Inheritance (Genetics, Mutations and Evolution)

Question 1

What are alleles?

Answer 1

• Variations of the same gene
• We have two alleles of each gene, one in each chromosome within a chromome pair
• Therefore, we have two alleles which control each characteristic
• One comes from our mother, and one comes from our father

Question 2

What is the phenotype of an organism?

Answer 2

The observable characteristics of the organism

Question 3

What is the genotype of an organism?

Answer 3

The combination of alleles that controls each characteristics in an organism (what genes the organism has)

Question 4

What is a dominant allele?

Answer 4

An allele which only needs to be inherited from one parent in order for the characteristic to show up in the phentotype

Dominant alleles are represented by upper-case letters

Question 5

What is a recessive allele?

Answer 5

An allele which needs to be inherited from both parents in order for the characteristic to show up in the phenotype

Recessive alleles are represented by lower-case letters

Question 6

What does homozygous (dominant and recessive) mean?

Answer 6

• If an individual is homozygous for a gene, that means their two alleles of the gene are the same
• If these two alleles are dominant, they are homozygous dominant, if these two alleles are recessive, they are homozygous recessive

Question 7

What does heterozygous mean?

Answer 7

• If an individual is homozygous for a gene, that means their two alleles of the gene are different
• This means they have a dominant and a recessive allele for the gene

Question 8

If height was controlled by two alleles, T and t, T being tall, and t being short, and T was dominant, what are all of the possible offspring if:
* One parent was homozygous dominant and the other was homozygous recessive
* Both parents were heterozygous
* One parent was homozygous recessive, and the other was heterozygous
* One parent was homozygous dominant, and the other was heterozygous

Give your answer in terms of their phenotype

Answer 8

• Tall, Tall, Tall, Tall
• Tall, Tall, Tall, Short
• Tall, Tall, Short, Short
• Tall, Tall, Tall, Tall

Question 9

What is codominance, using blood groups as an example?

Answer 9

• When both alleles within a genotype are different but both expressed in the phenotype
• For example, to determine blood group, there are 3 alleles instead of the usual 2
• The A allele will produce the A antigen, the B allele will produce the B antigen and the O allele will produce no antigens
• A and B and both dominant over the O allele, but are not dominant over eachother
• Therefore, if an individual has both A and B, they will produce both antigens

This can happen with other characteristics, including characteristics which are determined by the regular 2 alleles

Question 10

What are polygenic characteristics?

Answer 10

• Characteristics which are controlled by multiple genes
• In reality, most characteristics are determined by polygenic inheritance, but for simplicity things have been earlier explained as monohybrid (there only being one gene controlling each characteristic)
• Polygenic characteristics can therefore show a much wider range of different combinations

Question 11

How is sex determined in humans?

Answer 11

• The 23rd chromosome pair will either be two X chromosomes, or an X chromosome and a Y chromosome
• XX is female, XY is male
• As you can see, the mother will always pass on an X chromosome, but the father could pass on either an X or a Y - therefore the father is responsible for determining the sex in a human
• As with all characteristics, it is 50/50 because, as a result of meoisis, half of all the male gametes (sperm) will contain the X chromosome, and the other half will contain the Y chromosome

Sex is controlled by the entire chromosome pair as opposed to most other characteristics which are controlled by one or a number of genes

Question 12

What is a mutation?

Answer 12

A rare, random change in genetic material, specifically the sequence of bases

Question 13

How does a mutation change the genetic material?

Answer 13

• Insertions - a random base is added to the sequence of bases, changing the amino acid that would have been coded for in the triplet in which the mutations occurred, and having a knock on effect down the strand
• Deletions - a random base is deleted from the sequence of bases, having similar (but logically different) effects to insertions
• Subsitutions - A base in the DNA sequence is randomly changed for another base - unlike insertions and deletions, this will only affect the amino acid which the affected triplet codes for

In all of these: order of bases are changed, different triples/codons are therefore present so the amino acids present in the final protein will be different

Question 14

How can mutations affect the phenotype of an organism?

Answer 14

• Changing the sequence of amino acids can change the protein a gene codes for
• It can lead to a significantly altered protein, or a completely new protein
• For example, if the affected protein is an enzyme, the shape of the active site can change and therefore it cannot fit the substrate
• On rare occasions, a mutation can make the gene into a completely different allele, drastically changing the phenotype of an organism

These changes can be positive - like being taller, or negative - like sickle cell anaemia

Question 15

How often do mutations affect the phenotype of an organism and why?

Answer 15

Most mutations have no effect, with only some having a small effect and very rarely do they have a significant effect
This could be because:
* The changed bases could still code for the same amino acids
* The changed amino acids may not be relevant to the function of the protein
* The protein may still be fairly similar and therefore perform same function, therefore no there is no effect on the phenotype
* The gene changed could be a recessive allele

Question 16

What things increase the incidence of mutations?

Answer 16

• Exposure to ionising radiation - gamma rays, x-rays, ultraviolet rays, alpha + beta
• Exposure to certain carcinogens - chemicals in tobacco such as tar

Question 17

What is Darwin’s theory of evolution by natural selection?

Answer 17

• Due to differences in genes, individuals within a population of a species show variation in characteristics
• Those with characteristics suited to the environment have a higher chance of survival and therefore are more likely to reproduce more and have more offspring
• Therefore, the characteristics which favour survival are passed onto more offspring than the characteristics which do not
• Over many generations, these beneficial characteristics become more common in the phenotypes of the population of that species (the gradual change in phenotypes in a population of species is evolution)

Question 18

What are antibiotics and how do they work?

Answer 18

• Antibiotics are chemical substances used medicinally to treat bacterial infections
• They work by disrupting bacterial cell structure of functions, or by preventing them from reproducing
• They are only effective against bacterial infections - not virus or fungi

• Penicillin, for example, inteferes with bacteria building their cell walls
• As they are bacteria-cell specific, they generally do not affect animal or plant cells
• Antibiotics will not affect viruses as they are not living and do not have cell processes, instead will live inside a host cell

Question 19

How does antibiotic resistance develop and what effects does it have?

Answer 19

• Bacteria can undergo mutations, some of these mutations may give them resistance to antibiotics
• The overuse of antibiotics can lead to bacteria whose mutations gives them resistance surviving and reproducing more than other bacteria
• Therefore, the bacterial population of those with resistance to antibiotics will increase, and these bacteria will become even more prevalent as their infections cannot be treated with antibiotics
• This makes infections difficult to control as many types of antibiotics will not work on certain bacterial infections