Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

​ ​PRINCIPLES​ ​OF​ ​INHERITANCE​ ​AND​ ​VARIATION
Genetics
Genetics​ ​is​ ​a​ ​branch​ ​of​ ​biology​ ​dealing​ ​with​ ​inheritance​ ​and​ ​variation
of​ ​characters​ ​from​ ​parents​ ​of​ ​offspring
...


● He​ ​used​ ​garden​ ​pea​ ​as​ ​his​ ​sample
...


● Garden​ ​pea​ ​plant​ ​possessed​ ​certain​ ​completely​ ​opposite​ ​traits
...
​ ​No
...


● Fourteen​ ​true​ ​breeding​ ​pea​ ​lines​ ​were​ ​selected​ ​as​ ​pairs,​ ​which​ ​were
similar​ ​except​ ​for​ ​one​ ​character​ ​with​ ​contrasting​ ​traits
...


Inheritance​ ​of​ ​One​ ​Gene
● After​ ​hybridisation,​ ​the​ ​F​1​​ ​generation​ ​so​ ​obtained​ ​resembled​ ​only​ ​one
of​ ​its​ ​parents​ ​(say,​ ​all​ ​tall;​ ​no​ ​dwarf)
...


● Revival​ ​of​ ​unexpressed​ ​trait​ ​(dwarf)​ ​was​ ​observed​ ​in​ ​some​ ​F​2​​ ​progeny
...


● Mendel​ ​proposed​ ​that​ ​something​ ​is​ ​being​ ​passed​ ​unchanged​ ​from

generation​ ​to​ ​generation
...


● Factors​ ​contain​ ​and​ ​carry​ ​hereditary​ ​information
...
​ ​(e
...
,​ ​tall​ ​and​ ​dwarf)
Monohybrid​ ​Cross
● Cross​ ​that​ ​considers​ ​only​ ​a​ ​single​ ​character​ ​(e
...
,​ ​height​ ​of​ ​the​ ​part)

●
● Studying​ ​the​ ​cross:

○ TT,​ ​tt,​ ​and​ ​Tt​ ​are​ ​genotypes​ ​while​ ​the​ ​traits,​ ​tall​ ​and​ ​dwarf,​ ​are
phenotypes
...


○ Even​ ​if​ ​a​ ​single​ ​‘T’​ ​is​ ​present​ ​in​ ​the​ ​genotype,​ ​phenotype​ ​is​ ​‘tall’
...
​ ​Therefore,​ ​T​ ​(for​ ​tallness)​ ​is
dominant​ ​trait​ ​while​ ​t​ ​(for​ ​dwarfness)​ ​is​ ​recessive​ ​trait
...


○ From​ ​the​ ​cross,​ ​it​ ​can​ ​be​ ​found​ ​that​ ​alleles​ ​of​ ​parental​ ​pair

separate​ ​or​ ​segregate​ ​from​ ​each​ ​other​ ​and​ ​only​ ​one​ ​allele​ ​is
transmitted​ ​to​ ​the​ ​gamete
...


● Punnett​ ​square

○ Graphical​ ​representation​ ​to​ ​calculate​ ​the​ ​probability​ ​of​ ​all
possible​ ​genotypes​ ​of​ ​offsprings​ ​in​ ​a​ ​genetic​ ​cross

○ Possible​ ​gametes​ ​are​ ​written​ ​on​ ​two​ ​sides,​ ​usually​ ​at​ ​top​ ​row

and​ ​left​ ​columns,​ ​and​ ​combinations​ ​are​ ​represented​ ​in​ ​boxes
...
​ ​From​ ​the​ ​above​ ​given​ ​Punnet​ ​square,​ ​it​ ​is​ ​evident
that​ ​genotypic​ ​ratio​ ​TT:​ ​Tt:​ ​tt​ ​is​ ​1:2:1
...

​

○ Gamete-bearing​ ​genes​ ​are​ ​in​ ​equal​ ​frequency​ ​of​ ​


...


Law​ ​of​ ​Dominance
● According​ ​to​ ​this​ ​law,​ ​characters​ ​are​ ​controlled​ ​by​ ​discrete​ ​units​ ​called
factors,​ ​which​ ​occur​ ​in​ ​pairs​ ​with​ ​one​ ​member​ ​of​ ​the​ ​pair​ ​dominating
over​ ​the​ ​other​ ​in​ ​a​ ​dissimilar​ ​pair
...

​

Test​ ​Cross
● Cross​ ​between​ ​F​2​​ ​progeny​ ​and​ ​its​ ​homozygous​ ​recessive​ ​parent

● This​ ​cross​ ​determines​ ​whether​ ​the​ ​dominant​ ​character​ ​is​ ​coming​ ​from
homozygous​ ​dominant​ ​genotype​ ​or​ ​heterozygous​ ​genotype
...
g
...
​ ​Whereas​ ​when​ ​Tt​ ​is​ ​crossed​ ​with​ ​tt,​ ​we​ ​obtain​ ​Tt​ ​(tall)​ ​and​ ​tt
(dwarf)​ ​individuals​ ​in​ ​the​ ​progeny
...
​ ​We​ ​obtain​ ​both​ ​tall​ ​and​ ​dwarf​ ​varieties​ ​in​ ​test
cross,​ ​if​ ​tallness​ ​is​ ​coming​ ​from​ ​Tt
...


● In​ ​homozygous​ ​parents,​ ​all​ ​gametes​ ​produced​ ​are​ ​similar;​ ​while​ ​in
heterozygous​ ​parents,​ ​two​ ​kinds​ ​of​ ​gametes​ ​are​ ​produced​ ​in​ ​equal
proportions
...


● Example​ ​−​ ​Flower​ ​colour​ ​in​ ​dog​ ​flower​ ​(snapdragon),​ ​where:
○ RR​ ​−​ ​Red​ ​flowers

○ rr​ ​−​ ​White​ ​flowers
○ Rr​ ​−​ ​Pink​ ​flowers

● Here,​ ​genotypic​ ​ratio​ ​remains​ ​same​ ​as​ ​in​ ​Mendelian​ ​crosses,​ ​but

phenotypic​ ​ratio​ ​changes​ ​since​ ​complete​ ​dominance​ ​is​ ​not​ ​shown​ ​by​ ​R
(hence,​ ​incomplete​ ​dominance)
...


● Suppose​ ​a​ ​normal​ ​gene​ ​produces​ ​a​ ​product​ ​P
...


● The​ ​altered​ ​version​ ​of​ ​the​ ​gene​ ​must​ ​not​ ​perform​ ​the​ ​functions​ ​that​ ​a
normal​ ​gene​ ​performs
...


● The​ ​original​ ​gene​ ​is​ ​said​ ​to​ ​be​ ​dominant​ ​while​ ​the​ ​modified​ ​gene​ ​is
recessive
...

● Example:​ ​ABO​ ​blood​ ​groups​ ​in​ ​human​ ​beings

● ABO​ ​blood​ ​groups​ ​are​ ​controlled​ ​by​ ​gene​ ​I​
...
​ ​A​ ​person​ ​possesses​ ​any​ ​two​ ​of​ ​the​ ​three​ ​alleles
...
​ ​But​ ​with​ ​each​ ​other,​ ​IA​​ ​and​ ​IB​​ ​are
​
​
​
​
co-dominant
...


Allele​ ​from

Allele​ ​from

Genotype​ ​of

Blood​ ​type​ ​of

I​ A
​

I​ A
​

I​ A​​ ​I​ A
​ ​

A

I​ A
​

I​ B
​

I​ A​​ ​I​ B
​ ​

AB

I​ A
​

i

I​ A​​ ​i
​

A

I​ B
​

I​ A
​

I​ A​​ ​I​ B
​ ​

AB

I​ B
​

I​ B
​

I​ B​​ ​I​ B
​ ​

B

I​ B
​

i

I​ B​​ ​i
​

B

i

i

i​ ​i

O

Parent​ ​1

Parent​ ​2

offspring

offspring

● Multiple​ ​alleles:​ ​When​ ​more​ ​than​ ​two​ ​alleles​ ​control​ ​a​ ​character,​ ​as​ ​in
human​ ​blood​ ​groups

○ Multiple​ ​alleles​ ​are​ ​used​ ​in​ ​population​ ​studies
...
​ ​(e
...
,​ ​seed​ ​colour​ ​and
seed​ ​shape)

● Yellow​ ​colour​ ​and​ ​round​ ​shape​ ​is​ ​dominant​ ​over​ ​green​ ​colour​ ​and
wrinkled​ ​shape
...


● In​ ​a​ ​dihybrid​ ​cross​ ​between​ ​two​ ​plants​ ​having​ ​round​ ​yellow​ ​(RRYY)

and​ ​wrinkled​ ​green​ ​seeds​ ​(rryy),​ ​four​ ​types​ ​of​ ​gametes​ ​(RY,​ ​Ry,​ ​rY,

ry)​ ​are​ ​produced
...

​Chromosomal​ ​Theory​ ​of​ ​Inheritance
Rediscovery​ ​of​ ​Mendel’s​ ​Work
● Mendel’s​ ​work​ ​remained​ ​unrecognised​ ​for​ ​several​ ​years​ ​because​ ​of​ ​the
following​ ​reasons
...


○ Mendel’s​ ​approach​ ​to​ ​explain​ ​biological​ ​phenomenon​ ​with​ ​the
help​ ​of​ ​mathematics​ ​was​ ​also​ ​not​ ​accepted
...


Chromosomal​ ​Theory​ ​of​ ​Inheritance
● By​ ​1900,​ ​due​ ​to​ ​the​ ​advancement​ ​in​ ​microscopy,​ ​chromosomes​ ​were
also​ ​discovered
...


● Chromosomes​ ​and​ ​genes​ ​both​ ​occur​ ​in​ ​pairs—two​ ​alleles​ ​of​ ​a​ ​gene
pair​ ​are​ ​located​ ​on​ ​homologous​ ​sites​ ​of​ ​homologous
chromosomes​
...


● Union​ ​of​ ​knowledge​ ​of​ ​chromosomal​ ​segregation​ ​with​ ​Mendelian
principles​ ​constitutes​ ​chromosomal​ ​theory​ ​of​ ​inheritance
...


● He​ ​worked​ ​on​ ​fruit​ ​flies,​ ​Drosophila​ ​melanogaster​
...

○ Their​ ​life​ ​cycle​ ​is​ ​complete​ ​in​ ​two​ ​weeks
...


○ Clear​ ​differentiation​ ​of​ ​sexes​ ​−​ ​Easily​ ​distinguishable​ ​male​ ​and
female

○ Hereditary​ ​variations​ ​clearly​ ​visible​ ​with​ ​low​ ​power​ ​microscopes

● Morgan’s​ ​experiment

○ Dihybrid​ ​cross​ ​was​ ​carried​ ​out​ ​on​ ​fruit​ ​flies
...


○ F​1​​ ​progeny​ ​was​ ​obtained,​ ​which​ ​were​ ​inter-crossed
...


○ F​2​​ ​ratio​ ​was​ ​observed​ ​to​ ​be​ ​significantly​ ​different​ ​from​ ​9:3:3:1
as​ ​observed​ ​in​ ​Mendelian​ ​dihybrid​ ​cross
...


○ When​ ​two​ ​genes​ ​are​ ​located​ ​on​ ​the​ ​same​ ​chromosome,​ ​the

proportions​ ​of​ ​parental​ ​gene​ ​combinations​ ​were​ ​much​ ​higher
than​ ​those​ ​of​ ​non-parental
...


● In​ ​this​ ​way,​ ​genetic​ ​maps​ ​were​ ​prepared,​ ​which​ ​are​ ​extensively​ ​used

today​ ​for​ ​genome​ ​sequencing​ ​projects​ ​as​ ​in​ ​human​ ​genome​ ​project
...
​ ​He​ ​observed​ ​specific​ ​nuclear
structures​ ​during​ ​spermatogenesis​ ​in​ ​insects
...


● He​ ​observed​ ​that​ ​after​ ​spermatogenesis,​ ​50%​ ​of​ ​the​ ​sperm​ ​obtained
these​ ​structures,​ ​while​ ​50%​ ​did​ ​not
...


● Chromosomes​ ​involved​ ​in​ ​sex​ ​determination​ ​are​ ​called​ ​sex

chromosomes,​ ​while​ ​the​ ​other​ ​chromosomes​ ​are​ ​called​ ​autosomes
...


○ Some​ ​sperms​ ​contain​ ​X​ ​chromosomes,​ ​while​ ​some​ ​do​ ​not
...
​ ​So,​ ​females​ ​have​ ​two​ ​X​ ​chromosomes
...
​ ​So,​ ​males​ ​have​ ​only​ ​one​ ​X​ ​chromosome
...
​ ​Hence,​ ​males​ ​are​ ​XY
...
​ ​Hence,​ ​females​ ​are​ ​XX
...


○ In​ ​XO​ ​type,​ ​some​ ​gametes​ ​have​ ​X​ ​chromosomes,​ ​while​ ​some
gametes​ ​are​ ​without​ ​X​ ​chromosomes
...


● Female​ ​heterogamety​ ​−​ ​ZW​ ​type​ ​of​ ​sex​ ​determination​ ​is​ ​an​ ​example
of​ ​female​ ​heterogamety
...


Mutation,​ ​Pedigree​ ​Analysis,​ ​&​ ​Genetic​ ​Disorders
Mutation
● Alteration​ ​of​ ​DNA​ ​sequence​ ​resulting​ ​in​ ​changes​ ​in​ ​genotype​ ​and
phenotype​ ​of​ ​organisms

● DNA​ ​helix​ ​runs​ ​in​ ​a​ ​chromatid,​ ​hence​ ​any​ ​change​ ​(insertion​ ​or
deletion)​ ​in​ ​the​ ​DNA​ ​sequence​ ​affects​ ​the​ ​chromosome
...


● A​ ​particular​ ​trait​ ​under​ ​study​ ​is​ ​represented​ ​in​ ​a​ ​family​ ​tree
...


● DNA​ ​is​ ​believed​ ​to​ ​be​ ​the​ ​carrier​ ​of​ ​genetic​ ​information,​ ​which​ ​passes
unaltered​ ​from​ ​generation​ ​to​ ​generation
...


● Standard​ ​symbols​ ​in​ ​pedigree​ ​analysis​ ​are​ ​as​ ​follows:

● Pedigree​ ​chart​ ​is​ ​represented​ ​as​ ​follows:

Chart​ ​(a)​ ​represents​ ​inheritance​ ​of​ ​an​ ​autosomal​ ​dominant​ ​trait​ ​as​ ​in
muscular​ ​dystrophy
...

Genetic​ ​Disorders
● Include​ ​Mendelian​ ​disorders​ ​and​ ​chromosomal​ ​disorders

Mendelian​ ​Disorders
● Characterized​ ​by​ ​mutation​ ​in​ ​a​ ​single​ ​gene

● Their​ ​mode​ ​of​ ​inheritance​ ​follows​ ​the​ ​principles​ ​of​ ​Mendelian​ ​genetics
...


○ In​ ​this​ ​disease,​ ​protein​ ​involved​ ​in​ ​blood​ ​clotting​ ​is​ ​affected
...


● Sickle​ ​cell​ ​anaemia

○ Autosomal​ ​recessive​ ​disease

○ Transmission​ ​−​ ​From​ ​parent​ ​to​ ​offspring​ ​when​ ​both​ ​parents​ ​are
carriers​ ​of​ ​disease

○ Pair​ ​of​ ​alleles​ ​Hb​A​​ ​and​ ​Hb​S​​ ​controls​ ​the​ ​expression​ ​of​ ​this
disease
...


○ The​ ​mutant​ ​haemoglobin​ ​so​ ​formed​ ​polymerises​ ​at​ ​low​ ​oxygen
tension,​ ​resulting​ ​in​ ​change​ ​in​ ​shape​ ​of​ ​RBC​ ​to​ ​sickle-like
...


○ Phenylalanine​ ​accumulates
...


Chromosomal​ ​Disorders
● Total​ ​number​ ​of​ ​chromosomes​ ​in​ ​humans​ ​=​ ​46​ ​(23​ ​pairs)

● Total​ ​23​ ​pairs​ ​=​ ​Autosomes​ ​(22​ ​pairs)​ ​+​ ​Sex​ ​chromosomes​ ​(1​ ​pair)
● Monosomy​ ​−​ ​Lack​ ​of​ ​any​ ​one​ ​pair​ ​of​ ​chromosomes

● Trisomy​ ​−​ ​Inclusion​ ​of​ ​an​ ​additional​ ​copy​ ​of​ ​chromosomes

● Aneuploidy​ ​−​ ​Loss​ ​or​ ​gain​ ​of​ ​chromosomes​ ​due​ ​to​ ​failure​ ​of
segregation​ ​of​ ​chromatids​ ​during​ ​cell​ ​division

Chromosomal​ ​Disorders

● Total​ ​number​ ​of​ ​chromosomes​ ​in​ ​humans​ ​=​ ​46​ ​(23​ ​pairs)

● Total​ ​23​ ​pairs​ ​=​ ​Autosomes​ ​(22​ ​pairs)​ ​+​ ​Sex​ ​chromosomes​ ​(1​ ​pair)
● Monosomy​ ​−​ ​Lack​ ​of​ ​any​ ​one​ ​pair​ ​of​ ​chromosomes

● Trisomy​ ​−​ ​Inclusion​ ​of​ ​an​ ​additional​ ​copy​ ​of​ ​chromosome

● Aneuploidy​ ​−​ ​Loss​ ​or​ ​gain​ ​of​ ​chromosomes​ ​due​ ​to​ ​the​ ​failure​ ​of
segregation​ ​of​ ​chromatids​ ​during​ ​cell​ ​division

● Down’s​ ​Syndrome

○ Cause:​ ​Presence​ ​of​ ​an​ ​additional​ ​copy​ ​of​ ​chromosome​ ​21
(Trisomy​ ​of​ ​21)

○ Affected​ ​individual​ ​has​ ​short​ ​stature,​ ​small,​ ​round​ ​head,
furrowed​ ​tongue,​ ​partially​ ​opened​ ​mouth,​ ​palm​ ​crease,
congenital​ ​heart​ ​disease​ ​and​ ​mental​ ​retardation
...
e
...
e
...


○ Affected​ ​females​ ​are​ ​sterile;​ ​have​ ​rudimentary​ ​ovaries;
secondary​ ​sexual​ ​characters​ ​are​ ​absent



---