1. EXTENSIONS OF MENDELIAN INHERITANCE
CHAPTER 2
EXTENSIONS OF MENDELIAN INHERITANCE
MISS NUR SHALENA SOFIAN

2. INTRODUCTION
Mendelian inheritance describes inheritance patterns that obey two laws
Law of segregation
Law of independent assortment
Simple Mendelian inheritance involves
A single gene with two different alleles
Alleles display a simple dominant/recessive relationship

3. INTRODUCTION (cont.)
In this chapter we will examine traits that deviate from the simple dominant/recessive relationship
The inheritance patterns of these traits still obey Mendelian laws
However, they are more complex and interesting than Mendel had realized

4. 2.1 INHERITANCE PATTERN OF SINGLE GENES
There are many ways in which two alleles of a single gene may govern the outcome of a trait
Mendelian inheritance describes several patterns that involve single gene. What are they? (submit 19/1/2011)
These patterns are examined with two goals in mind
1. Understanding the relationship between the molecular expression of a gene and the trait itself
2. The outcome of crosses

5. Prevalent alleles in a population are termed wild-type alleles
These typically encode proteins that
Function normally
Are made in the right amounts
Alleles that have been altered by mutation are termed mutant alleles
These tend to be rare in natural populations
They are likely to cause a reduction in the amount or function of the encoded protein
Such mutant alleles are often inherited in a recessive fashion

6. Consider, for example, the traits that Mendel studied
Wild-type (dominant) allele
Mutant (recessive) allele
Purple flowers
White flowers
Axial flowers
Terminal flowers
Yellow seeds
Green seeds
Round seeds
Wrinkled seeds
Smooth pods
Constricted pods
Green pods
Yellow pods
Tall plants
plants
Another example is from Drosophila
Wild-type (dominant) allele
Mutant (recessive) allele
Red eyes
White eyes
Normal wings
Miniature wings

7. Genetic diseases are caused by mutant alleles
In many human genetic diseases , the recessive allele contains a mutation. What kind of genetic diseases that you know of containing recessive alleles? (Submit 19/1/2011)
Preventing the allele from producing a fully functional protein
In a simple dominant/recessive relationship, the recessive allele does not affect the phenotype of the heterozygote
So how can the wild-type phenotype of the heterozygote be explained?

8. FIGURE 1

9. Lethal Alleles
Essential genes are those that are absolutely required for survival
The absence of their protein product leads to a lethal phenotype
Nonessential genes are those not absolutely required for survival
A lethal allele is one that has the potential to cause the death of an organism
Resulting mutations in essential genes
Inherited in a recessive manner

10. Many lethal alleles prevent cell division
These will kill an organism at an early age
Some lethal alleles exert their effect later in life
Huntington disease
Characterized by progressive degeneration of the nervous system, dementia and early death
The age of onset of the disease is usually between 30 to 50
Conditional lethal alleles may kill an organism only when certain environmental conditions prevail
Temperature-sensitive (ts) lethals
A developing Drosophila larva may be killed at 30 C
But it will survive if grown at 22 C

11. An example is the “creeper” allele in chicken
Semilethal alleles
Kill some individuals in a population, not all of them
Environmental factors and other genes may help prevent the detrimental effects of semilethal genes
A lethal allele may produce ratios that seemingly deviate from Mendelian ratios
An example is the “creeper” allele in chicken
Creepers have shortened legs and must creep along
Such birds also have shortened wings
Creeper chicken are heterozygous

12. Creeper is a dominant allele
Creeper X Normal
Creeper X Creeper
1 creeper : 1 normal
1 normal : 2 creeper
Creeper is lethal in the
homozygous state
Creeper is a dominant allele

13. Incomplete Dominance
In incomplete dominance the heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes
Example:
Four o’clock (Mirabilis jalapa) flower color plant
Two alleles
CR = wild-type allele for red flower color
CW = allele for white flower color

14. 1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian inheritance
In this case, 50% of the CR protein is not sufficient to produce the red phenotype
Figure 2

15. Incomplete Dominance
Whether a trait is dominant or incompletely dominant may depend on how closely the trait is examined
Take, for example, the characteristic of pea shape
Mendel visually concluded that
RR and Rr genotypes produced round peas
rr genotypes produced wrinkled peas
However, a microscopic examination of round peas reveals that not all round peas are “created equal”

16. FIGURE 3

17. Multiple Alleles Many genes have multiple alleles
Three or more different alleles

18. An interesting example is coat color in rabbits
Four different alleles
C (full coat color)
cch (chinchilla pattern of coat color)
Partial defect in pigmentation
ch (himalayan pattern of coat color)
Pigmentation in only certain parts of the body
c (albino)
Lack of pigmentation
The dominance hierarchy is as follows:
C > cch > ch > c
Figure 4 illustrates the relationship between phenotype and genotype

19. Agouti (wild type) c+c+, c+cch, c+ch, c+c Chinchilla (mutant) cchcch
Phenotype Genotype
Agouti (wild type) c+c+, c+cch, c+ch, c+c
Chinchilla (mutant) cchcch
Himalayan(mutant) chch,chc
Light grey cchch, cchc
Albino (mutant) cc
FIGURE 4

20. HOW CAN THIS BE??? Caused by tyrosinase; producing melanin
Two types of melanin: eumelanin (black pigment) and phaeomelanin (orange/yellow pigment)

22. The himalayan pattern of coat color is an example of a temperature-sensitive conditional allele
The enzyme encoded by this gene is functional only at low temperatures
Therefore, dark fur will only occur in cooler areas of the body
This is also the case in the Siamese pattern of coat color in cats

23. The ABO blood group provides another example of multiple alleles
It is determined by the type of antigen present on the surface of red blood cells
WHAT ARE ANTIGENS? (Submit 19/1/2011)
As shown in Table 1, there are three different types of antigens found on red blood
Antigen A, which is controlled by allele IA
Antigen B, which is controlled by allele IB
Antigen O, which is controlled by allele i

24. Allele i is recessive to both IA and IB
Alleles IA and IB are codominant
They are both expressed in a heterozygous individual
N-acetyl-
galactosamine B

25. A fourth can be added by the enzyme glycosyl transferase
The carbohydrate tree on the surface of RBCs is composed of three sugars
A fourth can be added by the enzyme glycosyl transferase
The i allele encodes a defective enzyme
The carbohydrate tree is unchanged
IA encodes a form of the enzyme that can add the sugar N-acetylgalactosamine to the carbohydrate tree
IB encodes a form of the enzyme that can add the sugar galactose to the carbohydrate tree
Thus, the A and B antigens are different enough to be recognized by different antibodies

26. For safe blood transfusions to occur, the donor’s blood must be an appropriate match with the recipient’s blood
For example, if a type O individual received blood from a type A, type B or type AB blood
Antibodies in the recipient blood will react with antigens in the donated blood cells
This causes the donated blood to agglutinate
A life-threatening situation may result because of clogging of blood vessels