Transmission Genetics

Darwin:

Pangenesis - reproductive cells contain gemmules, bearing hereditary attributes, derived from all cells by moving through the blood

Gemmules - units representing each body part collected by blood to semen

William Harvey - Epigenesis - organism derives from substances in egg

Jean-Baptiste Lamarck:

Use and disuse - more frequent and continuous use strengthens and develps organs e.g. giraffe's neck

Inheritance of aquired characteristics - all characteristics preserved by reproduction to new individuals

Mendel - bred pea plants for inheritance

Gene - unit of inheritance often responsible for one trait

Allele - alternative states of a 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 others

Recessibe - allele trait expressed if homozygous

Phenotype - expression of character in organism

Geneotype - allelic hereditary constitution

• P1 - parental generation
• F1 - first filial generation, progeny from parental cross
• F2 - second filial generation, progeny from F1 cross

Mendel selected paired characteristics all inherited in dominant/recessibe fashion

Advantages of pea plants to study inheritance:

• Several discrete traits
• Self-fertilising
• Numberous viable and fertile progeny
• Short generation time
• Crosses between different individuals by emasculation of anthers before pollen is ripe and transfer pollen from alternate parent

Mendel started w/ two pure breeding (homozygous) parents

Discrete phenotype/traits:

• Seed shape/colour
• Flower colour/position
• Stem height
• Pea pod shape/colour

Unit factors in pairs (1 loci w/ two allels) - for each character or locus, organism inherits 2 alleles (1 from each parent) 

Alternative versions of alleles account for variations in inherited characters caused by mutations in DNA code

Cross involving 1 trait called monohybrid cross

Pure-breed homozygous tall stemmed plants with a short stem producing variety - resulting seed produced tall plants (heterozygous dominant)

Dominance/recessive:

If 2 alleles on gene locus differ, then dominant one determines appearance while other has no noticable effect

Depending on traits, uniform feature is either one or parent's traits or intermediate

Postulate = not always true (e.g. codominance can occur etc)

Complete dominance = recessive allele in heterozygote not expressed

F1 phenotype same as dominant parent

Parental Generation (P) - homozygous dominant x homoygous recessive

Offspring (F1) - heterozygous,dominant phenotype

Cross (F2) - 1:2:1 genotypic ratio, 3;1 phenotypic ratio

Segregation

At gamete formation, 2 alleles segregate randomly

If homozygous alleles identical so so are gametes

If heterozygous 50% gametes contain 1 allele type and 50% other

Mendel's contribution unique because of methodical approach to problem and application of stats

• F1 offspring showed 1 of 2 parental traits and always the same traits
• Results always same regardless of which parent donated pollen
• Trait not show in F1 appear in F2 in 25% of offspring
• Traits remained unchanged when passed to offspring - no blending
• Recipricol crosses showed each parent made equal contribution to offspring

Mendel's conclusions:

• Recessive traits - factors could be hidden/unexpressed
• Phenotype - outward appearance of trait
• Genotype - genetic makeup of organism
• Male and females contributed equally to offspring's genetic makeup - simplest solution is that number of traits probably 2
• Factors must segregate from each other during gamete formation
• Upper case letters for dominance, lower case letters for recessives

Test complete dominance by crossing F2 back to recessive parent

Alleles from fully recessive will no influence phenotype

Progny phenotype reflects the diffrent genotype of unknown parent's alleles

Dominant - All preogeny heterozygous dominant

Heterzygous dominant - 50% progeny dominant phenotype 50% recessive phenotype

Incomplete dominance - form of intermediate inheritance where 1 allele for specific trait not completely expressed over paired alleles = 3rd phenotype which is combination/intermediate of both allele phenotypes 

Intermediate phenotype cannot breed true - if bred with other intermediate will give 1:2:1 ratio for homoDOM:heteroDOM:homoRECESS

Example - red (RR) flower with white (rr) gives pink (Rr) flowers

Example - Andalusian fowl chickens. blue x blue (intermediate phenotype) crosses will produce 25% blacks, 50% blues and 25% splash birds 

Codominance - where neither alleles is dominant or recessive = both get expressed in phenotype (effects additive)

Example - blood groupings

• Three alleles IA, IB, i
  • A and B both dominant to i
  • A and B codominant
• A and B alleles present when both produce own specific A and B antigens. Both fully expressed, effects additive, do not result in intermediate expressive

Lethal alleles - alleles that cause the death of the organism carrying them (mutations in genes essential to growth/development)

Always have dominant nomenclature while non-lethal alleles have recessive ones

Dihybrid inheritance - involved 2 genes on different chromosomes each with two alleles

When Mendel considered 2 traits per cross, he started with true-breeding plants - short w/ yellow pods, and crossed with true-breeding plants - tall w/ green pods

Seeds in F1 all tall w/ green points - expressed all dominant traits of each allele

• Tall plants dominant over short plants - T locus is on large chromosome
• Green pods dominant over yellow pods - G locus on small chromsome

Resulted in Mendel's 4th postulate (and second law)

Genes at different loci on different chromosomes segregate independently

Other examples include corn kernels and snake skin colour.

Test cross w/ fully recessive for dihybrid inheritance would get:

• 4 different genotypes
• 4 different phenotypes