Inherited Change (no meiosis/genetic diagrams)

Question 1

What leads to GENETIC VARIATION? (5)

Answer 1

1) INDEPENDENT ASSORTMENT of chromosomes (therefore alleles) during meiosis
2) Crossing over between chromatids of homologous chromosomes (meiosis)
3) Random mating
4) Random fertilisation of gametes
5) Gene mutation

Question 2

What causes gene mutation and what are its effects on phenotype? Give an example

Answer 2

Caused by a base deletion, substitution or base addition.

If in a fertilised gamete, new allele is often recessive and does not show up in popln until generations after mutation occuring.

eg a change in gene for production of BETA POLYPEPTIDE of Hb (valine instead of glutamic acid AA, gene is HbS) lead to the recessive sickle cell allee.

Question 3

What are the effects of gene mutation in somatic cells?

Answer 3

Mutations in somatic cells often have no effect, most are recognised as foreign and destroyed. However if a mutation affects regulation of cell division, and the cell escapes attack of immune system, it produces a tumour, which may be MALIGNANT and cause cancer.

Question 4

What are CHROMOSOME MUTATIONS?

Answer 4

A change in the structure or number of whole chromosomes.

Question 5

What can increase the chances of mutation?

Answer 5

Ionising radiation, ultraviolet radiation, mustard gas, all damage DNA molecules, altering their base structure.

Question 6

What causes FRAME SHIFTS in the DNA code, and what are its effects?

Answer 6

Base addition or deletion. The protein that is made is totally useless, or a stop triplet code may be introduced, and the complete protein never made at all.

Question 7

What kind of mutations does base substitution cause?

Answer 7

A SILENT MUTATION. THis is because many AAs have more than one triplet code. However base substitution can also have a significant effect.

Question 8

Give an example of a base substitution with a significant effect on phenotype. Describe its effect.

Answer 8

SICKLE CELL ANAEMIA. Hb is made of four polypeptide chains, each with one iron containing haem group in the centre. Two are alpha, two are beta.

Gene which codes for AA in the Beta chain mutated from CTT to CAT. The AA valine is made instead of glutamic acid. This change means when the Hb is not combined with oxygen, it is much less soluble. Molecules tend to stick to each other, forming long fibres inside RBC. RBCs are pulled out of shape (sickle shape). Distorted cells useless at oxygen transport. Can also get stuck in small capillaries, stopping passage of unaffected cells.

Person with sickle cell anaemia can suffer severe anaemia (lack of oxygen transported to cells) and may die.

Question 9

What is PKU, what are its effects, and what causes it?

Answer 9

PHENYLKETONURIA. Caused by abnormal base sequence in part of DNA molecule, affecting coding of enzyme PHENYLALANINE HYDROXYLASE (catalase conversion of AA phenylalanine to tyrosine, which is then converted to melanin by another route). People with PKU lack melanin and have lighter hair/skin.

Phenylalanine accumulates in blood and tissue fluid, causes severed brain damage in small children, who become severely mentally retarded without treatment. Can be avoided by testing for phenylalanine in blood at birth, and put on diet with no phenylalanine.

Question 10

How does the environment affect the phenotype? Give examples (2)

Answer 10

1) Human height - if diet is poor, cells will not have enough nutrients to develop to full height potential of alleles.
2) Himalayan rabbits - develops dark tips on nose, ears, paws, and tail. Caused by allele which allows formation of dark pigment in low temp. Parts of rabbit that grow dark are the coldest parts.

Question 11

Define locus

Answer 11

Position of gene ( or other specific DNA such as a marker) on a chromosome. Smae gene always found at the same locus of the same chromosome ( unless mutation). Designated by chrom number, arm, and place (eg gene for ABO blood group is 9q34).

Question 12

Define allele

Answer 12

Variant forms of a gene. One or more alternative nucleotide sequences at a single gene locus.

Question 13

What is a dominant allele?

Answer 13

An allele with a phenotype that is expressed even when present with an allele that is recessive to it.

Question 14

What is a recessive allele?

Answer 14

A allele with a phenotype not expressed when an allele that is dominant to it is present.

Question 15

What is a codominant allele? GIve an example

Answer 15

Alleles both expressed if they are present together in a heterozygous person.

Eg A person with Hb(A) Hb(S) has the less severe sickle cell trait while someone with Hb(S) Hb(S) has the more severe sickle cell trait.

Question 16

Define homozygous

Answer 16

A diploid organism that has the same allele of a gene at the gene’s locus on both copies of the homolohoug chromosome in its cells. That organism is a homozygote.

Question 17

Define heterozygous

Answer 17

A diploid organism with different alleles of a gene at the gene’s locus on both copies of homologous chromosomes in its cells.

Question 18

Define phenotype

Answer 18

The physical, detectable expression of the particular alleles of a gene or genes present in an individual.

Question 19

Define genotype

Answer 19

The particular alleles of a gene at the appropriate locus on both copies of the homologous chromosomes of its cells.