Summary notes for AQA A-level Biology inherited change topic. Includes clear information on genetic crosses, monohybrid inheritance, polygenic inheritance, sex linkage, epistasis, autosomal linkage, chi squared test, crossing over. Summarised from class notes and the official course textbook. From ...
Studying inheritance
Mendel
• Experimented on pea plants
• Controlled pollination
• Worked out the basic principles of inheritance: dominant and recessive genes and
the 3:1 ratio
Key definitions:
• Genetics: the study of inheritance, controlled by genes
• Gene: length of DNA that carries a coded base sequence to synthesise a single polypeptide, or
a functional RNA molecule
• Alleles: different forms of the same gene, e.g. height in pea plants is controlled by two alleles
• Genome: total genetic make up of an organism, made up of genes on chromosomes
• Polygenes: a single feature that is determined by several genes, e.g. human height
• Gene mapping: a technique used to locate the position of a gene on a chromosome
• Chi squared: stats test used to find out whether offspring phenotypes fit an expected ratio
• Homologous chromosomes: 2 chromosomes of a pair carrying the same genes but different
alleles
• Co-dominance: Where both alleles are expressed in the phenotype
• Multiple alleles: there are more than two types of allele for a particular gene, e.g. blood groups
Dominant alleles: always appear Recessive alleles: only expressed if
in the phenotype both alleles in the pair are recessive
Genotype: genetic make up of an Phenotype: the features resulting
organism, made up of alleles which from expression of the genes and
determine a particular feature their interaction with the
environment. [Genotype +
environment = phenotype]
Homozygous: two alleles of a pair Heterozygous: two alleles of a pair
are the same are different
Sex linked genes: genes found on Autosomal genes: genes found on
the sex chromosomes all chromosomes apart from the X
and Y chromosomes
, Monohybrid inheritance.
Monohybrid inheritance is a single pair of contrasted characteristics (e.g. tall and short/dwarf
plants) that are controlled by a single gene
The position of a gene on a particular chromosome is known as the gene locus. In a diploid
organism, chromosomes occur in homologous pairs and so there will be two alleles present for a
specific gene (one inherited from each parent).
This means that there will be 3 possible combinations of alleles for the characteristic:
Monohybrid crosses
Steps to follow:
1. Parental phenotypes and genotypes
2. Gametes - represent with a letter and a circle around it
3. Punnett square diagram
4. Offspring genotypes and phenotypes
Pure breeding organisms will produce the same phenotype with successive self crosses as they
are homozygous
If a cross is carried out between 2 pure breeding parents the offspring = the F1 generation.
Crossing 2 individuals from the F1 generation leads to an F2 generation
Crossing two heterozygotes:
The ratio of dominant to recessive phenotypes is 3:1 and this is Mendel’s first law.
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