chapter 3 transmission of genes from generation to generation · chapter 3 human heredity by...

Chapter 3 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Chapter 3Transmission of Genes fromGeneration to Generation


Johann Gregor Mendel1822-1884

• Performedexperimentswith peas tounderstand thefundamentalprinciples ofgenetics

Figure 3.1

Fig. 3.1


Mendel’s Peas

Why peas?• Identifiable traits• Self fertilizing with a flower structure that

minimizes accidental pollination• Can be artificially fertilized• Short growth period


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


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


Monohybrid cross


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.


•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


Mendel’sResults

Fig. 3.4


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


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


Punnett Squares

•Indicate possiblegametecombinationsto produce offspring


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)


Single Gene Cross

• One gene withtwo alleles

• Smooth (S) isdominant towrinkled (s)

Fig. 3.7


Crosses with TwoTraits

• Seed shapegene with twoalleles: smoothand wrinkled

• Seed color genewith two alleles:yellow and green

Fig. 3.8


Analyzing the F2 Results

• Two parental phenotypes

• Two new phenotypes


CrossingTwo Genes

Fig. 3.9


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


Meiosis Explains Mendel’s Results

Fig. 3.10


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

chapter 3 transmission of genes from generation to generation · chapter 3 human heredity by...

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