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Summary A level biology topic 7 mark scheme notes
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Genotype and phenotype- Genotype: genetic constitution of an organism
- e.g. BB, Bb, bb (eye colour)
- Phenotype: the expression of this genetic constitution and its interaction with the environment
- E.g. brown eyes
- Explains why genetically identical twins may have diverging phenotypes, especially as get
older and are exposed to more environmental stimuli
Alleles
- Allele: different version of a gene (sequence of bases different)
- In a diploid organism, the alleles at a specific locus may be either…
- Homozygous
- Both alleles at a specific locus on each homologous chromosome are the same
e.g. BB or bb
- Heterozygous
- Both alleles at a specific locus on each homologous chromosome are different
e.g. Bb
- Alleles may be…
- Dominant
- In heterozygous state, it’s the allele that is expressed in the phenotype
- Recessive
- In heterozygous state, it’s the allele that isn’t expressed in the phenotype. Only
expressed if homozygous recessive
- Codominant
- Both alleles for same gene in heterozygous organism contribute to phenotype
Genetic diagrams
- Used to illustrate possible genotypes (and thus, phenotypes) of offspring from two parents, and
the probability of occurrence of each
- Genetic inheritance diagram should contain:
- Parent phenotypes / genotypes
- Parent gamete genotypes
- Punnet square – offspring genotypes, labelled with phenotypes
,Genetic diagrams: monohybrid crosses
- Monogenic inheritance: inheritance of characteristic controlled by a single gene
- Example: Inheritance of wing length in fruit flies
N – normal wings allele (dominant); n – vestigial (little) wings allele (recessive)
Parents genotypes: Parent 1 – NN; parent 2 – nn.
- Step 1: work out parents’ gametes (haploid; 1 allele)
Parent 1 – N, N (parent 1 only produces gametes for normal wings)
Parent 2 – n, n (parent 2 only produces gametes for vestigial wings)
- Step 2: work out possible genotypes/phenotypes of F1 generation offspring
N N
n Nn Nn
n Nn Nn
Genotype(s): Nn (100% heteroxygous)
Phenotype(s): Normal wings
- Step 3: work out parents’ (F1 generation) gametes
Parent 1 – N, n
Parent 2 – N, n
(both parents produce gametes containing allele for normal or vestigial wings)
- Step 4: work out possible genotypes/phenotypes of F2 generation offspring
N n
N NN Nn
n Nn nn
Genotype(s): NN, Nn, nn
Phenotype(s): Normal wings, vestigial wings
- Step 5: work out ratio in F2 offspring
3:1 (normal : vestigial)
i.e. 75% chance offspring will have normal wings; 25% change offspring will have
vestigial wings
i.e. 1 in 4 chance of offspring having vestigial wings
Genetic diagrams: dihybrid crosses
- Inheritance of two different characteristics controlled by two different genes
- An allele from one pair of chromosomes can enter a gamete with either allele from the other
pair because of independent segregation, so…
- Possible genotypes of gametes from an AaBb adult: Ab, AB, ab, aB
Crosses involving codominance
- Both alleles in a heterozygous individual contribute to phenotype
- Example: coat colour in cattle
- Red and white allele = roan coat, speckled white and red coat as both alleles expressed
, Crosses involving multiple alleles
- Individuals only have 2 alleles of a gene (2 homologous chromosomes) but there may be more
than 2 alleles in the population
- There is usually a dominance hierarchy
Example exam question: multiple alleles
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.
(a) Give all the genotypes which would result in a brown-shelled snail. (1 mark)
ü CB CB , CB CP , CB CY
(b) A cross between two pink-shelled snails produced only pink-shelled and yellow-shelled snails.
Use a genetic diagram to explain why. (3 marks)
ü Two genotypes (as parents) shown as CP CY
or
Two sets of gametes shown as CP and CY
ü Genotypes of offspring shown as CP CY, CP CP and CY CY
ü Above genotypes of offspring correctly linked to phenotypes i.e. pink and yellow
Crosses involving sex linkage
- A gene is ‘sex-linked’ if its locus is on a sex-chromosome
- Specific characteristic more likely to be inherited in either male or female offspring
- Genes are more likely to be X linked (found on X chromosome) …
- Females (XX) have two copies of the gene / allele
- Only express recessive allele if homozygous recessive
- Males (XY) have 1 copy of the gene / allele
- Can’t be heterozygous; express recessive X-linked allele if only one copy present
(inherited from mother)
Example exam question: sex linkage and codominance
In cats, males are XY and females are XX. A gene on the X chromosome controls fur colour in cats. The
allele G codes for ginger fur and the allele B codes for black fur. These alleles are codominant.
Heterozygous females have ginger and black patches of fur and their phenotype is described as
tortoiseshell.
(a) Male cats with a tortoiseshell phenotype do not usually occur. Explain why. (1 mark)
ü Only possess one allele / the Y chromosome doesn’t carry the allele/gene / can’t be
heterozygous
(b) A tortoiseshell female was crossed with a black male. Use a genetic diagram to show all the
possible genotypes and the ratio of phenotypes expected in the offspring of this cross. (3
marks)
Use XG to indicate the allele G on an X chromosome.
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