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Unit IV : Extension to Mendelian Genetics

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1 Unit IV : Extension to Mendelian Genetics
Lecture 7 Unit IV : Extension to Mendelian Genetics  

2 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.

3 Dominant vs. Recessive= interaction

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

5 Her eyelashes are short
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

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

7 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

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

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

10 Flower Color in Four’O clocks:
Gametes R W RR R RR RW Gametes W F1 generation All RW RW WW F2 generation WW 1 : 2 : 1

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

12 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’.

13 Application: The genes for fur design of Panthers are codominant
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

14 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.

15 Codominance 1. type A = IAIA or IAi 2. type B = IBIB or IBi
Ex. ABO blood group both alleles expressed, IAIB = type AB blood IA allele expresses A antigen on the surface of red cell IB allele expresses B antigen on the surface of red cell Individuals with AB (IA IB ) blood group express both A and B antigens on their red cell surface. 1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii

16 Codominance Application
Mendelian Genetics 4/12/2017 Codominance Application Example: homozygous Type B (IBIB) x heterozygous Type A (IAi) IB IA i IAIB IBi 1/2 = IAIB 1/2 = IBi copyright cmassengale

17 Codominance Application
Mendelian Genetics 4/12/2017 Codominance Application If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (IAIB) IB IA i IAIB ii Parents: genotypes = IAi and IBi phenotypes = A and B

18 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

19 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: IB, its enzyme adds galactose; IA, its enzyme adds galactosamine; and i, which codes for a protein that does not add a sugar.

20 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.

21 ABO blood types Both clump clump clump No clump

22 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....

23 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

24 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.

25 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.

26 Epistasis E_ Dark pigment in fur ee No dark pigment in fur E_bb E_B_ eebb eeB_ Yellow fur Yellow fur Brown fur Black fur 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

27 Interaction 7: Environmental Effects on gene expression
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

28 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).

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

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

31 Interaction 8: Lethal alleles
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

32 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”.

33 Environmental effects on gene expression Sex linkage
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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