Clear and concise Cornell styled notes for Unit 7 Biology A Level - Genetics, Populations, Evolution and Ecosystems. The notes include diagrams and colours to make revision more visually appealing. The whole course is covered by my notes and is written in a detailed way that is still easily underst...
Basics of Genetics Eve Holland
Key Points: Notes:
Gene = section of chromosome that Genes & Alleles:
codes for a polypeptide - Each chromosome is a long DNA molecule that codes for multiple
proteins
Gene position = locus - A section of DNA which codes for a particular protein is called a
gene
Different forms of same gene = allele - The position of a gene on the chromosome is called its locus
- Different forms of a gene that take the same locus are called alleles
Genotype & Phenotype:
Genotype is the genetic makeup - The chromosomes of eukaryotic cells occur in homologous pairs
(there are two copies of each chromosome) e.g XY
- The genotype is the set of allele of gene possessed by an organism
Phenotype is the visible characteristics
- When the 2 allele copies are identical - homozygous
- When the 2 allele copies are different - heterozygous
2 identical allele copies = homozygous
- Phenotype is the visible characteristics of an organism
2 different allele copies = heterozygous
Dominance and Codominance:
- Dominant alleles are ALWAYS expressed in the phenotype
Dominant = always expressed - Recessive alleles are only expressed in the phenotype if there are
Recessive = only expressed when no no dominant alleles are present (homozygous for recessive allele)
dominant allele is present
- Sometimes both alleles can be expressed in the phenotype at the
2 allele copies expressed in phenotype same time - Codominance
at the same time = codominant - When writing the genotype for codominance the gene is
symbolised as the capital letter and the alleles are represented by
different superscript letters, for example IA
Sex linkages genes are only found on Linkage:
the X & Y Sex linkage:
- Women have XX and men have XY, sex genes are found on the X&Y
X is bigger than Y so has more genes so - X chromosome is a lot larger than Y chromosome so some alleles
the X allele is always expressed are found on a region of X that is not present on the Y
- Therefore the gene on the X is always expressed
Autosomes are non sex chromosomes Autosomal linkage:
- Autosomes are non sex chromosomes
Autosomal linkage is when 2+ alleles - It is when two or more genes are on the same autosomal
chromosome. These genes do not assort independently during
are on the same chromosome so they
meiosis
are linked together and stay in the
- Thus they are linked and they stay together in the original parental
original parental combination.
combination
Summary:
, Monohybrid vs dihybrid crosses Eve Holland
Key Points: Notes:
Monohybrid Crosses:
Monohybrid looks at how a SINGLE - Monohybrid inheritance looks at how the alleles for a single gene
gene is passed on are passed on from one generation to the next
- A genetic diagram that can be used to show monohybrid crosses
could be a punnett square.
Predicted ratio of phenotypes in offspring– 3
black coat : 1 chestnut coat
Predicted ratio of genotypes in offspring – 1 BB :
Diagram → is called a punnett square 2 Bb : 1 bb
- The predicted genotypes that genetic diagrams produce are all
based on chance - There is no way to predict which gametes will
Diagrams are only predictions as fuse so sometimes the observed or real-life results can differ from
gamete fusion is completely random the predictions
Dihybrid Crosses::
- Dihybrid Crosses look at the alleles of 2 genes cross and transfer
Dihybrid looks at how the alleles of 2 across generations
genes cross and transfer down
generations
- Dihybrid crosses and their predictions rely on the assumption that
independent assortment effects the genes being investigated behave independently of one another
independent assortment during meiosis
- Not all genes assort independently during meiosis - Some genes
which are located on the same chromosome display autosomal
linkage and stay together in the original parental combination
- Linkage between genes affects how parental alleles are passed
If alleles are independently assorted
onto offspring through the gametes
then they should be kept in a bracket
- When writing linked genotypes it can be easier to keep the linked
alleles within a bracket, e.g genotype FFGG if there is linkage
between the two genes then it would be written as (FG)(FG)
Note:
Use FOIL to find gametes in dihybrid - To find the gametes to cross in the punnett squares use foil on the
genotype - this is only for dihybrid
Summary:
Gregor Mendel did the
pea experiments →
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