Mendel and the Gene Idea

1
Chapter 14

• Mendel and the Gene Idea

2
Differences Among Individuals

• diet and environment
• genetic inheritance

3
Diet/Environment.Genetic

• fractures .x..
• sunburn x..
• PKU x..
• heart disease x..
• Tay-Sachs ...x
• Down syndrome..x

4
Multifactorial Inheritance

• Genotype does not rigidly define the phenotype,
  but a range of possibilities over which the
  environment can influence
• genes environment phenotype

5
Early Ideas about Heredity

• Knight and others
• basic facts of heredity
• studied garden pea plant
• formed hybrids
• found recessive traits in some offspring

6
Gregor Mendel

• father of genetics
• mentors - physics, botany
• quantified results
• subject - garden pea plant

7
Mendels Experimental Design

• 1. true-breeding plants
• (self-pollination)
• 2. crossed alternative traits to produce
  hybrids (hybrids expressed only dominant
  traits)
• 3. hybrids were self-pollinated (produced 31
  ratio)

8
Symbols

• P parental generation
• F1 first filial generation
• F2 second filial generation

9
Terminology

• homozygous RR, rr
• homozygous dom RR
• homozygous rec rr
• heterozygous Rr
• genotype and phenotype
• complete dominance

10
Monohybrid Cross

• parental varieties differ in a single character
• character flower color (P)
• allelles purple, dom (PP,Pp)
  
  white, rec
  (pp)

11
Monohybrid Cross

• P purple white
• true-breeding X true-breeding dominant
  recessive
• F1 offspring are all purple
• (then self-pollinate to produce F2)
• F2 offspring show 31
• (3 purple 1 white)

12
Monohybrid Cross Ratio in F2

• phenotypic ratio 31
• 3 dom 1 rec
• PP Pp are indistinguishable in complete
  dominance
• genotypic ratio 121

13
Monohybrid Cross Problem

• Eyebrows
• P not connected x connected
• (homo. dom) (homo. rec)
• determine phenotypic ratios for
• F1 F2
• determine genotypic ratios for
• F1 F2

14
Testcross

• to determine unknown genotype
• utilizes a homozygous recessive
• phenotypic ratio of offspring determines genotype

15
Mendels First LawLaw of Segregation

• allele pairs separate during gametogenesis then
  randomly reform pairs at fertilization

16
Dihybrid Cross

• parental varieties differ in two characters
• characters
• seed shape (R)
• seed color (y)

17
Dihybrid Cross, cont.

• seed shape alleles
• round dom (RR, Rr)
• wrinkled rec (rr)
• seed color alleles
• yellow dom (YY, Yy)
• green rec (yy)

18
Dihybrid Cross

• P round, yellow wrinkled, green
• true-breeding X true-breeding dominant
  recessive
• F1 offspring
• (self-pollinate to produce F2)
• F2 offspring (9331)

19
Dihybrid Cross Ratio in F2

• phenotypic ratio 9331
• 9 dom, dom R_Y_
• 3 dom, rec R_yy
• 3 rec, dom rrY_
• 1 rec, rec rryy

20
Dihybrid Cross Problem

• Dimples dom, freckles dom
• P dimples, freckles x none
• (homozygous) (homozy.)
• determine phenotypic ratios for
• F1 F2
• determine genotypic ratios for
• F1

21
Dihybrid Cross, Budgies

• Melopsittacus undulatus
• Characters for plumage color
• outer zone color (Y)
• core color (B)

22
Budgies cont.

• outer zone alleles
• yellow dom (YY,Yy)
• no pigment rec (yy)
• core zone alleles
• melanin dom (BB,Bb)
• no melanin rec (bb)

23
Budgies, dihybrid cross

• P
• true-breeding X true-breeding dominant
  recessive
• determine F1 and F2

24
Budgies, dihybrid cross cont.

• P YYBB x yybb
• green white
• F1 all YyBb, all green
• F2
• 9 Y_B_ 3Y_bb 3yyB_ 1yybb
• green yellow blue white

25
Dihybrid Cross Ratio in F2

• phenotypic ratio 9331
• 9 dom, dom Y_B_
• 3 dom, rec Y_bb
• 3 rec, dom yyB_
• 1 rec, rec yybb

26
Mendels Second LawLaw of Independent Assortment

• each allele pair segregates independently during
  gamete formation
• true when genes are located on different pairs
  of homologous chromosomes

27
Patterns of Inheritance not described by Mendel

• incomplete dominance
• codominance
• pleiotropy
• epistasis
• polygenic inheritance

28
Incomplete Dominance

• intermediate phenotype
• examples
• snapdragons
• hypercholesterolemia

29
Codominance

• Both alleles are expressed
• examples
• Landsteiner blood groups
• (A x B AB)
• shorthorn cattle
• (white x red roan)
• calico cat

30
Pleiotropy

• single gene has multiple effects
• example
• in Siamese cats, one gene causes abnormal fur
  pigmentation and esophoria (crossed-eyed)

31
Epistasis

• one gene alters the expression of another gene
  that is independently inherited
• example
• albinism in animals (mice)
• Indian corn

32
Polygenic Inheritance

• An additive effect of 2 or more genes on one
  character
• example
• skin pigmentation
• hair color

33
Chapter 13

• The End