4B: Natural Selection

Question 1

Describe the steps involved in Meiosis

Answer 1

After interphase, a reproductive cell undergoes meiosis’s. In humans, this results in 4 haploid cells, each with half the number of chromosomes as the parent cell.
Meiosis 1: chromosomes carrying the same genes pair up and then are separated into 2 daughter cells
Meiosis 2: Sister chromosomes move apart to form a new pair of daughter cells

Question 2

Main differences between Mitosis and Meiosis?

Answer 2

Mitosis involves 1 division whilst meiosis involves 2
Mitosis produces 2 genetically identical daughter cells whilst meiosis produces 4 genetically different cells
Mitosis produces Diploid cells, meiosis produces haploid cells

Question 3

What is independent segregation?

Answer 3

In meiosis, homologous pairs of chromosomes line up opposite each other at the equator of the cell. It’s random which side of the equator the maternal and paternal chromosomes line up. The pairs are separated so one of each homologous chromosomes end up in the daughter cell

Question 4

What is crossing over?

Answer 4

Chromatids can become twisted around each other. Tension on the chromatid causes some of it to snap and fall off. Where they break off, they can recombine on the opposite chromatid creating a new combination of alleles.

Question 5

Explain how a mutation can result in new alleles of a gene

Answer 5

Many mutations are harmful but in certain environments the new allele might benefit its processor leading to reproductive success. Advantageous alleles are inherited by members of the next generation. Over many generations new alleles are increased in the population frequency

Question 6

what are the three causes of mutations?

Answer 6

Deletion, addition and substitution of bases

Question 7

What type of substance can increase the rate at which mutations occur

Answer 7

Mutagenic agents

Question 8

What is the percentage difference in the amino acid sequence between mammals and insects?

Answer 8

33%

Question 9

How are gene technologies used to study genetic diversity

Answer 9

Allow the alleles in an individual to be directly measured
Allow genetic diversity to be directly measured for example the number of different alleles for a gene and the number of individuals with each allele directly shows the genetic diversity

Question 10

What statistical measures should be used to ensure accurate results when investigating variation?

Answer 10

Random sampling
Calculating the mean
Standard deviation

Question 11

When does crossing over occur?

Answer 11

Prophase I

Question 12

What is genetic diversity?

Answer 12

The degree of variation in the genes that exist in a certain population

Question 13

What are genes?

Answer 13

Multiple versions of genes

Question 14

2 ways in which genetic diversity is increased

Answer 14

Mutations
Gene flow ( introduction of new alleles by migration)

Question 15

What is evolution

Answer 15

A change in the allle frequency over time

Question 16

What is selection pressure?

Answer 16

An external factor that influences the reproductive success of an individual

Question 17

What environment does stabilising selector take place in?

Answer 17

Non-changing

Question 18

What are the 4 principles of natural selection?

Answer 18

Mutations are spontaneous
Changes in allele frequency
Some mutations are beneficial
Genes are inherited by offspring

Question 19

What phenotype in favoured in bacterial populations that are expressed to antibiotics?

Answer 19

Antibiotic resistance

Question 20

What does natural selection favour in stabilising selection?

Answer 20

average phenotype

Question 21

Explain the steps of natural selection

Answer 21

1 new alllels for a gene are created by random mutations
2 If the new allele increases the chance of that individual surviving then they are more likely to survive and reproduce
3 This reproduction passes on the advantageous allele to the next generation
4 As a result over many generations the new allele increases in frequency in the population

Question 22

Describe what happens to chromosomes in meiosis (6 marks)

Answer 22

1. Chromosomes condense
2. Chromosomes associate in homologous pairs
3. Crossing over and a chiasma forms
4. Joins to spindle fibres
5. At the equator
6. Homologous chromosomes move to opposite poles
7. Chromatids separated in 2nd division

Question 23

How to we calculate the number of possible different combinations of chromosomes following meiosis?

Answer 23

2^n

Question 24

Describe and explain the processes that occur during meiosis that increase genetic variation

Answer 24

1. Homologous chromosomes pair up
2. Independent segregation leading to maternal and paternal chromosomes are reshuffled in any combination
3. Crossing over leads to the exchange of alleles between homologous chromosomes
4. Creating a new combination of alleles
5. Random fertilisation of haploid gametes creates genetic variation between zygotes

Question 25

What does the term ‘Bivalent’ mean

Answer 25

consists of two homologous chromosomes one from each parent aligned closely together. Allows for the exchange of genetic material between the chromosomes via chiasma formation

Question 26

Random assortment?

Answer 26

The random organisation of chromosomes during the first stage of meiosis increasing genetic diversity