Genetic variation occurs due to three processes during meiosis: 1) Independent assortment of chromosomes, which randomly distributes maternal and paternal chromosomes to gametes, resulting in 16 possible combinations. 2) Crossing over allows exchange of genetic material between homologous chromosomes, generating recombinant chromosomes. 3) Random fertilization, where a single egg is fertilized by one of millions of possible sperm, combining the genetic variation from both parents. Together, these three sources maximize genetic diversity.

1. Genetic Variation Due to Meiosis
• What makes us unique compared to someone
else?
• What makes you YOU?

2. The 3 Sexual Sources of Genetic Variation
1. Independent assortment of chromosomes
2. Crossover of homologues
3. Random fertilization of eggs by sperm

3. 1. Segregation and independent assortment of chromosomes
during meiosis:
• Formation of tetrad during Prophase I of meiosis is random
• Maternal and paternal chromosomes are randomly distributed
• This diagram shows the two possibilities of chromosome distribution.
Alleles (or the different variations of the trait) are represented by letters: Y =
yellow seed color, y = green seed color, R = round seed shape, r = wrinkled seed
shape

4. Allele pairs are segregated during meiosis
• Distributed to different gamete
• A gamete has either the parental or maternal allele, but not both. Some
chromosomes may include elements of each due to synapsis (crossing over)
• Alleles on different pairs of homologous chromosomes are assorted
independently of each other
Gamete
contains
either the R
or the r, not
both

5. Net result:
• Each gamete has a single allele for each trait
• Allele present is one of four possible parental alleles
And you say, there is only 8 gametes shown here. We calculated
there should be 24 or 16 possibilities. These possibilities would
occur in the female and male, making the 16 possible gametes.

6. Calculating the Number of different gametes resulting from
independent assortment
2n, where n is the number of homologous pairs
The previous cell: 24 or 16 possible gametes
Human’s: 223 or 8.5 million possible gametes

7. 2. Crossing over (synapsis) of homologues and gene
recombination
Genes are arranged linearly along a chromosomes
length
 Genes on the same chromosomes are said to be
linked.
 During meiosis, paternal chromosomes can precisely
exchange gene segments with their homologous
maternal counterparts
This process gives rise to recombinant chromosomes
occurs during prophase I.
 Occurs at specific locations along the chromosome
called the Chiasmata

8. 3. Random fertilization
•A single human egg will be fertilized by a
single sperm
•Variation resulting from independent
assortment and random fertilization: 8.5
million x 8.5 million or 72 trillion