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Lecture 7 Unit IV : Extension to Mendelian Genetics.

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2 Lecture 7 Unit IV : Extension to Mendelian Genetics

3 Mendel Inheritance: How do alleles interact with each other? Dominant allele: An allele that expresses its phenotype in the heterozygous state. Recessive allele: An allele whose phenotypic effect or trait is not expressed in a heterozygout state.

4 Dominant vs. Recessive= interaction

5 Dominant/Recessive interactions The dominant allele will “cover up” the recessive allele.

6 Application: A girl inherits: a long eyelash allele from her father and a short eyelash allele from her mother. The short eyelash allele is dominant and the long eyelash allele is recessive. What is the length of the girl’s eyelashes? Her eyelashes are short

7 Some alleles are neither dominant nor recessive Some exceptions to Simple Mendelian Genetics= Non Mendelian interactions Beyond dominant and recessive alleles Is dominance the only type of inheritance patterns?

8 Interaction 1: Incomplete Dominance When the 2 different alleles are present, the phenotype will look like a BLENDING of the two. Neither allele is dominant and heterozygous individuals have an intermediate phenotype between two parents

9 For height of the Sapping Evergreen Tree: the interaction of genes show incomplete dominance. Application: A tall tree is crossed with a short tree, what will the height of the new tree be? Medium height

10 Another question, a short tree is crossed with another short tree, what will the height of the new tree be? Short height

11 F 1 generation F 2 generation RR All RW WW 1 : 2 : 1 Gametes R W RW RR RW WW Flower Color in Four’O clocks:

12 Incomplete dominance in carnations Red flowers - 2 alleles to make a red pigment White flowers – 2 alleles; can’t make red pigment Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

13 Interaction 2: Codominance Neither allele can mask the other and both are fully expressed in the heterozygous offspring and not in an “intermediate” form. Example: red flowers that are crossed with blue flowers that yield red and blue flowers. A Capital Letter is used for first allele (R) and a Capital Letter with apostrophe (R’) for second allele. Given the Genotype RR’.

14 Application: The genes for fur design of Panthers are codominant. The 2 possible designs for fur are spotted and striped. A spotted panther is crossed with a striped panther. What will the fur design of the baby panther be? Spotted with stripes

15 Application: in cattle, the heterozygous (Rr) determines roan coat color (mixed red(RR) and white(rr) hairs). When two roan cattle are crossed, the phenotypes ratio is 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle? A) roan x roan B) red x white C) white x roan D) red x roan E) All of the above crosses would give the same percentage of roan.

16 Codominance – Ex. ABO blood group – both alleles expressed, I A I B = type AB blood I A allele expresses A antigen on the surface of red cell I B allele expresses B antigen on the surface of red cell Individuals with AB (I A I B ) blood group express both A and B antigens on their red cell surface. 1.type A= I A I A or I A i 1.type A= I A I A or I A i 2.type B= I B I B or I B i 2.type B= I B I B or I B i 3.type AB= I A I B 3.type AB= I A I B 4.type O= ii 4.type O= ii

17 16 Codominance Application Example:homozygous Type B (I B I B ) x heterozygous Type A (I A i) IAIBIAIB IBiIBi IAIBIAIB IBiIBi 1/2 = I A I B 1/2 = I B i IBIB IAIA i IBIB copyright cmassengale

18 17 Codominance Application IAIBIAIB ii Parents: genotypes genotypes = I A i and I B i phenotypes phenotypes = A and B IBIB IAIA i i If a boy has a blood type O and his sister has blood type AB, If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (I A I B ) boy - type O (ii) X girl - type AB (I A I B )

19 Interaction 3: Multiple Alleles  A gene may have more than 2 alleles for a trait in a population.  Often, no single allele is dominant; each allele has its own effect.  Multiple alleles (can occur in complete and incomplete dominance and in codominance) Example: ABO blood system

20 ABO blood system Four phenotypes (A,B,AB,O) and three alleles (A, B, and i) each individual only inherits two alleles: one from each parent  The gene encodes an enzyme that adds sugar on the surface of red blood cells.  This gene, designated I, has three common alleles: I B, its enzyme adds galactose; I A, its enzyme adds galactosamine; and i, which codes for a protein that does not add a sugar.

21  If a type A receives a transfusion of type B blood, the immune system recognizes that the type B blood cells possess a “foreign” antigen (galactose) and attacks the donated blood cells, causing the cells to clump, or agglutinate.  This also happens if the donated blood is type AB.  However, if the donated blood is type O, no immune attack will occur, as there are no galactose antigens on the surfaces of blood cells produced by the type O donor.  In general, any individual’s immune system will tolerate a transfusion of type O blood.

22 ABO blood types Both clump clump No clump

23 Interaction 4: Pleiotropy Single gene that causes multiple phenotypic traits (ex. sickle-cell disease) In sickle cell anemia, a defect in the oxygen carrying hemoglobin molecule causes anemia, heart failure, kidney failure, enlargement of the spleen....

24 Interaction 5: Polygenic Inheritance 2 or more genes combine their effects to produce single phenotypic trait Example: skin and eye color, obesity, recurrent aborption and heart disease

25 Polygenetic Inheritance Indicated by quanitative variation usually This occurs when there is an additive effect from two or more genes. Pigmentation in humans is controlled by at least three (3) separately inherited genes.

26 Interaction 6: Epistasis It’s a type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus. Example: In Labrador retrievers (dog) one gene (E,e) affects one other of coat color (B,b) by controlling the amount of the pigment eumelanin in the fur.

27 ee No dark pigment in fur eebbeeB_ Yellow fur E_ Dark pigment in fur E_bbE_B_ Brown furBlack fur Epistasis A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat However, a second gene controls whether any eumelanin is deposited in the fur. The homozygous recessive dogs (ee) will have yellow fur

28  The degree to which an allele is expressed may depend on the environment factors.  Some alleles are heat-sensitive, for example: such alleles are more sensitive to temperature than other alleles.  The arctic foxes make fur pigment only when the weather is warm. Arctic fox in winter in summer Interaction 7: Environmental Effects on gene expression

29 Temperature Effects on Phenotype In Himalayan rabbits, Melanin is produced in cooler areas of body. an enzyme of melanin production is heat- sensitive (Homozygous genotype).

30 pH of the soil will change the color of hydrangea flowers from blue to pink The effect of environment of phenotype

31 Environmental Effects brown eye color requires phenylalanine from diet (alimentation) to produce melanin, the eye pigment

32 Alleles implicate with survival of individuals carrying this genes  those carrying homozygous allele die. Ex. Creeper hen have vestigial wings and legs. If two creeper hen are crossed, there is a ratio of 2 creepers : 1 normal hen among offsprings (not 3 : 1). Creeper allele (C) is dominant in its effect on wings and legs length, but recessive on its effect on viability. CC  lethal, die before hatching Cc  creeper hen cc  normal hen, survive after hatching on Interaction 8: Lethal alleles

33 Non- Mendelian Inheritance Mitochondria is organell with its own DNA (is not interfeared by DNA genom). During fertilization sperm penetrate its head into the egg & leave the tail containing mitochondria outside.  zygote carry only mitochondria from egg (maternal)  every individual mitochondrial DNA is inherited down the maternal line : from mother to children, and so on. Sex linked inheritance Alleles (genes) which occupy loci within sex chromo –somes, though express traits of non sex-determiners are said to be “sex linked”.

34 Does these examples of inheritance patterns argue that Mendel was wrong? No, Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait). We know now that there is a range of dominance and that genes can work together and interact. Incomplete dominance Codominance Multiple alleles Polygenic traits Epistasis Pleiotropy Environmental effects on gene expression Sex linkage


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