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6.6 Meiosis and Genetic Variation
Mendel’s Laws
• Law of Independent Assortment: allele pairs separate
independently of one another during meiosis, therefore
traits are inherited separately. (He discovered this using
dihybrid crosses)
• Law of Segregation: Organisms inherit one gene from
each parent (a total of two copies). Organisms donate
only one copy to their offspring (so the paired copy
separates during gamete formation).
6.6 Meiosis and Genetic Variation
Most traits do not follow a simple dominance pattern
• The Punnett Square was to show the probability of
results from a simple dominance situation.
Homologous chromosomes are the set of chromosomes
donated by your parents…the matching traits. One
chromosome from your mom and one from dad.
• One of each from the set is what you donate to your
child. (Fertilization)
• Fertilization is the fusing of haploid sex cells.
6.6 Meiosis and Genetic Variation
Heredity patterns can be calculated with probability.
• Probability is the likelihood that something will happen.
• Probability predicts an average number of occurrences, not
an exact number of occurrences.
number of ways a specific event can occur
• Probability =
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
6.6 Meiosis and Genetic Variation
KEY CONCEPT
Independent assortment and crossing over
during meiosis result in genetic diversity.
6.6 Meiosis and Genetic Variation
Sexual reproduction creates unique combinations of
genes.
• Sexual reproduction creates unique combination of genes.
– independent assortment of chromosomes in meiosis
– random fertilization of gametes
• Unique phenotypes may give a reproductive advantage to
some organisms.
6.6 Meiosis and Genetic Variation
Crossing over during meiosis increases genetic diversity.
• Crossing over is the exchange of chromosome
segments between homologous chromosomes.
– occurs during prophase I of meiosis I
– results in new combinations of genes
6.6 Meiosis and Genetic Variation
• Chromosomes contain many genes.
– The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
– Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
• Genetic linkage allows the distance between two genes to
be calculated.
6.6 Meiosis and Genetic Variation
Genotype from phenotype?
• No you cannot determine genotype from phenotype,
especially if the trait is more dominant.
• You could perform a testcross to determine an unknown
organisms genotype.
• Testcross: crossing an organism with an unknown
genotype with an organism with a known phenotype to
determine the genotype of the unknown organism.