Chapter 9 - Genetic Diversity

Question 1

What is a gene mutation and how does it occur?

Answer 1

• a change in the base sequence of DNA which results in the formation of a new allele
• can arise spontaneously during DNA replication.

Question 2

What is an allele?

Answer 2

A specific version of a gene

Question 3

What are the 3 types of mutations?

Also describe role of mutagenic agents

Answer 3

1. Substitution - swapping 1 base for another
2. Deletion - deleting a base
3. Addition - adding a base

• mutagenic agents can increase rate of gene mutations

Question 4

What are the positives of gene mutations?

Answer 4

• results in a change of polypeptide = beneficial change in properties of protein
• increases chance of organisms surviving to reproduce

Question 5

What are the negatives of gene mutations?

Answer 5

• results in a change of polypeptide = harmful change in properties of protein
• decreases chance of organisms surviving to reproduce

Question 6

Why do some mutations have no effect?

Answer 6

• genetic code is degenerate so amino acid sequence may not change
• does change amino acid but no effect on tertiary structure
• mutation occurs in introns so is non coding
• new allele is recessive so does not influence phenotype.

Question 7

What is meiosis vs mitosis?

Answer 7

Meiosis = produces four daughter cells, each with half number of chromosomes as parent cell

Mitosis = produces two genetically identical daughter cells each with same number of chromosomes as parent cell

Question 8

What is the importance of meiosis?

Answer 8

Ensures the number of chromosomes is halved to result in the formation of gametes otherwise the number of chromosomes in each generation would continue to double

Question 9

What are homologous chromosomes?

Answer 9

Chromosomes that have the same genes with the same loci

Question 10

Describe the process of meiosis [2]

Answer 10

1. In the first division, homologous chromosomes pair up and their chromatids wrap around each other:
   - Equivalent portions of chromatids may be exchanged (crossing over).
   - by the end of this division, the homologous pairs have separated with 1 chromosome from each pair going into 1 of the 2 daughter cells
2. In second meiosis division, the chromatids move apart:
   - at end, 4 cells have been formed containing 23 chromosomes

Question 11

When does crossing over occur in meiosis and what is it?

Answer 11

Occurs at the start of meiosis

• homologous pairs of chromosomes pair up to form a bivalent
• chiasmata form between them (point at which DNA exchange occurs)
• equal length of non-sister chromatids are exchanged resulting in new combinations of alleles

Question 12

When does independent segregation of homologous chromosomes occur during meiosis and what is it?

Answer 12

During meiosis 1

• each chromosome lines up alongside its homologous pair
• these pairs arrange themselves in a line randomly
• one of each pair will pass into each daughter cell
• this is done at random based on how they line up in the parent cell

Question 13

How does genetic variation come about?

Answer 13

• random fusion of gametes
• crossing over during meiosis
• independent segregation of homologous chromosomes
• new genetic combinations (recessive/ dominant alleles)

Question 14

What is the equation to determine the possible number of combinations of chromosomes for each daughter cell?

Answer 14

2^n

n = number of pairs of homologous chromosomes

Question 15

What is the equation to determine the total number of different combinations of chromosomes in the offspring?

Answer 15

(2^n)²

n = number of homologous pairs