1. Chapter 14: Mendel & The Gene Idea

2. Mendelian genetics Character = heritable feature (ex. fur color)
Trait = character variant (ex. brown)
True-bred = all offspring same variety
Hybridization = crossing 2 different true-breds
P generation = parents
F1 generation = first filial generation

3. Law of Segregation Alternative versions of genes: alleles
account for variation
Per character:
Offspring gets 2 alleles (1/parent)
If the two alleles differ:
Dominant fully expressed/detectable
Recessive not expressed/detectable
Alleles for each character segregate (separate) during gamete production (meiosis).
Mendel’s Law of Segregation

5. Genetic vocabulary……. Punnett square: Homozygous: Heterozygous:
Predicts results of crossing (if parents genotypes known)
Homozygous:
pair of identical alleles for a character
Heterozygous:
2 different alleles for a character
Phenotype:
organism’s expressed traits
Genotype:
organism’s genetic makeup
Testcross:
breeding rec homo X dom phenotype (unknown genotype)

7. The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2+ characters?
Monohybrid cross vs. dihybrid cross
The two pairs of alleles segregate independently of each other.
Mendel’s Law of Independent Assortment

9. EXPERIMENT P Generation F1 Generation Hypothesis of Hypothesis of
Fig. 14-8a
EXPERIMENT
P Generation
YYRR
yyrr
Gametes YR  yr
F1 Generation
YyRr
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Predictions
Sperm or
Predicted
offspring of
F2 generation
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1

10. Non-single gene genetics, I
Incomplete dominance:
appearance between phenotypes of 2 parents
Ex: snapdragons (red x white = pink)
Codominance:
2 alleles affect the phenotype in separate, distinguishable ways (red + white = red & white)
Ex: sickle cell
Multiple alleles:
more than 2 possible alleles for a gene
Ex: human blood types: A B O

13. Non-single gene genetics, II
Pleiotropy:
genes w/ multiple phenotypic effects
Ex: sickle-cell anemia
Epistasis:
gene at 1 locus affects expression of gene at 2nd locus
Ex: mice coat color
Polygenic Inheritance:
additive effect of 2+ genes on a single character
Ex: human skin pigmentation and height

14. Pleiotrophy

15. Epistasis

16. Polygenic traits

17. Human disorders The family pedigree Recessive disorders:
Cystic fibrosis
Tay-Sachs
Sickle-cell *
Dominant disorders: rare
Huntington’s
Testing:
Amniocentesis
Chorionic villus sampling (CVS)

18. Pedigrees

19. Video: Ultrasound of Human Fetus I
Genetic testing
Video: Ultrasound of Human Fetus I

20. Heterozygous phenotype same as that of homo- zygous dominant PP Pp
Fig. 14-UN2
Degree of dominance
Description
Example
Complete dominance
of one allele
Heterozygous phenotype
same as that of homo-
zygous dominant PP Pp
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
CRCR
CRCW
CWCW
Codominance
Heterozygotes: Both
phenotypes expressed
IAIB
Multiple alleles
In the whole population,
some genes have more
than two alleles
ABO blood group alleles
IA , IB , i
Pleiotropy
One gene is able to
affect multiple
phenotypic characters
Sickle-cell disease

21. Relationship among genes Description Example Epistasis
Fig. 14-UN3
Relationship among
genes
Description
Example
Epistasis
One gene affects
the expression of
another
BbCc
BbCc BC bC Bc bc BC bC Bc bc 9
: 3
: 4
Polygenic
inheritance
A single phenotypic
character is
affected by
two or more genes
AaBbCc
AaBbCc

22. You should now be able to:
Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F1 generation, F2 generation
Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype
Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

23. Define and give examples of pleiotropy and epistasis
Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance
Define and give examples of pleiotropy and epistasis
Explain why lethal dominant genes are much rarer than lethal recessive genes
Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings