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Flashcards in Genetics Deck (36):

Colour blindness is caused by a recessive allele. A woman and her partner have normal vision. Their first child has colour blindness. What is the probability of their second child having colour blindness if it is a son?
A. 100%
B. 25%
C. 50%
D. 0%



One type of gene mutation involves a base substitution. What are the consequences of the base substitutions in the two new sequences of DNA?
A. Both are mutations that would result in different polypeptides.
B. Sequence 2 would result in a changed polypeptide but sequence 1 would not.
C. All three DNA sequences would translate into the same polypeptide.
D. Only the original DNA and sequence 2 would translate into the same polypeptide.



The sequence of the first six amino acids of the normal beta haemoglobin chain are listed.
Valine - Histidine - Leucine - Threonine - Proline - Glutamic Acid

What sequence of amino acids could there be in the first six amino acids of the sickle-cell beta haemoglobin chain?



A mouse breeder performs a test cross using a brown mouse to determine the unknown genotype of a black mouse. There are four offspring produced by the cross. What conclusions can be drawn?
A. 1/4 brown offspring if heterozygous parent
B. 3/4 black offspring if heterozygous parent
C. All black offspring = MUST be homozygous parent
D. Any brown offspring = Parent MUST be heterozygous



What evidence from the pedigree chart confirms that ACHOO syndrome is not X-linked dominant?
A. 4 females in pedigree affected and X-linked do not affect females.
B. Affected male in generation II
C. Affected female in generation II
D. If X-linked dominant, affected mom in gen II couldn't produce unaffected son



Which of the following would be possible only if the ACHOO syndrome is inherited as autosomal recessive?
A. 2 unaffected parents have child with condition
B. At least 1 parent has to be affected to produce children that have condition
C. Affected and unaffected parent have affected child.
D. 2 unaffected parents have no affected children.



A test cross resulted in these recombinants:
tB Tb
-- --
tb tb
Which of the following was the parental cross?
A. TB//tb x tb//tb
B. TB//tB x tb//Tb
C. Tb//tB x tb//tb
D. TB//tb x TB//tb



What is the difference between the alleles of a gene?
A. Their position on the chromosome
B. Their amino acid sequence
C. Their pentose sugars
D. Their base sequence



Describe the use of DNA profiling in forensic investigations.

Restriction endonuclease used to cut satellite DNA into fragments. PCR then used to amplify DNA segments and separated by size by gel electrophoresis. Pattern of bands compared to bands of suspects.


Using a named example, discuss the effects of genetically modifying an organism.

Genetic modification is the transfer of genes between organisms such as when modifying corn crops using Bt toxin gene.
- makes corn resistant to pests like corn-borers
- less pesticide used, saves money
- more yield
- less land needed for crops, could be used for wildlife instead as crops more efficient

- non-target insects like monarch butterflies might be killed
- gene might spread to other plants by cross pollination
- possibility of allergic reactions when crop eaten
- only favoured countries have tech to do so.


Outline the effects of a base substitution mutation in the case of sickle-cell anaemia.

DNA changes from GAG to GTG, which is a single base missense substitution. This means that the mRNA changes from GAG to GUG, which when translated produces valine rather than glutamic acid and this changes the shape of haemoglobin and hence RBCs. Reduces efficiency of RBCs and can also block capillaries making blood clots more likely. Causes severe anaemia.


Factor IX is a blood clotting protein which some haemophiliacs lack. In the future haemophilia could be treated using clotting factors synthesised by gm bacteria. Outline the basic technique used for gene transfer.

mRNA coding for factor IX extracted from human cell and copied to cDNA using reverse transcriptase. Gene transferred using plasmids. Restriction endonuclease are used to open plasmid and to cut DNA. This produces sticky ends on gene and plasmid, which is easily sealed using ligament. Recombinant plasmid taken up by bacteria and then cultured to produce factor IX


Explain how males inherit haemophilia and how females can become carriers for the condition.

Haemophilia due to X-linked recessive allele. Y chromosomes as a result don't have the allele, so males only inherit from mothers, meaning trait cannot be masked by another (dominant) allele from father. As females inherit one X chromosome from each parent, the disease can be inherited from both mother and father. Carriers can only be female as only they can be heterozygous for the recessive allele.


Explain how meiosis results in an effectively infinite genetic variety of gametes.

1 homologous chromosome comes from mother and one from father; and these pair up in prophase I (synapsis) to form bivalents. Crossing-over is the exchange of genes between non-sister chromatids and this also occurs in prophase I. Junctions where genes cross-over are chiasmata. Crossing-over occurs at random positions and may occur more than once on same bivalent. This means chromosomes aren't identical. In metaphase I, chromosomes line up in middle of cell and are separated in anaphase I depending on how they are orientated, which is random. The assortment of one chromosome is independent of others. Leads to 2^23 possible combinations


Explain the use of karyotyping in human genetics.

Karyotyping is the number and type of chromosomes in a cell; cells collected from chorionic villus or by amniocentesis; chromosomes then arranged in pairs according to size.
Karyotypes have many uses:
- identifying gender of embryo as males will have differently sized sex chromosomes, whilst females have equally sized.
- used for pre-natal diagnosis of chromosome abnormalities/non-disjunction; helping to identify trisomies like trisomy 21 (down); which may lead to decisions to abort foetuses.


Outline the method that can be used to amplify small quantities of DNA to obtain large enough quantities for DNA profiling.

PCR; DNA obtained from blood or hairs and combined with Taq polymerase in thermal cycled and DNA is replicated many times


Deduce, with reasons for your answer, whether the chromosomes are (i) autosomes or sex chromosomes and (ii) homologous or non-homologous

(i) Autosomes as X and Y are different lengths
(ii) Homologous chromosomes as they have the same genes in same sequence.


Determine the combinations of alleles that would be present on each chromatid. Use the diagrams to indicate your answer.

A - B - C
a - b - c
A - b - c
a - B - C


Explain why carriers of sex-linked genes must be heterozygous.

Carrier has 2 copy of a recessive allele, but not affected by condition as it is to be masked by dominant allele.


Outline the formation of chiasmata during crossing over.

Crossing-over occurs in prophase I of meiosis. In prophase I, homologous chromosomes pair up (synapsids) and form a bivalent. Non-sister chromatids exchange genes during cross-over and X-shaped junctions form called chiasmata. Chiasmata occurs where crossing-over occurs and crossing over occurs at any point in chromosome and can occur more than once. Chiasmata hold chromosomes together.


Explain how an error in meiosis can lead to Down syndrome.

Non-disjunction is when chromosomes do not separate in anaphase I due to incorrect spindle attachment or when chromatids do not separate during anaphase II because of centromeres not dividing. Non-disjunction is less common in sperm than egg formation. Down syndrome is caused due to an extra chromosome 21 (trisomy 21) caused by non-disjunction. Gamete receives 2 chromosomes of same type, so zygote has 3 chromosomes.


Describe codominance and multiple alleles using inheritance of ABO blood groups as an example of them.

Codominant alleles both affect the phenotype. I^A, I^B and i are the 3 alleles controlling blood groups; where I^A and I^B are codominant and i is recessive. When A and B are both expressed, they give blood group AB rather than A or B. On the other hand, i is recessive to both I^A and I^B and so gives rise to blood group A and B respectively and blood group O when gene is homozygous recessive. Punnet square.


Research is being undertaken by scientists in some countries to develop mess of therapeutic cloning. Discuss the ethical issues of therapeutic cloning in humans.

Therapeutic cloning is the creation of an embryo to supply embryonic stem cells for medical use; involves the transfer of nucleus from somatic cell into an egg; stimulated by electric shock to begin cell division.



Distinguish between autosomes and sex chromosomes in humans.

Sex chromosomes are either X or Y and they determine sex of organism. Females have XX, whilst males have XY. X carried by both males and females and is larger than Y whereas Y only carried by males. 22 types of autosomes exist and males and females have the same types of autosomes.


Define the terms chromosomes, gene, allele and genome.

Chromosome: Structure formed by DNA and proteins.
Gene: Heritable factor that controls a specific trait.
Allele: Alternate versions of the same gene, hence occupying the same gene locus.
Genome: The whole of genetic information of an organism.


Define Single Nucleotide Polymorphism

Differences between base sequences of different alleles in a gene.


Define Silent Mutations

A mutation that doesn't affect polypeptide produced (due to degeneracy of genetic code)


Define nonsense mutation

When a mutation creates an early stop codon.


Define missense mutation

When a mutation results in codon that codes for different amino acid


Define frameshift mutation

When nucleotides are inserted or deleted, which changes entire reading frame of mRNA codons and can affect polypeptide production.


Define homologous chromosome

Carry same gene sequence, but not necessarily same alleles of those genes.


Why might genome size not be directly proportional to organism's complexity?

- Proportion of DNA that acts as functional genes is variable.
- Amount of gene duplication varies.


What is the genome size?

Total number of DNA base pairs in 1 copy of haploid genome.


Compare Amniocentesis and Chorionic Villus Sampling

- Fetal cells obtained from amniotic fluid.
- Ultrasound used to guide large needle through abdomen and into amniotic sac.
- 1% risk of miscarriage

Chorionic Villus Sampling
- Fetal cells obtained from chorion, part of placenta.
- Sampling tool uterus through vagina - can be done earlier in pregnancy.
- 2% risk of miscarriage


Describe the process of meiosis

Prophase I:
Nuclear membrane brakes down
Crossing over can occur

Metaphase I:
- Spindle fibres move homologous pairs along equator of cell.
- Orientation is random and independent

Anaphase I:
- Disjunction
- Independent and Random Assortment

Telophase I:
- chromosomes uncoil and 2 new nuclei form
- Reduction division complete
- Sister chromatids different if crossing over and recombination occurs.

The rest is same as mitosis...


What is gel electrophoresis?

Used to separate DNA fragments according to size by separating charged particles in an electrical field.