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