Offspring vary genetically from both parents and siblings
Sperm fertilizes egg to form zygote
Creates clones (offspring genetically IDENTICAL to each other and parent)
-Variations can arise through mutations
No sperm or egg required
What do human chromosomes inherit?
How many chromosomes do humans have?
46 total chromosomes (23 pairs)
-22 pairs of autosomes (homologous chromosomes)
-1 pair of sex chromosomes
Two copies of each chromosome (1 from mom and 1 from dad)
One set of chromosomes
Gametes (sperm and eggs)
Visual representation of chromosomes in a cell
Having more than two paired sets of chromosomes (triploid, tetraploid)
Very common in plants (30-80%)
Chromosome count doesn't mean complexity
Privet shrubs and humans each have a diploid number of 46 chromosomes per cell, why are the two species so dissimilar?
The two species have different genes
Why are triploid plants seedless?
They are seedless because they are sterile
There is an odd number of chromosomes so the gametes (seeds never form properly)
Is one duplicated chromosome
What is a homologous pair?
Two nonsister chromatids
A set from the mother and a set from the father
the region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.
What statement correctly describes homologous chromosomes?
D) They carry the same genes
What does meiosis do to chromosomal content?
It reduces chromosomal content from 2 n to n
Occurs in 2 steps (Meiosis I and Meiosis II)
Results in the formation of 4 haploid cells (may become sperm or eggs)
Why is meiosis important for organisms who reproduce sexually?
You need to ensure that the zygote has the appropriate chromosome count
The pair of homologous chromosomes in diploid parent cell duplicate to become a pair of homologous chromosomes consistent of sister chromatids
Homologous chromosomes pair and exchange segments (crossing over)
Synapsis: pairing of homologs to form tetrad
The pairs of homologous chromosomes (bivalents) line up at the metaphase plate and the spindle fibers attach to the chromosomes
The homologous pairs are separated.
Sister chromatids remain attached
What results at the end of Meiosis I
2 haploid cells
Each chromosome still has two sister chromatids
During anaphase 1, which of the following separate?
The chromosomes uncondenses and the nuclear envelope reforms
When the cleavage furrow splits the cell
When the cytoplasm splits off and the cell membrane pinches off
Two haploid cells form; chromosomes are still double
The 2 daughter cells, each with 23 chromosomes condense into X-shaped structures
The membrane around the nucleus disolves and meiotic spindle forms again
The pair of (nonidentical) sister chromatids line up along the metaphase plate and the mitotic spindle fibers attach to each of the sister chromatids
The (nonidentical) sister chromatids are then pulled to opposite poles
The separated chromatids are now individual chromosomes
The chromosomes complete their move to the opposite poles of the cell
A membrane forms around each set of chromosomes to create two new cell nuclei
The cytoplasm splits creating 4 granddaughter cells
With 23 chromosomes
Events unique to meiosis
Synapsis and crossing over
Line up of homologous pairs
Separation of homologs not chromatids
Sources of genetic variation
Independent assortment of chromosomes
Orientation of maternal and paternal homologs is random
Each homologous pair is sorted independently of the others
How to figure out the number of combos for independent assortment
n= # of chromosomes
223= 8.4 million possible gametes
Produces recombinant chromosomes
-contains DNA derived from two different parents
1-3 crossover events occur per homologous pair
How and at what stage do chromosomes undergo independent assortment?
Meiosis I at metaphase alignment
Crossing over contributes genetic variability between...
Any sperm (~84 million possibilities) can fertilize any egg (~8.4 million possibilities)
=70 trillion possible zygote combinations + variation due to crossing over
The genotype of a human zygote will differ from that of both parents. Which of the following does not contribute to this variation?
E) presence of dominant genes
What happens when meiosis goes wrong?
Gametes end up with extra or missing chromosomes
-often results in spontaneous abortion of the fetus
Better tolerated in plants than animals
What happens when nondisjunction occurs on homologous chromosomes in meiosis I
Products: n+1, n+1, n-1, n-1
number of chromosomes
What happens when nondisjunction occurs on sister chromatids in meiosis II
Products: n+1, n-1, n, n
number of chromosomes
The failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.
Abnormal chromosome number (i.e one extra or one missing)
Most are so disastrous that the embryo doesn't survive to term
What are the most common autosomal abnormalities in live births
Trisomy 21, 18 and 13
Where is aneuploidy better tolerated?
XXY, XYY, XXX, XO
XO- individual that only has 1 X so they are likely to be steril
1/700 children born in the US
Symptoms range from mild to severe (characteristic facial features, developmental delays, short stature, steril)
Life expectancy has increased to around 55
Link between maternal age and down syndrome
As a women's age increases so does her risk of having a child with down syndrome
This may be because of the way eggs develop
-Meiosis I begins when female is in the womb (arrests in prophase I and restarts when puberty is reached)
About 80% of babies with down syndrome are born to women under age 35. Why?
There are more women having more kids under 35
C) produces offspring genetically identical to the parent
Sexual and asexual reproduction are alike in that they both...
They both can occur in multicellular organizms
How many copies of chromosome 14 do your muscle cells contain?
Where do the 2 copies of chromosome 14 in your muscle cells come from?
One from mother and one from father
The paradox of sex
Asexual reproduction is more efficient
-no males (who cannot give birth) needed
-every individual capable of reproducing
Why does sexual reproduction exist?
Purifying selection hypothesis
Genetically identical offspring not likely to thrive if the environment changes (i.e new pathogen)
If off spring are genetically diverse, they are more likely to at least some will be resistant and can survive to pass on beneficial alleles
Purifying selection hypothesis
Natural selection against deleterious alleles (in asexual reproduction, all offspring would receive defective allele)