MENDEL’S LAWMENDEL’S LAW

Gregor Mendel Gregor Mendel 1822-18841822-1884 An An Austrian scientistAustrian scientist

=research in a =research in a Czechoslovakian Czechoslovakian monastery.monastery.

Established the Established the basis basis of modern genetic of modern genetic science. science.

Objective = Objective = understand how understand how physical traits are physical traits are passed from a parent passed from a parent to its offspring. to its offspring.

                                   

Important conceptsImportant concepts

Dominance vs RecessivenessDominance vs RecessivenessPrinciple of Segregation. Principle of Segregation. Principle Independent Assortment Principle Independent Assortment

Monohybrid and Dihybrid crossMonohybrid and Dihybrid cross

Important TermsAlleles – Genetic factors that controls a trait, Dominant = ability to mask the expression of a recessive allele (Capital)

Recessive = masked by a dominant allele

Homozygous- 2 identical alleles (TT, tt)

Heterozygous- 2 different alleles (Tt)

Genotype- genes of an organism = alleles received during fertilization.  

Phenotype = the physical appearance of a trait in an organism . Eg blue eyes, yellow flower

EgsEgs

Genotype = genes present in an organism (usually abbreviated as two letters

TT- Homozygous Dominant (PURE) tt-Homozygous Recessive (PURE) Tt- Heterozygous

AllelesAlleles

Alternative forms of the same gene.  Eg….Eg…. Gene for hair texture (curly? Straight?)Gene for hair texture (curly? Straight?) One form of the hair texture gene codes One form of the hair texture gene codes

for curly hair.  A different code for of for curly hair.  A different code for of the the same genesame gene makes hair straight.  makes hair straight. 

= So the gene for hair texture exists as = So the gene for hair texture exists as two alleles --- one curly code, and one two alleles --- one curly code, and one straight code. straight code.

tall stem x short stem = (F1) ALL tall stem x short stem = (F1) ALL TALLTALL

yellow seeds x green seeds = (F1) yellow seeds x green seeds = (F1) ALL YELLOWALL YELLOW

round seeds x wrinkled seeds = round seeds x wrinkled seeds = (F1)ALL ROUND(F1)ALL ROUND

Monohybrid crossMonohybrid cross

Punnet squarePunnet square

Conclusion of Exp 1Conclusion of Exp 1

phenotypes of the offspring resembled only one phenotypes of the offspring resembled only one of the parent plants. of the parent plants.

alleles compete randomly for expression in the alleles compete randomly for expression in the offspring depending on Dom/Rec statusoffspring depending on Dom/Rec status

F1 = T x tt = All Tt . the tall factor seems to F1 = T x tt = All Tt . the tall factor seems to DOMINATE the short factor". DOMINATE the short factor".

F2 = The trait not shown in the F1 reappeared in F2 = The trait not shown in the F1 reappeared in the F2 in about 25% of the offspringthe F2 in about 25% of the offspring

Different offspringDifferent offspring of the same parents have of the same parents have different sets of hereditary informationdifferent sets of hereditary information

Mendels LawMendels LawThe Law of Segregation The Law of Segregation

pair of alleles of each pair of alleles of each parent separate and parent separate and only one allele passes only one allele passes from each parent on from each parent on to an offspring. to an offspring. 

Which allele in a Which allele in a parent's pair of alleles parent's pair of alleles is inherited is a matter is inherited is a matter of chance.  of chance. 

segregation of alleles segregation of alleles occurs during the occurs during the process of sex cell process of sex cell formation = MEIOSISformation = MEIOSIS

Mendel's Law of Independent AssortmentMendel's Law of Independent Assortment

Alleles for Alleles for differentdifferent traits are distributed to sex traits are distributed to sex cells (& offspring) independently of one another. cells (& offspring) independently of one another.

Conseq ; new combinations of genes present in Conseq ; new combinations of genes present in neither parent are possible. neither parent are possible. 

Eg:Eg:

two two differentdifferent traits: traits: (1) seed texture (round or wrinkled) & (1) seed texture (round or wrinkled) & (2) pod color (green or yellow). (2) pod color (green or yellow). 

The The differentdifferent traits do not influence the inheritance of each other.  They traits do not influence the inheritance of each other.  They are inherited INDEPENDENTLYare inherited INDEPENDENTLY

Yellow round, green wrinkled vs Yellow wrinkled, green roundYellow round, green wrinkled vs Yellow wrinkled, green round

where where "G" = dominant allele for yellow pods "G" = dominant allele for yellow pods "g" = recessive allele for green pods "g" = recessive allele for green pods

““W" = dominant allele for round seeds W" = dominant allele for round seeds “w" = recessive allele for wrinkled seeds “w" = recessive allele for wrinkled seeds ..

GGWW x ggww vs GGww x ggWWGGWW x ggww vs GGww x ggWW

Dihybrid crossDihybrid cross

9:3:3:19:3:3:1

Eg. 2Eg. 2

Purple round, White wrinkled Purple round, White wrinkled vs vs

Purple wrinkled, white roundPurple wrinkled, white round

Purple dominant (P),Purple dominant (P),

Round dominant (W), Round dominant (W),

Eg. 3Eg. 3

Red Tall x White short vs Red Tall x White short vs

Red short, White TallRed short, White Tall

Red dominant (R), Red dominant (R),

Tall dominant (T) Tall dominant (T)

PPWW x ppww vs PPww x ppWWPPWW x ppww vs PPww x ppWW

RRTT x rrtt vs RRtt x rrTTRRTT x rrtt vs RRtt x rrTT

SummarySummary

11stst Law = Dominance Law = DominanceParent TT x tt (tall x short)Parent TT x tt (tall x short)Offspring = All tallOffspring = All tall

22ndnd Law = Segregation Law = Segregation

Parent = Tt x Tt (tall x tall)Parent = Tt x Tt (tall x tall)

Offspring = 75% tall 25% short Offspring = 75% tall 25% short

33rdrd Law = Independent assortment. Law = Independent assortment.2 traits2 traitsParent = GgWw x GgWwParent = GgWw x GgWwOffspring Offspring 9/16 yellow pods & round seeds 9/16 yellow pods & round seeds

3/16 green pods & round seeds 3/16 green pods & round seeds 3/16 yellow pods & wrinkled seeds 3/16 yellow pods & wrinkled seeds 1/16 green pods & wrinkled seeds &1/16 green pods & wrinkled seeds &