting
and going to separate cells like in mitosis, _____________________
__________split and are packaged into separate cells
In this cellular division __________cell forms __________cells
Second cellular division (____________________):____________________
__________split and are packaged into separate cells
In this cellular division _____________cells form __________cells
Fertilization Restores the Chromosome Number
Haploid gametes form by ____________________
The diploid chromosome number is restored at ____________________n when two haploid
gametes fuse forming a ____________________ which is _________________
____________________: cell formed by the fusion of two gametes
If meiosis did not precede fertilization, the chromosome number would ______________
with every generation
Chromosome number changes can have drastic consequences in animals
Meiosis I
Meiosis I: ____________________
Homologous chromosomes ____________________, ____________________, and
____________________
Spindle microtubules attach to ______________________________as the nuclear
envelope breaks up
Meiosis I: ____________________
The ______________________________pairs are aligned midway between spindle
poles
Meiosis I : ____________________
The ______________________________separate and begin heading toward the
spindle poles
Meiosis I: ____________________
Two clusters of ______________________________reach the spindle poles
A new nuclear envelope forms around each cluster, so _________________
____________________form
Meiosis II
Meiosis II: ____________________
The chromosomes ____________________
The ______________________________breaks up
Spindle microtubules attach to each ______________________________
Meiosis II: ____________________
The (still duplicated) ______________________________are aligned midway
between poles of the spindle
Meiosis II: ____________________
All ______________________________separate
The (now unduplicated) ____________________ head to the spindle poles
Meiosis II: ____________________
A complete set of____________________clusters at each spindle pole
A new ______________________________encloses each cluster
__________ __________ (n) nuclei form
Two events in meiosis introduce novel combinations of alleles into gametes:
____________________________________________________________
____________________________________________________________
Along with fertilization, these events contribute to the ____________________ in
combinations of traits among the offspring of sexually reproducing species
Crossing Over in Prophase I
During prophase I chromatids of homologous chromosomes ____________________ and
align along their length and ______________________________
This exchanging of segments is called ______________________________and it introduces
novel combinations of traits among offspring by creating new chromatids composed of a
________________________________________
In humans between 46 and 95 crossovers occur per meiosis, with an average of at least one
crossover per chromosome
Chromosome Segregation
When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to
each of the two new nuclei
For each chromosome pair, the maternal or paternal version is equally __________
________________________________________
Human gametes have 23 pairs of homologous chromosomes
Each time a human germ cell undergoes meiosis the four gametes that form end up
with one of __________ __________ (or 223) possible combinations of homologous
chromosomes
______________________________increases these combinations
The chance that the maternal or paternal version of any chromosome will end up in a
particular nucleus is 50 percent due to the way the spindle segregates the homologous
chromosome during meiosis I