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- A specific version of a gene
- Diff alleles may code for diff forms of same protein → result in different characteristics


Law of segregation

- Alleles will separate during meiosis
→ Each gamete will receive one allele


Models of inheritance: particulate inheritance

- offspring are a combination of both parents and that the characteristics of parents are passed on as a separate entities
- parents pass on discrete heritable units (genes) that retain their separate identities in thei r==r children
- particulate inheritance maintains variation; genes w/ diff alleles that are present in each parent are recombined and passed on


Gregor Mendel - Reason studying pea plants

1. Other research showed that pea hybrids could be produced
2. many pea varieties were available
3. peas are small plants and easy to grow
4. peas can self-fertilise or be cross-fertilised


Meldel- experimental method

1. produce TRUE-BREEDING STRAINS for each trait he was studying
2. cross-fertilise true breeding strains having alternate forms of a trait → also perform RECIPROCAL CROSSES
3. allows hybrid offspring to self fertilise for several gens & count no. of offspring showing each form of the trait


monohybrid crosses

- cross to study only 2 variations of a single trait
- mendel produced true-breeding pea strains for 7 diff traits
→ flower colour, seed colour, seed texture, pod colour, pod shape, flower position, plant height


F1 Gen

- first filial gen
- offspring produced by crossing 2 true-breeding strains
- for every trait mendel studied, all F1 plants resembled only 1 parent
→ reffered to this trait as dominant
→ alternative trait was recessive
- No plants w/ characteristive immediate betw 2 parents were produced


F2 gen

- second filial gen
- offspring resulting from the self-fertilisation of F1 plants
- although hidden in the F1 gen, recessive trait had reappeared among some F2 individuals
- counted proportions of traits
→ 3:1


3:1 , really 1:2:1

F2 plants
- 3/4 plants w/ dominant
- 1/4 plants w/ recessive
-ratio is actually:
→1 true-breeding dom plant
→ 2 not-true-breeding dom plant
→ 1 true-breeding recessive plant


Principles of segregation

1. parents transmit discrete factors (genes) to their offspring
2. each individual receives onecope of a gene from each parent (relates to gametes being haploid)
3. Not all copies of gene are identicle
4. allenes remain discrete -no blending
5. presence of alleles does not guarantee expression



- cond. : pigment melanin is not produced
- pedigree for form of albinism due to nonfunctional allele of enzyme tyrosinase
- M & F affected equally
- most affected individuals have unaffected parents


mendel - extensions

- Mendel's model of inheritance assumes that
→ each trait is controlled by a single gene
→ each gene has only 2 alleles
→clear dom-reces relationship betw alleles


multiple alleles

- may be more than 2 alleles for a gene in a population
- ABO blood types in humans (3 alleles)
- each individual can only have 2 alleles
- no. of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an out breeding population


Polygenic inheritance

- when multiple genes are involved in controlling the phenotype of a trait
- phenotype is an accumulation of contributions by multiple genes
- traits show continuous variation & referred to as quantitative traits
→ eg. human hright


Environmental influence

- coat colour in himalayan rabbits and siamese cats
→ allele produces an enzyme that allows pigment production only at temp below 30 degC


incomplete dominance

heterozygote is intermediate inphenotype betw 2 homozygotes
- red x white = pink


Blood type A

Sugar- galactosamine


Blood type B

sugar- Galactose


Bloodtype AB

both galactose and galactosamine


Blood type O

no sugar