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Transmission of genetics
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Transmission GeneticsMonohybrid and Dihybrid Crosses
Terminology:
Gene- Unit of inheritance often responsible for one trait
Allele- Alternatives states (mutations) of particular gene
Homozygous- Identical alleles at gene loci- produces identical gametes- true breeding
Heterozygous- Different alleles at gene locus- produces unlike gametes
Dominant- Allele trait expressed regardless of other allele(s)
Recessive- Allele trait only expressed if homozygous
Phenotype- expression of character in organism
Genotype- Allelic hereditary constitution
P1- parental generation
F1- First filial generation- progeny from parents
F2- Second filial generation-progeny resulting from F1 cross
Punnet Square- Probability diagram illustrating possible offspring of mating
Gene Notation- A- dominant allele (upper-case), a- recessive allele (lower-case)
In Drosophila letter- letter denotes mutant allele “+”- wildtype allele
Mendelian Inheritance
Complete Dominance- Monohybrid & Dihybrid
RULE 1- Always start with TWO parents that are pure breeding (homozygous)
Genetics using Pea (Pisum sativum)
Advantages
1) Number of discreet traits that can be studied
2) Self- fertilising
3) Numerous viable and fertile progeny
4) Short generation time
5) Can carry out crosse between different individuals by emasculation of anthers before pollen ripe and transfer of
pollen from alternate parent
Monohybrid & Dihybrid Inheritance
Complete Dominance- Autosomal
Mendel’s four postulates & laws of inheritance
1) Principles of Paired Factors
2) Principle of Dominance
3) Law of Segregation/ Law of Purity of Gametes- Mendel’s First Law of Inheritance
4) Law of Independent Assortment- Mendel’s Second Law of Inheritance
Incomplete Dominance- Co-dominance
Lethal
Epistasis
1. Mendel Choose Discrete Phenotypes/ Traits
,Mendel’s first Postulate
Unit factors in pairs- one gene loci with two alleles
For each character/ gene locus- organism usually inherits two alleles- one from each parent
Alternative versions of alleles account for variations in inherited characters- traits – these different versions are usually
caused by mutations in DNA code
Character or gene locus for stem length TT (tall phenotype)
- Tt- tall
- tt- short
Crossing involving only one trait is referred to as monohybrid cross
Pure breeding- Homozygous tall stemmed cross with variety that produced short stems
- All resulting seeds were collected and planted and produced tall plants
Parental generation is denoted as P generation
Offspring of P generation are F1 generation- First filial
Mendel’s Second Postulate
If two alleles at gene locus differ (heterozygous)
- then one- dominant – determines appearance- phenotype
- the other – recessive – has no noticeable effect
Depending on traits- uniform feature is either one of parent’s traits
– dominant-recessive pair of characteristics or it is intermediate
Complete dominance- Monohybrid
Inheritance
Complete dominance- recessive allele in heterozygote not
expressed
F1 phenotype- is same as dominant parent
Behaviour of T and t alleles in F2 generation
Seeds from self-fertilised F1 were planted and grew
into F2 (second filial) generation
, Mendel’s third postulate- First law- Segregation
At gamete formation- two alleles segregate randomly – one to each pole
- If homozygous- alleles are identical – so are all gametes
- If heterozygous- 50% gametes will contain one allele type and 50% the other
General Pattern of Monohybrid inheritance with
complete dominance
Dihybrid Inheritance
Monohybrid- one gene- two alleles
Dihybrid- two genes on different chromosomes each
with two alleles
Complete Dominance- Dihybrid Inheritance
F1 expressed dominant traits of each allele
When Mendel considered two traits per cross- Dihybrid as opposed to
monohybrid (single-trait crosses)
Mendel started with true-breeding plants- that were short with yellow pods
and crossed them with true-breeding plants- that were tall with green pods.
The seeds in F1 were all tall with green pods
Resulted in Mendel’s 4th Postulate- Second Law-
Independent Assortment
At gamete formation – meiosis; alleles on two or more
different gene loci on different chromosomes assort
independently from each other
-dihybrid inheritance- each pair of alleles at a gene
locus segregate independently of other pairs of alleles
during gamete formation.
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