16 Inheritance

Question 1

Explain the meanings of the terms haploid and diploid

Answer 1

1 haploid is one (complete) set of chromosomes ;
2 diploid is two (complete) sets of chromosomes ;

Question 2

Describe the behaviour of chromosomes during the main stages of meiosis 1 in animal cells

Answer 2

prophase I
chromosomes / chromatin, condense / shorten / fatten I DNA
or
homologous chromosomes pair up / bivalents form
or
crossing over / synapsis / chiasmata formation ;

metaphase I
homologous (pairs of chromosomes / bivalents, line up on the, equator / metaphase plate
or
random / independent, assortment, of homologous (pairs of) chromosomes / bivalents ;

anaphase I
chromosome(s) / one of each pair of chromosomes, moves (to pole / end of cell) ;

telophase I
chromosomes reach the opposite, poles / centrosomes / centrioles / end of cell
or
chromosomes decondense / chromatin formation / chromosomes become diffuse

Question 3

Explain how independent assortment occurs

Answer 3

1 genes on separate chromosomes / not linked ;
2 each, bivalent / pair of homologous chromosomes, orients itself separately ;
3 (at equator) in metaphase I of meiosis;
4 gives, four / different / new, combinations of alleles / gametes ;
5 parental allele combinations are not, preserved / fixed ;

Question 4

Explain role of gibberellin in stem elongation

Answer 4

1 Le gene codes for functional enzyme
2 le gene codes for a, non /less, functional enzyme ;
3 alanine replaced with threonine at active site in le gene;
4 inactive gibberellin/ GA20, not converted to, active gibberellin/ GA1 ;
5 no / less, gibberellin to bind to, (intracellular) receptor / GID1;
6 no / less, gibberellin-receptor-DELLA complexes formed ;
7 DELLA not broken down ;
8 DELLA (stays) bound to, transcription factors / PIF ;
9 transcription factor / PIF / RNA polymerase, cannot, bind to promoter ;
10 growth genes / XET gene, (stay) switched off / not transcribed / not expressed ;
11 stem does not elongate / internode length is small OR
no /less, cell division in stem ;

Question 5

Describe the differences between the functions of structural gene and regulatory gene

Answer 5

structural genes
code for, enzymes / structural proteins / non-regulatory proteins / IRNA / tRNA ;

regulatory genes:
code for, proteins / products, that control, gene expression / transcription;

Question 6

Describe how the expression of TYR gene leads to production of melanin

Answer 6

1 TYR codes for tyrosinase ;
2 converts tyrosine into DOPA ;
3 converts DOPA into dopaquinone ;
4 dopaquinone converted to melanin ;

Question 7

Define the terms dominant and recessive

Answer 7

dominant - allele always has effect on phenotype / allele expressed in homozygote and heterozygote / allele always
expressed ;
recessive - allele only has effect on phenotype if dominant allele absent / only expressed in homozygote;

Question 8

Define the terms gene and allele

Answer 8

gene
length of DNA coding for a polypeptide;
allele
alternative / different, form / version, of a gene;

Question 9

Explain how you would carry out a test cross

Answer 9

1 cross with homozygous recessive
2 If all offsprings exhibit dominant phenotype then parent genotype must be homozygous dominant
3 If some offsprings exhibit recessive phenotype then parent genotype must be heterozygous dominant

Question 10

Define the term locus

Answer 10

The position of gene or allele on a chromosome

Question 11

Define a transcription factor

Answer 11

Proteins that bind to DNA and control gene expression in eukaryotes by increasing or decreasing transcription rate

Question 12

Explain what is meant by alleles being co-dominant

Answer 12

Both alleles are expressed in phenotype

Question 13

Explain how gibberellin acts on DELLA proteins to stimulate the production of amylase in a germinating seed

Answer 13

1 DELLA proteins inhibit, transcription factor / PIF;

2 gibberellin binds to receptor;

3 in aleurone layer ;

4 ref. to enzyme involved;

5 DELLA proteins broken down ;

6 TF / PIF, binds to promoter region (of DNA) ;

7 transcription of gene coding for amylase / AW ;

8 ref. to translation (leading to amylase production) ;

Question 14

Describe the differences between repressible enzymes and inducible enzymes

Answer 14

repressible enzymes
1 (generally) produced continuously ;
2 synthesis can be prevented by binding of repressor protein to, specific site / promoter / operator;
3 named example; e.g. enzyme involved in tryptophan synthesis
inducible enzymes
1 synthesis only occurs when, substrate / inducer, is present;
2 idea that transcription of the gene only occurs when, substrate / inducer, binds to, transcription factor / repressor protein;
3 named example; e.g. B galactosidase / lactose permease /

Question 15

Explain what is meant by homologous pairs of chromosomes

Answer 15

1 Carry the same genes in the same position
2 Are the same shape

Question 16

Explain the need for reduction division during meiosis in the production of gametes

Answer 16

1 halves the chromosome number ;
2 (so) fertilisation / fusion of gametes, gives, diploid / 2n / 40;
3 prevents chromosome number doubling (each generation);

Question 17

Explain the genetic control of protein production in a prokaryote using the lac operon

Answer 17

1 ref. to regulatory gene ;
2 codes for repressor protein ;
3 (repressor protein) binds to operator ;

in presence of (allo)lactose:
4 (allo)lactose binds to repressor protein ;
5 which changes shape ;
6 moves away from operator / AW ;

in absence of (allo)lactose:
7 covers part of promoter
8 RNA polymerase cannot bind to promoter
9 structural genes cannot be transcribed / mRNA not synthesised
10 enzymes cannot be synthesised