Mendelian Genetics_16_ by pptfiles


									Mendelian Genetics
Chapter 2

Phenotype and Genotype

Genotype and Phenotype
• Genotype – genetic constitution of an organism • Phenotype – observable characteristic
• Genotype and environment
• Contribution of environment varies between genes

• Can be controlled by many genes • Random developmental events

Mendel’s Experimental Design

Mendelian Genetics
• Modern genetics began with Gregor Mendel’s quantitative genetic experiments • Austrian monk • Mathematician • Numerical and observational data • Several generations

Stamen Carpel

Mendelian Genetics
• Heritable, obvious traits • Simple crosses at first • Used peas because:
• Easy to grow and available • Many distinguishable characteristics • Self-fertilization

• True breeding peas

Pea Traits

Monohybrid Crosses and Mendel’s Principle of Segregation

Breeding Crosses
• Initial cross is the P generation
• Parents

• Progeny of parents is first filial generation
• F1 generation

• Inbreeding of first generation creates second filial generation
• F2 generation

Monohybrid Crosses
• Cross between truebreeding individuals with one different trait • Mendel’s first crosses • Resembled only one of the parents • Planted progeny and allowed self-fertilization
• Revealed both phenotypes

Monohybrid Cross
• Mendel determined that
• Particulate factors for genes, each contains a set of two • Transmitted by both parents • Alternate forms called alleles • True breeding forms contains identical set


All P

All p

F1 PLANTS (hybrids) Gametes
1/ 2

All Pp


1/ 2


Eggs F2 PLANTS p



Sperm p

Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp

P p p p

P p

Monohybrid Cross
• F1 generation had both alleles • Only one expresses • One allele masks
• Dominant • Recessive

• Identical alleles – homozygous • Different alleles - heterozygous

Monohybrid Cross

Principle of Segregation
• Recessive characteristics are masked
• Reappear in F2

• Members of a gene pair (alleles) segregated during gamete formation

How cells carry characteristics
• Genes on chromosomes
• At a specific loci

• Homologous pairs carry the same genes at the same locus
• Different versions

• Separation of homologous chromosomes yields separation of alleles

Branch Diagrams
• Punnett squares can become messy with more than one gene • Use branch diagram to figure out genotype and phenotype expected frequency

Test Cross
• Mendel did several crosses
• Followed over several generations




• Selfing also very important
• Allowed plants to reveal their genotype and not just their phenotye

Two possibilities for the black dog:
BB or Bb










All black

1 black : 1 chocolate

Test Cross

Recessive Alleles
• Wild-type allele – functional allele • Predominates in population
• Dominant allele

• Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional
• Recessive • Function of other in heterozygote is sufficient

Wrinkled Peas
• SS type contains more starch and lower sucrose
• Also more water • SBEI - starchbranching enzyme • Extra 800 bp piece in mutation

Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment

The Principle of Independent Assortment
• Factors for different traits assort independently of one another
• Genes are inherited independently of each other • Segregate randomly in gametes

• Dihybrid Cross

Branch Diagram of Dihybrid Cross
P h e n o t y p e

Genotype vs. Phenotype

Test Cross With Dihybrid

Trihybrid Cross

Tribble Traits Activity

Statistical Analysis of Genetic Data: The Chi-Square Test

Statistical Analysis
• Data from genetics is quantitative • Use statistics to show deviation of observed results from predicted results
• Chance factors cause deviations

• Null-hypothesis – no difference between the predicted and observed
• If not accepted then have to come up with a new hypothesis for deviation

Chi-Square Test
• Goodness of fit test
• How much observed number deviates from the expected number

Mendelian Genetics in Humans

Pedigree Analysis
• Inheritance patterns are studied using family trees
• Pedigree analysis • Phenotypic records
• Proband is where gene was discovered

Examples of Human Genetic Traits

Most genetic disorders are recessive
Due to lack of function


Homozygous recessive expression
Dominant usually selected out



Characteristics of Recessive Inheritance Traits
• Most have normal heterozygous parents • Heterozygotes have 3:1 ratio • When both parents have the trait then all progeny have the trait • Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs

Characteristics of Dominant Inheritance Traits
• Gain of function mutations
• New property of the mutant gene • No loss of function

• Must have one parent with disease • Does not skip generations • Will transmit to half its progeny
• Huntingtons disease, Marfan syndrome, achondroplasia

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