Topic 3 - Genetics

Question 1

Explain how sex-linked genetic disorders are inherited

Answer 1

A characteristic is sex-linked if the allele codes for it is located on the sex chromosomes (X or Y). The Y chromosome is smaller than the X chromosomes and carries fewer genes. So most genes are only carried on the X chromosomes. As men only have one X chromosome, they often have one allele for sex-linked genes. Because men have only one allele, the characteristics of these alleles are shown even if it is recessive. This makes men more likely than women to show recessive characteristics for genes that are sex linked. Disorders caused by faulty alleles located on sex chromosomes are called sex-linked genetic disorders.

Question 2

What is DNA?

Answer 2

It’s the genetic code that determines all the characteristics of a living thing. Made up of nucleotides which contains a sugar and phosphate molecule (backbone) connected to a base.

Question 3

What are the effects of mutations on the phenotype?

Answer 3

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

Question 4

What was the aim of the Human Genome Project?

Answer 4

To find every single human gene

Question 5

What are nucleotides?

Answer 5

Nucleotides consist of one sugar molecule, one phosphate molecule and one ‘base’. The sugar and phospate molecules form a ‘backbone’ to the DNA strands. The sugar and phosphate molecules alternate. One of the four different bases join to each sugar

Question 6

What decides the order of amino acids in a protein ?

Answer 6

The order of bases in a gene determines the order of amino acids. Each amino acid is coded for by a sequence of three bases in a gene - base triplet. The amino acids are joined together according to the order bases in the gene.

Question 7

What is a genotype?

Answer 7

Genotype is what makes the trait - the information within a gene, or the genetic makeup of a specific organism.

Question 8

How does mutation affect the non-coding part of DNA?

Answer 8

If a mutation happens in this region of DNA, then it could affect the ability of RNA polymerase to bind to it. It might make it easier to bind to, or more difficult. How much RNA is transcribed and therefore how much of the protein is produced. And, depending on the function of the protein, the phenotype of the organism may be affected by how much of it is made. So genetic variants in non-coding regions can still affect the phenotype of an organism, even if they do not code for proteins themselves.

Question 9

Why is there genetic variation in species?

Answer 9

Mutations

Question 10

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

Answer 10

Some alleles are dominant (shown with a capital letter, eg ‘C’) and some are recessive (shown by a small letter, eg ‘c’). To display a dominant characteristic, an organism can have either two dominant alleles for a particular gene or one dominant and one recessive allele for that gene. But for an organism to display a recessive characteristic, both its alleles must must recessive.

Question 11

What are the complementary base pairs joined up by?

Answer 11

Weak hydrogen bonds

Question 12

Explain some of the advantages (3) and disadvantages (2) of asexual reproduction

Answer 12

Adv

• Can produce lots of offspring very quickly because the reproductive cycle is fast
• Allows organisms to colonise a new area quickly
• Only one parent needed so can reproduce whenever the conditions are favourable.

Disadv

• No genetic variation
• If environment changes and conditions become unfavourable, whole population may be affected.

Question 13

What is the genome?

Answer 13

The genome is the base sequence of all the DNA in an organism.

Question 14

What were the possible medical applications of the Human Genome Project? (3)

Answer 14

Prediction and prevention of diseases
- If doctors knew which genes gave people a certain disease, we could all get individually tailored advice on diet and lifestyle solutions to prevent disease.

Testing and Treatment of inherited disorders
- Scientists can now easily identify the genes and alleles that cause a certain inherited disorder quickly. They can develop better treatments and create a cure for the disease.

New and better medicines
- Scientists can design new drugs which are specifically tailored to a person to stop them from getting an inherited disorder. They can also see how effective an existing drug is.

Question 15

How can DNA be extracted from a fruit? (3)

Answer 15

1) Grinding fruit with sand, using pestle and mortar, to separate cells.
2) Adding a detergent to break open the membranes
3) Adding ice-cold alcohol so that DNA precipitates out.

Question 16

What is a chromosome?

Answer 16

The structure made of DNA that codes for all the characteristics of an organism.

Question 17

What are the two stages of protein synthesis?

Answer 17

Transcription then Translation

Question 18

What is a Gene?

Answer 18

A gene is a short piece of DNA that codes for a specific protein. You have genes for hair structure, eye colour, enzymes and every other protein in your body.

Question 19

What is an amino acid?

Answer 19

Chains of molecules that make up a protein. Each protein has its own number and order of amino acids. The amino acid chains fold up to give each protein a different, specific shape allowing all proteins to have their own functions.

Question 20

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

Answer 20

Humans have four potential blood groups - O, A, B, AB. The gene for blood type in humans has three different alleles - I^a, I^o and I^b. I^a and I^b are codominant with each other so if they are together the blood type will be AB - one allele isn’t dominant over the other one. However, I^o is recessive so if I^a is paired with I^o the blood type will be A. You only get blood type O when u have two of the recessive alleles (I^o I^o)

Question 21

What is an allele?

Answer 21

Different forms of the same gene.

Question 22

What is a recessive allele?

Answer 22

Describes the variant of a gene for a particular characteristic which is suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene.

Question 23

How does a mutation affect the coding part of the DNA?

Answer 23

If a mutation happens in a gene, a gentic variant (allele) is produced - different version of the gene. The genetic variant may code for a different sequence of amino acids which may change the shape of the final protein and its activity. For example, the activity of an enzyme might increase, decrease or stop altogether. This could end up changing the characteristics (phenotype, of an organism.

Question 24

What is mutation?

Answer 24

A mutation is a rare, random change to an organism’s DNA base sequence that can be inherited.

Question 25

What were the three conclusions of Mendel from his work?

Answer 25

1) Characteristics in plants are determined by “hereditary units”
2) Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent.
3) Hereditary units can be dominant or recessive - if an individual has both a dominant and recessive unit for a characteristic, the dominant characteristic will be expressed.

Question 26

What is mRNA? and what is it for?

Answer 26

mRNA is a polymer of nucleotides but it’s shorter and only a single strand. Also uracil (U) is used instead of thymine (T). It gets the information from the DNA to the ribosome in the cytoplasm to be able proteins.

Question 27

What is complementary base pairing? and what are the 4 bases?

Answer 27

The four bases are A (adenine), T (thymine), C (cytosine) and G (Guanine). Each base links to a base on the opposite strand in the helix. A always pairs up with T, C always pairs up with G.

Question 28

Show the how the sex of a human offspring is determined using a Punnett square.

Answer 28

This shows that there is a 50:50 probability of getting a boy or a girl.

X | Y |
X | XX | XY|
X | XX | XY|

Question 29

What is a dominant allele?

Answer 29

An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself.

Question 30

What did Mendel show in his work with pea plants?

Answer 30

Mendel had shown that the height characteristic in pea plants was determined by separately inherited “hereditary units” passed on from each parent. The ratio of tall and dwarf plants in the offspring showed that the unit for tall plants, T, being dominant over the unit for dwarf plants, t.

Question 31

What are the stages of transcription? (4)

Answer 31

1) The two strands of the DNA helix are unzipped by breaking the weak hydrogen bonds between them.
2) RNA polymerase enzyme attaches to the DNA in the non-coding region just before the gene (just before the region where the DNA helix is unzipped)
3) RNA polymerase moves along the DNA strand
4) Free RNA nucleotides form hydrogen bonds with the exposed DNA strand nucleotides by complementary base pairing to form a strand of mRNA. RNA nucleotides have the same bases as DNA but the T is replaced with a U. It is a complementary copy of the opposite DNA strand.
5) The newly formed strand of mRNA is ready to leave the nucleus and travel to the ribosome through the cyptoplasm.

Question 32

What does homozygous mean?

Answer 32

This describes a genotype in which the two alleles for the characteristic are identical.

Question 33

What are most phenotypic features caused by?

Answer 33

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

Question 34

What was the difficulty in understanding Mendel’s work?

Answer 34

His work was cutting edge and new to scientists. We now know his “hereditary units” are genes. At the time, scientists didn’t have the background knowledge to understand Mendel’s findings - they had no idea about genes, chromosomes and DNA. It wasn’t until after his death that people realised how serious his work was and that the mechanism of interference could be fully explained.

Question 35

Describe the causes of variation that influence phenotype.

Answer 35

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 36

What are stages of translation? (5)

Answer 36

1) The mRNA strand attaches to the ribosome in the cell
2) For every mRNA base triplet, it is matched by a complementary triplet from the tRNA
3) tRNA transport specific amino acids to the ribosome to match with the mRNA
4) The tRNA molecules exit the ribosome as the amino acids link together
5) A chain of several hundred amino acids in the correct order according to the original DNA is then made. This is called a polypeptide.

Question 37

What does heterozygous mean?

Answer 37

This describes a genotype in which the two alleles for a particular characteristic are different.

Question 38

Explain some of the advantages (3) and disadvantages (2) of sexual reproduction

Answer 38

Adv

• Genetic variation - different characteristics
• If environmental conditions change, more of the population is likely to survive
• Species can become better adapted to environment.

Disadv

• More time and energy required.
• Two parents needed which can be a probelm if individuals are isolated.

Question 39

What is a phenotype?

Answer 39

Phenotype is what you see - the visible or observable expression of the results of genes, combined with the environmental influence on an organism’s appearance or behavior

Question 40

What is RNA polymerase?

Answer 40

This is the enzyme involved in joining RNA nucleotides to make mRNA.

Question 41

What is Meiosis?

Answer 41

Meiosis is a type of cell division that produces four daughter cells, each with half the number of chromosomes. Meiosis only happens in gamete-producing cells, producing genetically different haploid gametes.

Question 42

What does monohybrid inheritance mean? Show in Punnett square

Answer 42

The inheritance of a single characteristic.

        |   B   ||  B   |                  |  B  ||  b  |
   b  |  Bb  |  Bb  |              B | BB | Bb |
   b  |  Bb  |  Bb  |              b | Bb | bb |

All of the 1st Punnett square are Bb which means they are all ‘boring’

1/4 of the 2nd Punnett square is bb which means 25% of the next generation will be ‘superpowered’.