CHAPTER 5 : INHERITANCE

5.1 Concept of inheritance based on Mendels experiment

GeneticsStudy of genes, heredity, and variation in living organismsBased on experiments conducted by Gregor Johann Mendel

Gregor Johann Mendel

Gregor MendelConducted experiments on pea plants Pisum sativum

Why Pisum Sativum? It is easy to growLife cycle is shortPollination of this plant can be easily controlledTraits of this plant can be easily observed

As a result, Mendel made a statement:

A certain trait that is determined by internal factors that exist in pair of these factors, only one factor (now known as allele) is carried in every gamete

This statement is called Mendels 1st Law / Law of segregation

3:1 : Referred as Mendels Monohybrid Inheritance Ratio

GENETICS VOCABULARY

Gene & Alleles

GENESA stretch of DNA / RNA that determine a certain traits Units of inheritance that are passed on to the next generationEvery gene situated in specific position referred as locus

ALLELESGenes found at the same locus on different homologous chromosomes are alleles of one anotherGenes that are alleles of one another always control the same characteristic

Phenotype & Genotype

Phenotypeexpressed trait of an organismcharacteristics that are visible, detectable or measurable

Phenotypecan & may change throughout life due to environmental influencesE.g. HairHeightBody weight

Genotypegenetic composition of all the cellsdepends on the inheritance of genes from its parents

Phenotype = genotype + environment

E.g.Even though a child has inherited the genes for a big body size (genotype) form his parents, his growth might be stunted (phenotype) if he does not get sufficient & balanced food (environment)

DOMINANT & RECESSIVE GENE

Dominant genegene that can show the phenotypic expression for a trait that is under its control in either a homozygous / heterozygous state

Recessive Genescan only show its phenotypic expression for a particular trait that is under its control in a homozygous state

Genotypic combinationPhenotype (hair texture)CC (homozygous)Curly hairCc (heterozygous)Curly haircc (homozygous)Straight hair

MENDELS FIRST LAWMONOHYBRID INHERITANCE

Mendels First LawLaw of segregation

used pure-breeding pea plants

PurplePPWhitepp

Pure Breeding

plants that carries 2 same alleles for a particular traitThese genes are passed on from the parent to the progeny through self-fertilization onlyPurplePPWhitepp

Explanation:The inheritance of an organism is determined by genes that exist in pairsDuring the formation of gametes, every gamete only brings with it one factor from the factorsWhen to alternate for one factor exist in an organism, only the dominant factor will be seen as phenotype

Punnett squareMale gamete

Female gameteRrRRR (round)Rr (round)rRr ( round)rr (wrinkled)

Exercise 1

Figure shows the inheritance of height in plants.If T represents the dominant allele for tallness & t the recessive allele for dwarfness, complete the information given in the figure.If the F1, generation plants are self-pollinated, draw a genetic diagram to find out the percentage of the offspring of F2 generation that would be a tall plant

What is the probability of Rr X Rr producing wrinkled seed?What is the probability of Yy X yy producing green seed?

Mendelian traits in humans

NON MENDELIAN TRAITS IN HUMAN

THINKING TIME !!Is it possible for a brown-eyed parents to have a blue-eyed child??

MENDELS SECOND LAWDihybrid Inheritance

Crossing of Tall Pea Plants which have round seeds with dwarf pea plants which have wrinkled seeds

Gamete

GameteTRTrtRtrTRTTRR(tall, round)TTRr(Tall,round)TtRR(tall,round)TtRr(tall,round)TrTTRr(Tall,round)TTrr(tall,wrinkled)TtRr(tall,round)Ttrr(Tall,wrinkled)tRTtRR(Tall,round)TtRr(tall,round)ttRR(Dwarf, round)ttRr(dwarf,round)trTtRr(tall,round)Ttrr(Tall,wrinkled)ttRr(dwarf,round)ttrr(dwarf,wrinkled)

PhenotypeTall/roundTall/wrinkledDwarf/roundDwarf/wrinkledGenotype(Total Number)TTRR (1)TTRr (2)TtRR (2)TtRr (4)TTrr (1)Ttrr (2)ttRR (1)ttRr (2)Ttrr (1)Fraction9/163/163/161/16F2 Phenotype ratio9331

Mendels Second Law= Law of independent assortmentduring gamete formation, each member of a pair of alleles may combine randomly with either member of another pair of alleles

Exercise 1

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