Topic 3: Genetics

Question 1

Define the terms gene and allele and explain how they differ. [4]

Answer 1

 gene is a heritable factor / unit of inheritance
 gene is composed of DNA
 gene controls a specific characteristic / codes for a polypeptide / protein
 allele is a form of a gene
 alleles of a gene occupy the same gene locus / same position on chromosome
 alleles differ (from each other) by one / a small number of bases(s)/ base pair(s)

Question 2

Describe the consequences of a base substitution mutation with regards to sickle cell anemia. [7]

Answer 2

 the sequence of nucleotide bases in DNA codes for the sequence of amino acids in proteins
 DNA is transcribed into mRNA, which is translated into amino acids of protein
 normal (ß chain) hemoglobin gene / DNA produces normal (ß chain) hemoglobin protein / amino acids
 substitution= the replacement of one (or more) nucleotide base with another
 caused by a copying mistake during DNA replication
 as a result of a mutagen / X-rays / chemical / UV radiation / other mutagen
 mutation in normal (ß chain) hemoglobin gene alters the sequence of nucleotide bases
 normal nucleotide sequence = CTC altered to CAC
 resulting in altered mRNA (GAG to GUG) during transcription
 resulting in altered sequence of amino acids in (ß chain) hemoglobin protein (glutamic acid to valine) during translation
 causing red blood cells to change shape / sickle under low oxygen conditions
 causing sickle cells anemia when two copies of the mutated gene are inherited
 producing a sickle cell carrier when one copy of the mutated gene is inherited
 sickle cells anemia reduces oxygen flow to organs, leading to their deterioration

Question 3

Karyograms involve arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. [5]

Answer 3

application of karyogram {2 max}
 find gender / test for Down’s syndrome / other chromosome abnormality
 identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted
 XX = female and XY = male / third chromosome 21 indicates Down’s syndrome / other chromosome abnormality (e.g. Klinefelter’s syndrome)
obtaining chromosomes {3 max}
 fetal cells obtained from amniotic fluid / amniocentesis / other named source
 white blood cells obtained
 cells encouraged to divide
 cells accumulated / blocked in metaphase
 prepare slide / chromosomes examined

Question 4

Compare the processes of mitosis and meiosis. [6]

Answer 4

 Mitosis: one cell division & Meiosis: two divisions / reduction division
 Mitosis: chromosome number does not change & Meiosis: converts diploid to haploid cells
 Mitosis: products genetically identical & Meiosis: products genetically diverse
 Mitosis: separation of sister chromatids in anaphase & Meiosis: separation of homologous chromosomes in anaphase I and sister chromatids in anaphase II
 Mitosis: no crossing over & Meiosis: crossing over in prophase I
 Mitosis: no formation of tetrads / no synapsis & Meiosis: formation of tetrads / synapsis
 Mitosis: produce cells for growth/repair/asexual reproduction & Meiosis: produce sexual cells / gametes for sexual reproduction
 Mitosis: two cells produced & Meiosis: four cells produced
 Mitosis: daughter cells with both copies of chromosomes/random assortment does not occur & Meiosis: random assortment of maternal/ paternal chromosomes
 Mitosis: replication of DNA in interphase & Meiosis: replication of DNA in interphase I
 Mitosis: four phases: prophase, metaphase, anaphase, telophase & Meiosis: same four phases twice

Question 5

Outline one example of inheritance involving multiple alleles. [5]

Answer 5

 multiple alleles means a gene has three or more alleles / more than two alleles
 ABO blood groups / other named example of multiple alleles
 ABO gene has three alleles / equivalent for other example
 IA IB and i shown (at some point in the answer) / equivalent for other example
accept other notation for alleles if clear
 any two of these alleles are present in an individual
 homozygous and heterozygous genotype with phenotypes (shown somewhere)
 all six genotypes with phenotypes given (shown somewhere)
 example / diagram of a cross involving all three alleles

Question 6

Describe the inheritance of ABO blood groups including an example of the possible
outcomes of a homozygous blood group A mother having a child with a blood
group O father. [5]

Answer 6

	example of co-dominance
	multiple alleles / 3 alleles
	(phenotype) O has (genotype) ii
	B can be IB IB or IB i
	A can be IA IA or IA i
	AB is IA IB
	(P are) i i x IA IA
	(gametes) i and IA
	(F1 genotype) IA i
	(F1 phenotype) blood group A

Question 7

Outline sex linkage. [5]

Answer 7

 gene carried on sex chromosome / X chromosome / Y chromosome
 inheritance different in males than in females
 males have only one X chromosome therefore, only one copy of the gene
 mutation on Y chromosome can only be inherited by males
 women can be carriers if only one X chromosome affected
 example of sex linked characteristics (e.g. hemophilia / color blindness)
 example of cross involving linkage

Question 8

Explain, using a named example, why many sex-linked diseases occur
more frequently in men than women. [9]

Answer 8

 named example of sex-linked disease
 caused by recessive allele
 on the X chromosome
 example of pair of alleles (e.g. X H and X h) (reject if alleles do not correspond)
 females are XX and males are XY
 females have two alleles of the gene and males have only one
 allele causing the disease is rare / uncommon
 probability of females inheriting rare allele twice as low
 calculation of squaring the gene frequency
 female would have to inherit the allele from her father
 who would have suffered from the disease
 so females can carry the gene but still be normal
 but males (with the gene) will have the disease