Chapter Basic Genetics

Inheritance

Gregor Mendel

Mendels pea garden in Brno

Verhandlungen des naturforschenden Vereines in Brünn. Brünn: Vereines, Mendels paper

Why did Mendel choose peas?

Mendels 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. Mendels experimental design

P Generation (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers

3. Allowed progeny (F1) to self-fertilize and examined offspring 4. Count the number of offspring of each type Mendels experimental design

P Generation (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers

Monohybrid crosses: examining one trait at a time What Mendel found- monohybrid crosses

P Generation (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers Monohybrid Cross

What Mendel found F1 generation P Generation (true-breeding parents) F 1 Generation (hybrids) Purple flowers White flowers All plants had purple flowers

P Generation (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers F1 x F1

F2: purple and white 705 purple flowered plants 224 white flowered plants 3:1 ratio What Mendel found F2 generation

F 1 Generation (hybrids) All plants had purple flowers F2 Generation 705 purple 224 white 3:1 ratio

Mendels 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 Mendels 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) Mendels model

Allele for purple flowers Homologous pair of chromosomes 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 3 Genotype PP (homozygous Pp (heterozygous Pp (heterozygous pp (homozygous 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 pp p p P P Pp

Dominant phenotype, unknown genotype: PP or Pp? If PP, then all offspring purple: pp P P Pp If Pp, then 1 2 offspring purple and 1 2 offspring white: pp P P pp Pp Recessive phenotype, known genotype: pp The testcross

Different types of Inheritance

Dominant - Recessive

purple Pp PP white pp

Incomplete Dominance

Incomplete dominance When alleles are not fully dominant or fully recessive in heterozygotes

Multiple Alleles: ABO Blood Group Codominance No single allele is dominant Each allele has its own effect.

Codominance

Type A: adds galactosamine –I A I A, I A i Type B: adds galactose –I B I B, I B i Type AB: adds galactosamine and galactose –I A I B Type O: no sugar added –ii Multiple Alleles: ABO Blood Group

ABO Blood Groups

Pleiotropic Effects One allele have multiple effects

Environmental effects

THE CHROMOSOMAL BASIS OF INHERITANCE

Chromosomes and Mendelian Inheritance

Allele for purple flowers Homologous pair of chromosomes 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 XX female XY male

Human sex chromosomes

Color blindness Sex Linked traits Male pattern baldness

Chromosomal problems

Klinefelter syndrone

Recessive genes can be carried without being expressed

Queen Victoria and hemophilia

Sickle cell anemia