Mendelian Genetics_16_

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					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
GENETIC MAKEUP (ALLELES) P PLANTS PP pp

Gametes

All P

All p

F1 PLANTS (hybrids) Gametes
1/ 2

All Pp

P

1/ 2

p

Eggs F2 PLANTS p

P
P P

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
TESTCROSS:

GENOTYPES

B_

bb

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

Two possibilities for the black dog:
BB or Bb

B

B

b

b

Bb

b

Bb

bb

OFFSPRING

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

•

Albinism

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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