OCR Biology Patterns of Inheritance 6.1.2 Revision Summary - By A* Student
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Course
Patterns of Inheritance 6.1.2 (H420)
Institution
OCR
A revision summary of OCR A level Biology Patterns of Inheritance 6.1.2
Made by a student who achieved A* in A level Biology.
Covers all the points within the OCR Biology Specification.
Condenses 3 OCR specification books and class notes.
Gene A sequence of base nucleotides that determine a characteristic. Monohybrid inheritance – the inheritance of a single
Allele
A variation of a gene gene
Allele freq
Total no. of times a allele is in a gene pool at a given time Genetic Cross
LocusThe position of a gene on a chromosome 1. Parental phenotype Red Red
Genotype
Genetic makeup of an organism 2. Parental genotype Rr Rr
Phenotype
Visible characterises of an organism 3. Gametes (in circle) R or r R or r
Homozygous
Pure breeding: 2 identical alleles at gene locus on homologous chromo 4. Punnet Square R r
heterozygous
Have 2 different alleles at gene locus on homologous chromosome (genotype) R RR Rr
Monogenic
Determined by a single gene r Rr rr
Dominant
Gene is always expressed if present 5. F1 generation phenotype 3 red: 1
recessive
Only expressed if gene is homozygous white
F1 generation
Produced from crossing parents - Carrier = heterozygous individual who has
F2 generation
Produced from crossing F1 recessive gene that is not expressed but can still
Linkage
When 2 or more genes loci are on the same chromosome. pass it on.
- 4 phenotypes, no ratio Test Cross – used to identify unknown genotype
Sex Linkage Gene is present on one sex chromosomes - Cross unknown genotype with Homozygous
Codominance 2 alleles are equally dominant & expressed in phenotype. 1:2:1 recessive
Autosome Any chromosome that isn’t a sex chromosome - If F1 gen express recessive gene, unknown must be
Monohybrid Inheritance of one gene, 3:1 heterozygous.
Dihybrid Inheritance of two non-interacting genes, 9:3:3:1 Codominance - two different alleles for the same gene
Epistasis The interaction 2+ genes at different loci are equally dominant and both expressed in genotype.
Recessive = 9:3:4, Dominant = 12:3:1/ 13:3, complementary 9:7 - Allele is subscripted to the gene.
Chromosome mutation – during meiosis F1 cross.
- Deletion – portion is lost 1. Parental phenotype Pink Pink
- Inversion – section inverts, now to far away from regulator gene to be expressed Flower Flower
- Translocation – piece of one chromosome becomes detached & attaches to 2. Parental genotype C C
R w
CRCW
another 3. Gametes (in circle) CR or CW CR or CW
- Duplication – piece is duplicated & now over expressed 4. Punnet Square C R
CW
- Non-disjunction – chromosomes fail to separate. CR CRCR CRCW
Aneuploidy – chromones not exact multiple of haploid C W
C C
R W
CWCW
Polyploidy – diploid is fertilised by a haploid = triploid. 5. F2 generation phenotype 1 Red: 2 Pink: 1 White
Plant mutations from environmental factors. Blood type has multiple alleles for the gene (I) each
- Chlorotic plants – have genotype to make chlorophyll but not enough Mg 2+ in soil codominant.
- Etiolation – not enough light means plant grows long & spindly. - IA = antigen A, IB = antigen B, IO = no antigen
(recessive)
Sex chromosomes
Human female = XX = homogametic – only one gamete Sex linkage – inheritance of trait is limited by alleles located on sex chromosome
Human male = XY = heterogametic – different gametes Haemophilia – sex-linked genetic disorder caused by recessive allele only on X
which determine sex
X chromosome much larger than Y, in males:
Only the homologous region pairs up
In non-homologous region: X chromosome has alleles not
present in Y.
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