B13 - Genetics And Evolution

Question 1

Sexual reproduction

Answer 1

• two parents
• fusion of male and female gametes to form a zygote
• characteristics combination of both parents
• leads to genetic variation
gametes formed by meiosis, zygote divides by mitosis
• 23 pairs of chromosomes
• involves fertilisation

Question 2

Asexual reproduction

Answer 2

• one parent
• no fusion of gametes so no mixing of genetic information
• produce clones - genetic material identical to parent
• no genetic variation in offspring
• mitosis is the only type of cell division involved
• genes are passed on from parent to offspring
• no fertilisation

Question 3

What types of organisms can asexually reproduce?

Answer 3

• mainly bacteria - very fast! x10 faster than quickest animals
• starfish
• strawberries - grow stems called runners
• potatoes - tubers
• spider plants - grow new plants on their stems, if cut off and put in soil they grow

Question 4

Meiosis

Answer 4

• parent cell
• chromosomes make identical copies of themselves
• similar chromosomes pair up
• sections of DNA get swapped
• first cell division - chromosome pairs separate
• second cell division

Cells in the reproductive organs divide by meiosis to form the gametes (sex cells).
• Body cells have two sets of chromosomes, gametes have only one set.
• In meiosis, the genetic material is copied and then the cell divides twice to form four gametes, each with a single set of chromosomes.
• All gametes are genetically different from each other.
• Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases and as the embryo develops, the cells differentiate.

Question 5

How is meiosis different to mitosis?

Answer 5

• genetically varied
• 23 chromosomes in a gamete
• divides twice
• four daughter cells
• the types of cells that are made are gametes
• sexual reproduction
• used in sexual reproduction

Question 6

Similarities between meiosis and mitosis

Answer 6

• both types of cell division
• chromosomes duplicate in both
• all other subcellular structure have to replicate

Question 7

How is mitosis different to meiosis?

Answer 7

• genetically identical daughter cells
• each daughter cell has 46 chromosomes
• cell only divides once
• two identical daughter cells are made
• normal body cells are made
• daughter cells are for growth and repair
• used in asexual

Question 8

Number of chromosomes in one cell at the start/end of meiosis

Answer 8

Start = 16 (8 pairs) - doubles to 32 then splits 4 ways
End - 8

Question 9

Explain why the change in the number of chromosomes is important (meiosis)

Answer 9

Meiosis forms gametes, and two gametes need to fertilise each other to keep chromosome number the same generation after generation

Question 10

Describe how meiosis produces cells that are genetically different

Answer 10

Meiosis produces cells that are genetically different when DNA is exchanged between gametes, and when homologous chromosome pairs randomly orient themselves.

Question 11

Who discovered DNA?

Answer 11

• Watson and Crick won Nobel prize for DNA discovery
• Rosalind Franklin’s work
• DNA shape is a ‘double helix’
• genes are short sections of DNA that code for particular proteins

Question 12

What is the human genome?

Answer 12

All the DNA of an organism

Question 13

Human Genome Project

Answer 13

• started in 1986 - officially in 1990
• completed in 2003 - under budget, two years early
• composite derived from several individuals - sequences
• all information

Question 14

Aims of the human genome project

Answer 14

• identify all 20-25000 genes in human DNA
• find where each gene is located
• determine sequences of 3 billion chemical base pairs that make up human DNA
• took 15 years
• cost US 13 billion dollars
• helps prevent problems by manipulating DNA

(The project opens up controversial/ethical issues)

Question 15

What is an allele?

Answer 15

• different forms of the same gene

Question 16

What does homozygous mean?

Answer 16

An individual with two identical alleles for a characteristic

Question 17

What does heterozygous mean?

Answer 17

• an individual with different alleles for a characteristic

Question 18

What is a genotype?

Answer 18

Describes alleles present, genetic make up of an individual regarding a particular characteristic, eg Bb

Question 19

What does phenotype mean?

Answer 19

• physical appearance of an individual regarding a particular characteristic, eg hair or eye colour

Question 20

What does recessive mean?

Answer 20

Alleles that are only expressed if both are present, eg rr

Question 21

What does dominant mean?

Answer 21

A dominant allele is always expressed in the phenotype, even if only one copy is present

Question 22

Cystic fibrosis

Answer 22

• single gene disorder
• when our lungs make too much mucus
• no cure
• affects digestion - don’t produce enzymes
• genetic degenerative disease
• 9000 suffers in the UK
• life expectancy = 61
• nebulisers and other medication, eg vitamins and antibiotics
• physiotherapy - aims to remove secretions from the lungs to improve airway clearance and lung function
• no digestive enzymes - food passes through the intestines without nutrients being absorbed
• can lead to: weight loss, diabetes and osteoporosis
• linked to a gene on chromosome 7 - codes for a protein called CFTR
• defective alleles

Question 23

Polydactyl

Answer 23

Having extra fingers or toes
Most inherited disorders are caused by recessive alleles but this is caused by a dominate allele

Question 24

Embryonic Screening

Answer 24

• ivf
• eggs are extracted and mixed with sperm
• take DNA from embryos and look at it - if carrying a defective allele, they are destroyed
• healthy embryos implanted back into the uterus, hopefully will grow into a baby
• a way of detecting inherited disorders
• methods such as pre-implantation genetic diagnosis (PGD), and chronic villus sampling (CVS)

Question 25

Arguments for embryonic screening

Answer 25

• helps to stop people suffering from certain inherited disorders
• treating disorders costs gov lots of money (and tax payers) so could reduce healthcare costs
• during IVF, most embryos are destroyed - PGD just ensures selected one is healthy
• if an inherited disorder is diagnosed through CVS, parents have a choice

Question 26

Arguments against embryonic screening

Answer 26

• may come to a point where people want to pick the most desirable embryo
• increases prejudice - implies having a genetic disorder makes you ‘undesirable’
• after PGD, rejected are destroyed
• risk CVS causes miscarriage
• could lead to an abortion (CVS)
• expensive to screen embryos

Question 27

What is the male gamete formed in flowering plants?

Answer 27

Pollen (grain)

Question 28

What is the mean mass of DNA in arbitrary units in a sperm cell?

Answer 28

2

Question 29

What is the mean mass of DNA in arbitrary units in each cell of an embryo?

Answer 29

4

Question 30

DNA is a….

Answer 30

Polymer
made up of two strands forming a double helix

Question 31

What is a gene?

Answer 31

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

Question 32

What are most genetic characteristics a result of?

Answer 32

Most characteristics are the result of multiple genes interacting, rather than a single gene.
(However can be a single gene)

Question 33

Sex chromosomes

Answer 33

Ordinary human body cells contain
23 pairs of chromosomes; 22 control general body characteristics only, but the sex chromosomes carry the genes that determine sex.
• In human females the sex chromosomes are the same
(XX) whilst in males the sex chromosomes are different (XY).