1. Genetics: Review
1. Alternative versions of genes (alleles) account for variation in inherited characters
2. For each character, an organism inherits two alleles, one from each parent
3. If two alleles differ, one is dominant, the other recessive
4. The two alleles for each character segregate (separate) during gamete production.

2. Variations in Mendel’s Laws
In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.
F1 Generation RR rr
Gametes
P Generation
F2 Generation
Sperm
Red
White R r Rr
Pink
Eggs 1 2
Table 9.1 Some Autosomal Disorders in Humans

3. Variations in Mendel’s Laws
Hypercholesterolemia
Dangerously high levels of cholesterol in the blood.
Is a human trait that is incompletely dominant.
Heterozygotes have blood cholesterol levels about 2X normal.
Homozygotes have blood cholesterol levels about 5X normal. HH Hh hh
Homozygous
for ability to make
LDL receptors
Heterozygous
Homozygous
for inability to make
LDL receptors
GENOTYPE
LDL
LDL
receptor
PHENOTYPE
Cell
Normal
Mild disease
Severe disease

4. Variations in Mendel’s Laws
Multiple Alleles
Blood
Group
(Phenotype)
Genotypes
Red Blood Cells
Carbohydrate A
IAIA or
IAi A
Carbohydrate B
IBIB or
IBi B AB
IAIB O ii

5. Variations in Mendel’s Laws
Pleiotropy is the impact of a single gene on more than one character.
Pleiotropy
Multiple traits
(e.g., sickle-cell
disease)
Single
gene

6. Variations in Mendel’s Laws

7. Variations in Mendel’s Laws
*Epistasis
Example:
In Drosophila, gene:eyeless
*Black (B) is dominant to brown (b)
*Second gene responsible for allowing pigment to be deposited in hair
C = presence, c = absence (colorless)

8. Variations in Mendel’s Laws
Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
Multiple genes
Polygenic
inheritance
Single trait
(e.g., skin color)

9. Variations in Mendel’s Laws
F1 Generation
P Generation
F2 Generation
Sperm
AABBCC
(very dark)
Eggs
aabbcc
(very light)
AaBbCc 1 8 64 6 15 20

10. Variations in Mendel’s Laws
*Gene interactions

11. Sex Linkage *1909 Thomas Hunt Morgan II III XY IV or XX
*Sex chromosomes
*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male 11

12. Sex Linkage
Any gene located on a sex chromosome is called a sex-linked gene.
Most sex-linked genes are found on the X chromosome.

13. Variations in Mendel’s Laws
*Phenotype depends on environment and genes

14. How do we account for genetic variation?
*Independent assortment *Crossing over *Random fertilization
Cross over:
Independent Assortment: