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Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Chapter 3Transmission of Genes fromGeneration to Generation
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Johann Gregor Mendel1822-1884
• Performedexperimentswith peas tounderstand thefundamentalprinciples ofgenetics
Figure 3.1
Fig. 3.1
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Mendel’s Peas
Why peas?• Identifiable traits• Self fertilizing with a flower structure that
minimizes accidental pollination• Can be artificially fertilized• Short growth period
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Experimental Design
• Large sample size• One pair of traits at a time• Repeated experiments• Analyzed his data with probability and
statistics• Keys to his success were methodical
approach and lack of preconceived notions
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Terms• True Breeding – self
fertilization produces thesame traits for manygenerations
• P1 – parental generation• First Filial (F1 ) – offspring
of P1
• Second Filial (F2 ) –offspring of F1 X F1
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Monohybrid cross
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Conclusions:
The reciprocal crosses gave the same results.
All the F1s looked like one of the parents.
Traits remained discrete! No blending.
Trait that disappeared in F1 reappeared inF2.
Trait visible in F1 was present at about 3Xin F2.
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
•The two alleles segregate during formation of gametes (sperm and egg) resulting in each gamete containing only one copy of each gene.
• Male and female gametes unite at random during fertilization.
First law of segregation
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Mendel’sResults
Fig. 3.4
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
From Mendel’s Results
• Traits are not blended as they are passedfrom parent to offspring
• Each parent makes equal contribution
• Genes can be carried but not expressed
• Appearance may be similar but geneticmake up may differ
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
More Terms
• Factors (genes) determine traits and canbe carried but not expressed
• Recessive trait – trait not expressed in F1but expressed in F2
• Dominant trait – trait expressed in F1
• Genotype – genetic makeup of theorganism
• Phenotype – appearance of the organism
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Punnett Squares
•Indicate possiblegametecombinationsto produce offspring
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Terms
Alleles – alternative forms of a gene
Homozygous individuals carry the samealleles for a given gene (SS or ss)
Heterozygous individuals carry twodifferent alleles for a given gene (Ss)
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Single Gene Cross
• One gene withtwo alleles
• Smooth (S) isdominant towrinkled (s)
Fig. 3.7
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Crosses with TwoTraits
• Seed shapegene with twoalleles: smoothand wrinkled
• Seed color genewith two alleles:yellow and green
Fig. 3.8
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Analyzing the F2 Results
• Two parental phenotypes
• Two new phenotypes
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
CrossingTwo Genes
Fig. 3.9
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Mendel’s Second Law:Principle of Independent
Assortment• During gamete formation, different pairs of alleles segregate independently of one another
• Gametes combine randomly with each other during fertilization
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Meiosis Explains Mendel’s Results
Fig. 3.10
Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Genes Are Located onChromosomes
• Each gene is located at a specific site(locus) on a chromosome
• Humans have approximately 30,000 genes
• Distributed on 22 autosomes and the Xand Y chromosomes