III: 3 - Inheritance

Question 1

Define

genetics

Answer 1

Genetics is the study of inheritance

Question 2

Define

inheritance

Answer 2

The transmission of genetic information (DNA/genes) from generation to generation.

Question 3

Define

chromosome

Answer 3

A thread of DNA, made up of a string of genes

Question 4

Gene

Answer 4

A length of DNA that is the unit of heredity and codes for a specific protein. A gene may be copied and passed on to the next generation.

Question 5

Allele

Answer 5

Any of two or more alternative forms or variety of a gene

Question 6

Haploid nucleus

Answer 6

A nucleus containing a single set of unpaired chromosomes, e.g. in sperm cells and egg cells.

Question 7

Diploid nucleus

Answer 7

A nucleus containing two sets of chromosomes, e.g. in body cells.

Question 8

How is sex inherited in humans?

Answer 8

The 23^rd pair of chromosomes in humans is responsible for sex. Females have an XX chromosome pair, whilst males have XY.

The egg cell always carries an X chromosome, whilst the sperm will carry either X or Y. Therefore, it is the sperm that affects which sex the offspring will be.

Question 9

Define

mitosis

Answer 9

Nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosomes

Question 10

What is the role/significance of mitosis?

Answer 10

1. Growth
2. Repair of damaged tissues
3. Replacement of worn out cells
4. Asexual reproduction.

Question 11

Define

meiosis

Answer 11

Reduction division in which the chromosome number is halved from diploid to haploid

Question 12

What is the role/significance of meiosis?

Answer 12

1. Produces gametes
2. Results in genetic variation so the cells produced are not all genetically identical
3. Keeps chromosome number in a species that undergoes sexual reproduction

Question 13

What are the differences between mitosis and meiosis?

Answer 13

Question 14

Define

monohybrid inheritance

and

monohybrid cross

Answer 14

Monohybrid inheritance is the inheritance of a single characteristic for which two alleles are inherited.

A monohybrid cross is the study of the inheritance of one characteristic.

Question 15

Genotype

Answer 15

Refers to the genetic makeup of an organism in terms of the alleles present, e.g. Tt or GG.

Question 16

Define

phenotype

Answer 16

Refers to the physical or other features of an organism due to both it’s genotype an the environment, e.g. tall plant or blue flower.

Question 17

Homozygous

Answer 17

Having two identical alleles of a particular gene, e.g. TT or gg. Two identical homozygous individuals that breed together will be pure-breeding.

Question 18

Heterozygous

Answer 18

Having two different alleles of a particular gene, e.g. Tt or Gg. Two heterozygous individuals that breed together will not be pure-breeding, even if their phenotype is identical.

Question 19

Dominant allele

Answer 19

An allele that is always expressed if it is present.

Question 20

Recessive allele

Answer 20

An allele that is only expressed when there is no dominant allele of the gene present.

Question 21

Define

Codominance

Answer 21

When both alleles of a gene are dominant, e.g. BW or I^AI^B, and the phenotype is a blend of the two alleles, e.g. grey colour or AB blood group.

Question 22

Incomplete dominance

Answer 22

When an allele is not completely dominant over the recessive allele, i.e. the recessive allele is partly expressed, e.g. in sickle cell trait.

Question 23

What genotypic and phenotypic ratios would you expect in the offspring when a homozygous dominant (BB) individual is crossed with a homozygous recessive (bb) individual?

Answer 23

Genotypic ratio: 100% Bb

Phenotypic ratio: 100% B characteristic

Question 24

What genotypic and phenotypic ratios would you expect in the offspring when two heterozygous (Bb) individuals are crossed?

Answer 24

Phenotypic ratio: 3:1 (B to b characteristic)

Genotypic ratio: 1:2:1 (BB to Bb to bb)

Question 25

What genotypic and phenotypic ratios would you expect in the offspring when a heterozygous (Bb) individual is crossed with a homozygous recessive (bb) individual?

Answer 25

Phenotypic ratio: 1:1 (dominant to recessive characteristic)

Genotypic ratio: 1:1 (Bb to bb)

Question 26

What genotypic and phenotypic ratios would you expect in the offspring when a homozygous dominant (BB) individual is crossed with a heterozygous (Bb) individual?

Answer 26

Phenotypic ratio: 100% dominant characteristic

Genotypic ratio: 1:1 (BB to Bb)

Question 27

Define

continuous variation

Answer 27

Variation in phenotypic traits, influenced by both genes and environment, in which a series of types are distributed on a continuum rather than grouped into discrete categories, resulting in a range of phenotypes between two extremes.

For example, height or body mass.

Question 28

Define

discontinuous variation

Answer 28

Variation in phenotypic traits, caused by genes alone, in which types are grouped into a limited number of discrete categories with few or no intermediate phenotypes.

For example, e.g. A, B, AB and O blood groups in humans.

Question 29

Define

Mutation

Answer 29

An unpredictable change in the DNA of a gene or chromosome.

Question 30

Define

Gene mutation

Answer 30

A mutation in which only a single gene is affected, e.g. in sickle cell anaemia.

Question 31

Define

Chromosomal mutation

Answer 31

A mutation in which an entire chromosome is affected, e.g. in Down’s Syndrome.

Question 32

What factors may affect the rate of mutation in an organism when exposed to?

Answer 32

• Ionising radiation from UV light, X-rays or radioactive substances
• Mustard gas
• Harmful chemicals, e.g. tar

Question 33

Define

natural selection

Answer 33

Natural selection is the greater chance of the passing on of genes by the best adapted or ‘fittest’ organisms.

Question 34

Define

genetic engineering

Answer 34

Taking a gene from one species and putting it into another species

Question 35

What is humulin, and why was it made?

Answer 35

Humulin is biosynthetic insulin made by genetic engineering, where bacteria are the vector for producing human insulin.

It was made as the previous method of obtaining insulin for diabetics from animals (such as pigs) had a number of problems, including:

• Ethical problems as the animals had to be killed
• Religious problems as certain fatihs did not allow the consumptipon of pigs
• The insulin took longer to take effect compared to human insulin
• Sometimes the body rejected the insulin

Question 36

How is humulin made?

Answer 36

1. A human cell is taken out, and the chromosome with the gene coding for insulin is identified.
2. The gene on the chromosome is identified.
3. The gene is separated from the rest of the chromosome using restriction enzymes produced by bacteria.
4. A bacterium is obtained and isolated.
5. A DNA plasmid is taken from the bacterium.
6. The same restriction enzymes are used to cut the plasmid at the right place for the human gene to be inserted.
7. The human gene is inserted into the plasmid using ligase enzymes. The plasmid now has recombinant DNA.
8. The plasmid is re-inserted into the bacterium, which is now transformed and produces insulin inside itself.
9. The bacterium can reproduce by binary fission to make more insulin-producing bacteria. The bacteria ae kept in a fermenter to produce insulin, whic is later extracted using water and high pressure to burst the cells with high water potential.

Question 37

What are the advantages of using humulin?

Answer 37

• Human insulin produced from genetically engineering bacteria will not trigger an allergic reaction
• No ethical or religious problems
• The product is very pure
• Humulin can be made on a commercial scale, reducing costs