Cap 4 Flashcards
State three causes of genetic variation
Mutation
Crossing over
Independent segregation / assortment (of homologous chromosomes)
Random fusion of gametes / fertilisation / mating
What is meant by a genome?
- (all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’
- ‘the total number of DNA bases in a cell/organism’
- the full base sequence of all the genetic material of an organism
What is a ‘gene pool’?
All the alleles in a population.
How do multiple alleles of a gene arise?
mutations;
which are different / at different positions within the same gene;
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
- Small sample size;
- Fusion/fertilisation of gametes is random;
- Linked Genes; Sex-linkage / crossing over;
- Epistasis;
- Lethal genotypes;
What is meant by a recessive allele?
Only expressed (in the phenotype) when homozygous
Define dominant allele
Always expressed within the phenotype
What does Hardy Weinberg’s equation predict? [3]
- The frequency of alleles (of a particular gene);
- Will stay constant from one generation to the next/over
generations / no genetic change over time; - (3 of the following) Providing no mutation/no selection / population large / population genetically isolated / mating at random / no migration;
Define gene linkage
(Genes/loci) on same chromosome
Define epistasis
The allele of one gene affects the expression of another in the phenotype;
Define codominance
Both alleles (equally) expressed in the phenotype;
Rules for Dominant alleles
- Affected offspring MUST have at least one affected parent.
- Unaffected parents ONLY have unaffected offspring.
- If both parents are affected and have an unaffected offspring, both parents must be Heterozygous
Male offspring (XY) are more likely than females (XX) to show recessive sex-linked characteristics.
Explain why.
- (Recessive) allele is always expressed in males / males have
one (recessive) allele; - Females need two recessive alleles / females need to be
homozygous recessive / females could have dominant and
recessive alleles / be heterozygous;
Expected offspring phenotype ratio from heterozygous parents:
Monohybrid
3 : 1
Expected offspring phenotype ratio from heterozygous parents:
Dihybrid
9 : 3 : 3 : 1
Expected offspring phenotype ratio from heterozygous parents:
Epistasis
9 : 4 : 3
or
15 : 1 (Ratio adds to 16 but is not 9 : 3 : 3 : 1)
or
9 : 7
Expected offspring phenotype ratio from heterozygous parents:
Autosomal linkage
3:1 (if no crossing over)
(no other pattern other than 4 phenotypes with recombination of alleles)
What is meant by the term phenotype
(Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
(Expression / appearance / characteristic due to) interaction with environment;
Explain how a single base substitution causes a change in the structure of a polypeptide.
- Change in sequence of amino acid(s) / primary structure (of
polypeptide); - Change in (position of) hydrogen, ionic, disulfide bonds;
- Alters tertiary structure;
Describe the process of crossing over and explain how it increases genetic diversity.
- Homologous pairs of chromosomes associate / form a bivalent;
- Chiasma(ta) form;
- (Equal) lengths of (non-sister) chromatids / alleles are exchanged;
- Producing new combinations of alleles;
THIS IS ‘RARE’ = Unequal chance of recombinant alleles forming.
Less recombinant gametes form.
Give two differences between mitosis and meiosis.
Mitosis given first
- One division, two divisions in meiosis;
- (Daughter) cells genetically identical, daughter cells genetically different in meiosis;
- Two cells produced, (usually) four cells produced in meiosis;
- Diploid to diploid/haploid to haploid, diploid to haploid in meiosis;
- Separation of homologous chromosomes only in meiosis;
- Crossing over only in meiosis;
- Independent segregation only in meiosis;
A population of fruit flies contained 64% grey-bodied flies.
Body colour is controlled by gene G which has 2 alleles. G produces Grey and is dominant to g, which produces black.
Use the Hardy–Weinberg equation to calculate the percentage of flies heterozygous for gene G. [2]
48% [2]
q2=0.36 or (36/100)
so, q=0.6 and P=0.4
Therefore, 2 x 0.4 x 0.6 x [100] = 48%
2Pq [1]
0.48 [1]
There were 850 fruit flies in one population. In this population, 510 fruit flies had the genotype WNWN, 255 had the genotype WNWV and 85 had the genotype WVWV.
Calculate the actual frequency of the allele WV. Do not use the Hardy-Weinberg equation in your calculation. [1]
0.25; [1]
WNWV = 1x 255 = 255
WNWV = 2 x 85 = 170
Therefore, (255 + 170) divided by (1700) = 0.25
Frequency is always shown as a proportion of 1.
In a population, the frequency of the allele for tongue-rolling, T, is 0.4.
Use the Hardy-Weinberg equation to calculate the percentage of people in this population that are heterozygous for tongue-rolling.
48% [2]
P=0.4 so, q=0.6
2Pq [1]
0.48 [1]
= 2 x 0.4 x 0.6 x [100] = 48% [2]
In a species of snail, shell colour is controlled by a gene with three alleles. The shell may be brown, pink or yellow. The allele for brown, CB, is dominant to the other two alleles. The allele for pink, CP, is dominant to the allele for yellow, CY.
Give all the genotypes which would result in a brown-shelled snail.
CBCB, CBCP and CBCY;
What are the most common LIMITATIONS to a conclusion?
- No STATISTICAL TEST: Differences could be due to CHANCE
- Data is for only ONE (named) species
- Data only collected in a lab / In vitro
- Only tested on males / females. Observations may differ in
females / males - Data is subjective
- Data only collected for ………….days / weeks / years
- Long term side effects are not known
- At no point is the ……… 100% effective.
Explain what is meant by the terms totipotent and pluripotent?
Totipotent cells can give rise to all cell types;
Pluripotent can only give some cell types;
Explain how cells produced from stem cells can have the same genes yet be of different types.
- {not all / different} genes are switched {on / off} /active /
activated ; - correct and appropriate reference to factors / mechanisms
for gene switching ;
e.g. reference to promoters / transcription factors
Describe the mechanism by which a signal protein causes the synthesis of mRNA (6)
1) signal protein {binds to / joins to / interacts with / activates}
2) receptor on surface membrane;
3) messenger molecule moves from cytoplasm and enters nucleus;
4) {produces / activates} transcription factor;
5) binds to promoter region;
6)RNA polymerase transcribes target gene;
Explain how oestrogen enables RNA polymerase to transcribe its target gene (7)
1)Oestrogen is lipid soluble so diffuses through the 2)Oestrogen diffuses through the phospholipid cell
membrane;
3) attaches to ERα receptor;
4)ERα receptor changes shape / Tertiary structure;
5) ERα receptor leaves protein complex which inhibited it’s
action;
6) oestrogen receptor binds to promoter region;
7)enables RNA polymerase to transcribe target gene.
Compare the structure of dsRNA and DNA (8)
Similarities; 2 max
Polynucleotides/polymer of nucleotides;
Contain Adenine, Guanine, Cytosine;
Have pentose sugar/5 carbon sugar;
Double stranded/hydrogen bonds/base pairs.
Differences; 2 max
dsRNA contains uracil, DNA contains thymine;
dsRNA contains ribose DNA contains Deoxyribose;
dsRNA is Shorter than DNA; fewer base pairs in length;