CHAPTER 2 EXTENSIONS OF MENDELIAN INHERITANCE MISS NUR SHALENA SOFIAN

1. 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 • Another example is from Drosophila

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. 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. 1 creeper : 1 normal 1 normal : 2 creeper Creeper X Creeper Creeper X Normal 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. 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. N-acetyl- galactosamine B • Allele i is recessive to both IA and IB • Alleles IA and IB are codominant • They are both expressed in a heterozygous individual

25. 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