Chapter 11 -- Basic Genetics. Inheritance Gregor Mendel 1822-1884.
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Transcript of Chapter 11 -- Basic Genetics. Inheritance Gregor Mendel 1822-1884.
Chapter 11 -- Basic Genetics
Inheritance
Gregor Mendel
1822-1884
Mendel’s pea garden in Brno
Verhandlungen des naturforschenden Vereines in Brünn. Brünn: Vereines, 1865.
Mendel’s paper
Why did Mendel choose peas?
Mendel’s Peas
Two Characters
Seed Coat
Flower color
Two traits per character
Rough
Smooth
Two traits per character
Purple flowers
White flowers
Traits are alternative forms of characters
white or purple
smooth or rough
“Brother Mendel! We grow tired of peas!”
Controlling the mating
No question as to parentage of offspring
Compare
• 1. Start with true breeding plants.
• 2. Cross alternative forms among these true-breeders.
Mendel’s experimental design
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants hadpurple flowers
• 3. Allowed progeny (F1) to self-fertilize and examined offspring
• 4. Count the number of offspring of each type
Mendel’s experimental design
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants hadpurple flowers
• Monohybrid crosses: examining one trait at a time
What Mendel found-monohybrid crosses
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants hadpurple flowers
Monohybrid Cross
What Mendel found F1 generation
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
Purpleflowers
Whiteflowers
All plants hadpurple flowers
P Generation
(true-breedingparents)
F1 Generation
(hybrids)
F2 Generation
Purpleflowers
Whiteflowers
All plants hadpurple flowers
F1 x F1
F1 x F1
F2: purple and white
705 purple flowered plants
224 white flowered plants
3:1 ratio
What Mendel found F2 generation
F1 Generation
(hybrids)All plants hadpurple flowers
F2 Generation
705 purple
224 white
3:1 ratio
Mendel’s 7 pea traits
1. Alternative versions of genes lead to variations in inherited characters. Alternative versions are called alleles
2. Each individual receives 2 alleles.– homozygotes: 2 copies of identical allele– heterozygotes: two different alleles
Mendel’s model
3. In heterozygotes only the dominant form of allele will be expressed.– genotype: alleles present– phenotype: physical appearance
4. Alleles segregate from one another into gametes (Law of segregation)
Mendel’s model
Allele for purple flowers
Homologouspair ofchromosomes
Allele for white flowers
Locus for flower-color gene
Chromosomes, genes and alleles ……..
Genes control characters
e.g., flower color
Genes control characters
Traits are alternative forms of characters
e.g., white or purple
Traits are alternative forms of characters
Alleles are alternative forms of a gene
Genes control characters
Human Chromosomes
Genes
Alleles
and
Variation within a population
• Capital letter for dominant allele; lower case letter for recessive allele
Example flower color:
P = dominant allele
p = recessive allele
Mendel used symbols to interpret results
Phenotype: purple
Genotype: PP or Pp
Phenotype: white
Genotype: pp
Phenotype
Purple
Purple3
Purple
Genotype
PP(homozygous
Pp(heterozygous
Pp(heterozygous
pp(homozygous
1
2
1
Ratio 1:2:1
White
Ratio 3:1
1
Cross plant with unknown genotype with homozygous recessive
The testcross
The Punnett square
Reginald C. Punnett
PP x ppp p
P
P
Pp Pp
Pp Pp
Dominant phenotype,unknown genotype:
PP or Pp?
If PP,then all offspring
purple:
p p
P
P
Pp Pp
Pp Pp
If Pp,then 1
2 offspring purpleand 1
2 offspring white:
p p
P
Ppp pp
Pp Pp
Recessive phenotype,known genotype:
pp
The testcross
Different types of Inheritance
Dominant - Recessive
purplePp
PP
white pp
Incomplete Dominance
Incomplete dominanceWhen alleles are not fully dominant or
fully recessive in heterozygotes
Multiple Alleles: ABO Blood Group
Codominance No single allele is dominantEach allele has its own effect.
Codominance
• Type A: adds galactosamine– IAIA, IAi
• Type B: adds galactose– IBIB, IBi
• Type AB: adds galactosamine and galactose– IAIB
• Type O: no sugar added– ii
Multiple Alleles: ABO Blood Group
ABO Blood Groups
Pleiotropic EffectsOne allele have multiple effects
Environmental effects
THE CHROMOSOMAL BASIS OF INHERITANCE
Chromosomes and Mendelian Inheritance
Allele for purple flowers
Homologouspair ofchromosomes
Allele for white flowers
Locus for flower-color gene
Chromosomes, genes and alleles ……..
and loci
Thomas Hunt Morgan (1866 – 1945)
Drosophila
Human Chromosomes
Sex Linkage
XXfemale
XY
male
Human sex chromosomes
Color blindness
Sex Linked traitsMale pattern baldness
Chromosomal problems
Chromosomal problems
Klinefelter syndrone
Recessive genes can be carried without being expressed
Queen Victoria and hemophilia
Sickle cell anemia