Topic 3 - Genetics

Question 1

Give examples of the advantages of asexual reproduction

Answer 1

-Only one parent is needed
-Time and energy efficient as you don’t need a mate
-Faster than sexual reproduction
-These factors allow for the population to increase rapid when conditions are favourable

Question 2

Give examples of the disadvantages of asexual reproduction

Answer 2

-It does not lead to variation in a population so the species may only be suited to one habitat and could be susceptible to one disease/selection pressure

Question 3

Give examples of the advantages of sexual reproduction

Answer 3

-It produces variation in offspring. This means that the species
can adapt to new environments due to variation, which gives them a survival advantage and a disease is less likely to affect all the individuals in a population

-Humans can speed up natural selection through selective breeding, which can be used, for example, to increase food production

Question 4

Give examples of the disadvantages of sexual reproduction

Answer 4

-Time and energy are needed to find a mate
-It is not possible for an isolated individual

Question 5

Explain the role of meiotic cell division

Answer 5

Meiosis produces four haploid, non-identical sex cells, or gametes which are all genetically different.

These fuse to form a diploid fertilised egg cell during fertilisation.
Meiosis produces sperm and egg cells in animals, and pollen and egg cells in plants.

Question 6

Describe DNA

Answer 6

DNA is a polymer made up of two strands coiled to form a double helix. These strands linked by a series of complementary base pairs which are joined together by weak hydrogen bonds. It is made up of nucleotides that consist of a sugar and phosphate group with one of the four different bases attached to the sugar.

(A - T and G - C)

Question 7

Describe the genome

Answer 7

The entire DNA of an organism

Question 8

Describe a gene

Answer 8

A section of a DNA molecule that codes for a specific protein

Question 9

Explain how DNA can be extracted from a fruit

Answer 9

*Crush the fruit - to break down the cell wall
* Add salt water - clumps the DNA together
* Add detergent - breaks down cell and nuclear membrane
* Filter - to remove insoluble material and separate out cell contents
* Add cold ethanol - cold slows down enzyme activity which will break the DNA apart and ethanol to precipitate the DNA out
Pick the DNA apart

*You will see strands of DNA with bubbles in them at the boundary between the filtrate and the chilled ethanol

Question 10

Explain how the order of bases in a section of DNA decides the order of amino acids…

Answer 10

-Each group of three bases (codons) codes for one particular amino acid.
-The amino acid molecules join together in a long chain and fold to make a protein molecule. The number and sequence of amino acids determines which protein is produced.
-Therefore, it is the order of bases in the DNA that determines which proteins are produced

(There are also non-coding parts of DNA that do not code for proteins)

Question 11

Describe transcription

Answer 11

The enzyme RNA polymerase breaks the hydrogen bonds between 2 DNA strands, exposing the bases. One DNA strand becomes the template strand. mRNA is made with complementary base pairs to the template DNA strand. The mRNA moves out of the nucleus into the cytoplasm.

Question 12

Describe translation

Answer 12

The mRNA moves to the ribosomes. The anticodon on the tRNA molecule binds to a codon (3 bases) on the mRNA by a complementary base pairing. The tRNA molecule carries a specific amino acid. Peptide bonds formed between amino acids to produce a polypeptide chain.

Question 13

Define transcription

Answer 13

The process in which a copy of DNA is made

Question 14

Define translation

Answer 14

Where amino acids are joined together to make a polypeptide chain, following the sequence.

Question 15

Describe the stages of protein synthesis

Answer 15

-RNA polymerase binds to non-coding DNA located in front of a gene
-RNA polymerase produces a complementary mRNA strand from the coding DNA of the gene
-the attachment of the mRNA to the ribosome
-the coding by triplets of bases (codons) in the mRNA for specific amino acids
-the transfer of amino acids to the ribosome by tRNA
-the linking of amino acids to form polypeptides

Question 16

Describe how genetic variants in the non-coding DNA of a gene can affect phenotype

Answer 16

Enzyme RNA polymerase binds to non-coding DNA and a change in the order of bases in this non-coding DNA can affect the amount of RNA polymerase that can bind to it. This means the enzyme is less likely to bind and so less protein is produced. If less protein is produce this can affect the phenotype of the organism.

Question 17

Describe how genetic variants in the coding DNA of a gene can affect phenotype

Answer 17

A genetic variant will alter the sequence of bases and therefore may change the sequence of amino acids which alters the final structure of the protein produced.

Question 18

Describe the work of Gregor Mendel in discovering the basis of genetics

Answer 18

He carried out breeding experiments on pea plants and in 1886 came to the conclusion that:
-Offspring have some characteristics that their parents have because they inherit ‘hereditary units’ from each.
-One unit is received from each parent. -Units can be dominant or recessive and cannot be mixed together.

Question 19

Why did people struggle to understand Mendel’s work?

Answer 19

Because it wasn’t until the 20th century that genes and chromosomes were discovered.

Question 20

Explain why there are differences in the inherited characteristics as a result of alleles

Answer 20

Different alleles code for different forms of the same protein. An allele that codes for a damaged form of a protein can cause illness. This leads the protein being folded incorrectly and causing the illness.

Question 21

Explain the term chromosome

Answer 21

A structure found in the nucleus which is made up of a long strand of DNA

Question 22

Explain the term gene

Answer 22

A section of DNA that codes for a protein

Question 23

Explain the term allele

Answer 23

Different versions of the same gene

Question 24

Explain the term dominant

Answer 24

AN allele that is always expressed, even if there is just one copy

Question 25

Explain the term recessive

Answer 25

An allele that is only expressed when there are two copies

Question 26

Explain the term homozygous

Answer 26

When both inherited alleles are the same

Question 27

Explain the term heterozygous

Answer 27

When one inherited allele is dominant and the other recessive

Question 28

Explain the term genotype

Answer 28

The combination of alleles an individual has

Question 29

Explain the term phenotype

Answer 29

The characteristic visibly seen in a person

Question 30

Explain the term gamete

Answer 30

An organism’s reproductive cell which has half the number of chromosomes (egg + sperm 23)

Question 31

Explain the term zygote

Answer 31

The stage of development immediately after fertilisation. A diploid cell formed from the fusion of 2 haploid gametes.

Question 32

What are the common features of a family pedigree?

Answer 32

-Squares usually represent males and circle females
-Black shapes represent an affected individual and white an unaffected individual
-A line through the shape means that the individual is deceased

Question 33

Describe the inheritance of the ABO blood groups with reference to codominance and multiple alleles

Question 34

Describe codominance

Answer 34

2 dominant alleles being expressed together in the same individual is codominance.

Question 34

Explain multiple alleles being present at the same locus

Answer 34

When 3≤ alleles are present at the same loci (but not necessarily expressed at once) we say that multiple alleles are present at the same position or locus.

Question 35

Describe the inheritance of the ABO blood groups

Answer 35

O is recessive
A and B are dominant

A and O alleles become type A
A and B alleles become type AB (codominantly)
A and A alleles become type A
B and B alleles become type B
O and O alleles become type O

Question 36

Explain how sex-linked genetic disorders are inherited

Answer 36

Genetic disorder who’s alleles and are found on chromosome pair 23 (which contains the sex chromosomes XY or XX) are called sex-linked genetic disorders.

These would be represented in a Punnett square by X^D or x^d (based on dominant/recessive with the letter being assigned to the genetic disorder)

Usually, X-linked recessive syndromes are more common in males because in females there is an extra X chromosome which (if dominant) can mask the effect of the recessive allele.

Question 37

State what most phenotypic features are a result of

Answer 37

Most phenotypic features are the result of multiple genes rather than single gene inheritance

Question 38

What are the causes of variation that influence phenotype?

Answer 38

Genetic variation – different characteristics as a result of mutation and sexual reproduction
Environmental variation – different characteristics caused by an organism’s environment (acquired characteristics)

Question 39

Discuss the outcomes of the Human Genome Project and its potential applications within medicine

Answer 39

The Human Genome Project had scientists map the entire human genome between 1990 and 2003. This improved our understanding of the genes linked to different types of disease, helped in the treatment of inherited disorders and helped us in tracing human migration patterns from the past.

Question 40

State a common feature of variation and what usually causes it

Answer 40

There is usually extensive genetic variation within a population of a species and these arise through mutations

Question 41

State what effect mutations have on the phenotype

Answer 41

Most genetic mutations have no effect on the phenotype, some mutations have a small effect on the phenotype and, rarely, a single mutation will significantly affect the phenotype