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How does meiosis result in daughter cells with half the number of chromosomes of the parent cell?
This process results in half the number of chromosomes of the parent cell because the chromosomes are copied once, but are divided twice (meiosis I separate homologous chromosomes and meiosis II separate sister chromatids)
Explain the process of meiosis I
Interphase: Chromosomes are replicated to form sister chromatids, which are genetically identical and join at the centromere. The single centriole is also replicated.
PMAT:
Prophase 1: During synapsis, chromosomes condense and homologous chromosomes form pairs to create tetrads. This is also when crossing over can occur in the chromatids of homologous chromosomes. A spindle forms from each centromere and spindle fibres move the tetrads around.
Metaphase 1: The tetrads are all arranged at the metaphase plate.
Anaphase 1: The homologous chromosomes are separated and are pulled to opposite ends of the cell.
Telophase 1: Movement of the homologous chromosomes continues until a haploid set is at each pole.
Cytokinesis 1: Two haploid daughter cells form. Chromosomes are still double-stranded.
Explain the process of meiosis II
*Note: Meiosis II is very similar to mitosis, but not identical
PMAT:
Prophase 2: Spindle fibres form, attaching to each sister chromatid and moving them around.
Metaphase 2: Sister chromatids are arranged at the metaphase plate and the sister chromatids face opposite ends.
Anaphase 2: The centromeres of sister chromatids separate and the now separated sisters travel toward opposite ends of the cell.
Telophase 2: The separated sister chromatids arrive at the opposite ends and nuclei form around the chromatids.
Cytokinesis 2: Cytokinesis separates the cytoplasm and 4 haploid daughter cells are produced.
How are mitosis and meiosis similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of daughter cells?
Both of these processes are similar in the way that the chromosomes separate at opposite ends of the cell.
However, they also differ in the number of cells produced as mitosis produces 2 diploid, identical daughter cells, whereas, meiosis produces 4 haploid, non-identical daughter cells.
What is nondisjunction?
It is when there are problems with the meiotic spindle causing errors in daughter cells, resulting in too many or too few chromosomes.
What is crossing over?
It is when the non-sister chromatids of homologous chromosomes exchange segments of their DNA. This occurs during synapsis in prophase 1 of meiosis.
What is independent assortment?
It is when homologous pairs line up on the equator of the cell independently of each other. This occurs during metaphase 1 of meiosis.
How do processes like crossing over and independent assortment increase genetic variation?
What is a gene?
A gene is a section of DNA that encodes for a certain trait e.g. eye colour
A part of a chromosome that controls the expression of a trait and is passed on to offspring; it has a specific DNA sequence
What is a chromosome?
It is a structure in the nucleus that contains DNA
What are homologous chromosomes?
It is a pair of chromosomes that contain the genetic code for the same traits. One chromosome comes from each parent. e.g. chromosomes contain information about hair colour
What is an allele?
An allele is a variant form of a gene e.g. blue eyes
What is the difference between a gene and an allele?
Genes encode for a certain trait, whereas, alleles are variant forms of a gene
e.g. a gene for hair colour, an allele for brown hair
How do DNA, genes, chromosomes and alleles account for the transmission of hereditary characteristics?
What is nondisjunction?
It is when a gamete with the incorrect number of chromosomes is fertilized, causing errors in daughter cells
e.g.
homologous chromosomes not separating properly during meiosis I
sister chromatids failing to separate during meiosis II
too many or too few chromosomes
How does nondisjunction result in the wrong number of chromosomes in a gamete?
What are some examples of human genetic disorders that can result from a gamete with the wrong number of chromosomes combining with a gamete with the correct number of chromosomes?
Down syndrome (trisomy 21), Turner syndrome (monosomy X or XO), Klinefelter’s syndrome (XXY male), Jacob’s syndrome (XYY male), trisomy X (XXX - doesn’t actually cause any health problems, just a female with an extra X chromosome)
What is down syndrome (trisomy 21)?
3 copies of chromosome 21 are made instead of 2
1 in 700 children born in the U.S. have trisomy 21
Chromosome 21 is the smallest human chromosome, but still has severe effects
Correlates to the age of the parents, as the mother and father grow older, the likelihood of trisomy 21 in a baby increases
What is Turner syndrome (monosomy X or XO)?
It is a female-only genetic disorder that occurs when an individual only has one X chromosome instead of 2
1 in every 5000 births
Degree of effects are varied: webbed neck, short stature, sterile
What is Klinefelter’s syndrome (XXY male)?
It is when a male is born with an extra X chromosome
1 in every 2000 births
Have male sex organs, but are sterile
Feminine characteristics: some breast development and lack of facial hair
Tall
Normal intelligence
What is Jacob’s syndrome (XYY male)?
It is when a male receives an extra Y chromosome from his father
1 in 1000 births
Slightly taller than average
More active
Normal intelligence or slight learning disabilities
Delayed emotional immaturity
Normal sexual development
What is trisomy X (XXX)?
It is when a female is born with an extra chromosome
1 in every 2000 births
Doesn’t have any negative effects and produces healthy females
Note 
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