CHAPTER 2

EXTENSIONS OF MENDELIAN INHERITANCE

MISS NUR SHALENA SOFIAN

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

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

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

• 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

Wild-type (dominant) allele Mutant (recessive) allelePurple 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

• Consider, for example, the traits that Mendel studied

Another example is from Drosophila

Wild-type (dominant) allele Mutant (recessive) alleleRed eyes White eyes

Normal wings Miniature wings

• 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?

FIGURE 1

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

• 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

• 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

Creeper X Normal

1 creeper : 1 normal

Creeper is a dominant allele

Creeper X Creeper

1 normal : 2 creeper

Creeper is lethal in the homozygous state

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

Figure 2

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

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”

FIGURE 3

Multiple Alleles

• Many genes have multiple alleles– Three or more different alleles

• 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

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

HOW CAN THIS BE???

• Caused by tyrosinase; producing melanin• Two types of melanin: eumelanin (black

pigment) and phaeomelanin (orange/yellow pigment)

• 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

• 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

• Allele i is recessive to both IA and IB

• Alleles IA and IB are codominant – They are both expressed in a heterozygous individual

B

N-acetyl-galactosamine

• 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

• 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