patterns of inheritance

Question 1

Discuss the ways in which genetic variation is produced, including the role of nuclear division.

Answer 1

A1 independent assortment / random segregation ,
of (homologous) chromosomes / bivalents;
A2 in , metaphase I / meiosis I ;
A3 of chromatids in , metaphase I I / meiosis I I ;
A4 (so) homologous chromosomes , have different
alleles / come from different parents ;
A5 produces large number of allele combinations ;
C1 crossing over / (formation of) chiasma(ta) ;
C2 in , prophase I / meiosis I ;
C3 (so) chromatids will have new combination of alleles ;
C4 amount of variation depends on distance between
crossover points ;
M1 mutation ;
M2 changes the (DNA) nucleotide/ base, sequence ;
M3 DNA checks (during duplication)
did not recognise damage ;
M4 idea of differences in (named) protein(s) ;
N1 non-disjunction ;
N2 homologous chromosomes do not separate
(in metaphase I) ;
N3 one , more / less , chromosome present ;
F1 random, mating / fusion of gametes/ fertilisation ;
F2 gametes are not genetically identical;
F3 produces large number of (allele) combinations ;

Question 2

Use this information and the value for χ2 that you have calculated in (i) to explain whether the original hypothesis should be accepted or rejected.

Answer 2

reject hypothesis because
calculated 
2
value / 1401 , is
(much) larger than , critical value / 11.35 ;

Question 3

The difference in the observed numbers from the cross compared with the expected numbers has not occurred by chance. Suggest a genetic explanation for this difference.

Answer 3

(autosomal) linkage
or
genes / alleles, are linked ;
on same chromosome ;
linked alleles inherited together ;
Ry and rY (on chromosomes in heterozygotes) ;
crossing-over produced (rare) recombinants ;
tight linkage / two genes close together ;

Question 4

Polyploidy is the possession of more than two sets of chromosomes in the nucleus. Polyploidy is common in plants.
Suggest an explanation for the significance of polyploidy in forming new species of plant such as bread wheat, Triticum aestivum.

Answer 4

1 (inter-species / triploid) hybrids ,
are sterile / cannot reproduce sexually;

2 polyploidy (in the hybrid) provides duplicate
of each chromosome ;
3 (polyploidy) allows the hybrid to ,
carry out meiosis / form gametes
or
(polyploidy) restores fertility / overcomes sterility ;
4 (hybrids are) reproductively isolated (from other
species);
5 increased, cell size / grain size, increases yield;
6 sterile hybrids expensive for farming
(especially in developing countries);
7 (plants) stronger/more vigorous/ healthier;

Question 5

Give two reasons why it was not appropriate to use the Hardy-Weinberg principle to estimate the frequencies of alleles in this group of rabbits in the pet shop.

Answer 5

in the pet shop
1 population is , small / not (sufficiently) large ;
2 not all members of the population are breeding ;
3 idea that mating is not random ;
4 idea that migration / emigration / immigration ,
is occurring ;
5 idea that the non-brown rabbits could be colours
other than white ;

Question 6

Some genes for feather colour and pattern in chickens are carried on the Z chromosome but not on the W chromosome. One such example is the gene for striped feathers (barring).
State the name given to this type of inheritance.

Answer 6

sex linkage / sex linked ;

Question 7

homozygous

gene locus

Answer 7

homozygous
(individual / cat / genotype with) 2 identical,
alleles / version of the gene / forms of the gene ;
gene locus
position / place / location, of, gene / allele, on chromosome ;

Question 8

Over time, the frequency of domestic cat mothers who perform essential maternal behaviour, such as licking the newborn kitten, has decreased.
Suggest and explain a reason for this change in frequency over time.

Answer 8

1 whether kittens, survive / breed ;
2 whether alleles, change in frequency / passed on / kept ;
3 correct reference to selection / how selection acts ;

Question 9

State the term used to describe a genotype that has two copies of the same allele at a particular gene locus.

Answer 9

homozygous

Question 10

Explain what is meant by the following terms:
genotype
allele

Answer 10

genotype
combination of alleles ;
possessed by organism ;
allele
alternative / mutant, form / version ;
of, a gene ;

Question 11

differences between:

prophase 1 of meiosis and prophase 2 of meiosis.

Answer 11

prophase 1
homologous chromosomes pair up / bivalents form ;
chiasmata / crossing-over / recombination ;

Question 12

State what is meant by the term recessive.

Answer 12

only expressed , when homozygous /
in absence of dominant (allele) ;
not expressed when heterozygous /
expression masked by dominant (allele) ;

Question 13

Explain why one individual can only have two of the different alleles of the DRD4 gene.

Answer 13

humans are, diploid / 2n ;
chromosomes, are in pairs / homologous ;
one, (copy / gene / allele),
from each parent / on each chromosome of pair ;