Genetic Variation

What are alleles?

Different versions of the same gene

Which two processes in meiosis increase genetic variation?

• Crossing over

• Independent segregation

How do both crossing over and independent segregation increase genetic variation?

By producing new allele combinations

What does independent segregation involve?

Homologous chromosomes lining up randomly during meiosis

What does crossing over involve?

• Homologous chromosomes wrap around each other

• Chromatids exchange alleles

How do you calculate the number of possible chromosome combinations following meiosis?

2ⁿ , where n is the number of pairs of homologous chromosomes

What is a chromosome mutation?

A mutation that leads to meiosis producing cells with an abnormal number of chromosomes (different to the haploid number)

What is an example of a chromosome mutation?

Non-disjunction

What is the difference between non-disjunction in meiosis I and non-disjunction in meiosis II?

In meiosis I, a pair of homologous chromosomes doesn’t separate properly. In meiosis II, a pair of sister chromatids doesn’t separate properly

What do the gametes look like after non-disjunction has taken place during meiosis I?

Two gametes have an extra chromosome, two gametes have a missing chromosome

What do the gametes look like after non-disjunction has taken place during meiosis II?

One gamete has an extra chromosome, one gamete has a missing chromosome, two gametes have the correct number of chromosomes

What could non-disjunction lead to? Give an example.

Could lead to a genetic condition in offspring such as Down’s syndrome

Why does fertilisation lead to genetic variation?

It is random- any two gametes may fuse

How do you calculate the number of possible chromosome combinations following random fertilisation?

(2ⁿ)²