Inheritance, Selective Breeding, Mutations, Evolution and Antibiotic Resistance Revision Summary
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Course
Biology
Institution
GCSE
Includes:
Inheritance
Selective Breeding
Mutations, Evolution
Antibiotic Resistance
Clear, conscise summaries for each section, in Q&A form, so can easily be turned into flashcards/ quizlet etc...
Why do offspring from sexual reproduction vary genetically?
1. Fertilization is random
2. Each gamete is genetically unique so the resulting zygote has a
unique combination of genes.
How are identical twins formed?
● They are formed when one zygote divides by mitosis however the
two genetically identical cells produced do NOT stay together.
● They are sometimes called monozygotic twins
What else can cause variation?
The environment (e.g. scars, accents, skin tone etc…)
What are Genes ?
● Sections of DNA which code for particular proteins
● These proteins make up different features (e.g hair colour, skin
colour)
What are Alleles?
● Different forms of a gene
● There are 2 alleles for each gene (2 types)
What are the 2 types of alleles?
● Dominant (A) and Recessive (a)
What is a genotype?
● The alleles an organism has for certain characteristics (e.g AA, aa,
or Aa)
The Genotypes:
Heterozygous = both dominant and recessive allele (e.g. Aa)
Homozygous Dominant = both alleles are dominant (e.g AA)
Homozygous Recessive = both alleles are recessive (e.g aa) - only
expressed in the absence of a dominant allele
What is a phenotype?
● The characteristic/ appearance of an organism resulting from its
genotype (e.g. hair colour)
, Monohybrid Inheritance:
Who was Gregor Mendel?
A monk who carried out hundreds of experiments with pea plants to find
the laws of inheritance.
What were the laws of inheritance he discovered?
1. The heritable unit (gene) is passed on from one generation to the
next
2. The heritable unit can have alternative forms
3. Each individual must have two alternative forms per feature
4. Gametes only have one alternative form per feature
5. One allele can be dominant over the other
Genetic Diagrams:
Tall = dominant
Short = recessive
E.g. Mendel crossed Homozygous Dominant with Homozygous
Recessive T T
t Tt Tt
t Tt Tt
= 100% tall offspring
Then he crossed their offspring…
T t
T TT Tt
t Tt tt
= Tall: short in the ratio 3:1
Why is the predicted ratio not always seen in actual
populations?
● Some plants may die
● Fertilisation is random
What is polygenic inheritance?
- When characteristics are controlled by two or more genes (e.g.
the amount of melanin ins someone’s skin is controlled by
several genes)
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