How does meisosis generate variability: 3 Mechanisms
( creates new combinations of alleles)
- Crossing Over
- Random alignment-Independent assortment
- Random Fertilization
Is a process in which 2 homologous chromosomes exchange genetic material
Where does the process of crossing over
Prophase 1 (late)
The process of crossing over
- In Prohase I, the homologs ( repicatid with sister chromatids) align themselves precisely gene by gene ( SYNAPSIS)
- The chromosomes are attached at a few points ( called CHIASMATA) where homologs exchange genetic material (exchanging alleles)
are diploid produce gametes that are haploid
Resulted from exchanging of alleles of two homologs in crossover process
how many chromatids take part in crossover
only 2 ( one sister chromatid from each homolog that is next to each other). ( recombinant)
those that do not experience chiasmata would remain unchanged ( parental)
only half of the chromatids contain allele combination that are new(
The result of crossing over
It ensures what
4 genetically different chromatids in place of 2 pairs of identical chromatids
Each will end up in a haplid cell (4)
meiosis takes place in which cell
germ cells that are diploid they produce gametes that are hapoid
It crossing over ensures what
that each haploid cell will be genetically different from the others
Independent assortment or random allignment occurs where
IN METAPHASE I
Where the homologs may align in the equator in any possible combination.
so that when in anaphase and 1/2 of a homolgous pair gets pulled to each pole it may have different genetic combinations of mom and dad
The number of possible arrangements in independent assortmnent are related to what
The number of chromosomes
the formula for random alignment
2^n ( where n is the haploid number)
eg. If there are 2 pairs of of homologs
(XX xx)= 4 chromosomes
n= 2n=4( the number of chromosomes)
eg 3. if their are 3 pairs
2^3 =8 possibilities/configurations can occur in a gamete
The purpose of Random Fertilization
It multiplies diversity
Why do identical twin exist
They are a result from just one fertilization event.
The resulting zygot or embryo splits in half creating seperate identical babies
They are (MONOZYGOTIC) resulting from one zygote
Fraternal twins are
DIZYGOTIC ( resulting from 2 zygotes) ( non identical )
How does crossing over shuffle genes?
Crossing over shuffles genes when part of one chromosome switches places with part of its homologous chromosome.
. Explain how events in metaphase I enable a human to produce over 8 million genetically different gametes.
During metaphase I, 23 homologous pairs of chromosomes (consisting of four chromatids) align at the equator of the cell. Each homologous pair can align in either of two ways. For 23 homologous pairs, there are 223 possible alignments (more than 8,300,000).
. What is random fertilization
During random fertilization, any one of the 8,388,608 possible combinations of gametes in a woman’s eggs can be fertilized by any one the 8,388,608 possible combinations of gametes in a man’s sperm, producing over 70 trillion offspring combinations.
How are identical twins different from fraternal twins?
Identical (monozygotic) twins arise from a single zygote and are therefore genetically identical to each other. Fraternal (dizygotic) twins arise from two separate fertilized eggs and are therefore as similar (or dissimilar) as any two non-identical siblings.
how many possibilities can a human cell have of chromosomes combinatiions
2^23 = 8388608 and equally as many gametes for either sex