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Genetics II

Genetics II. Mendelian Genetics. Complete dominance. Non-mendelian Genetics. Codominance Incomplete dominance Multiple alleles Pleiotropy Epistasis Polygenic inheritance Sex-related inheritance. Pleiotropy ( pleion , “more”). Ability of a gene to affect an organism in multiple ways

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Genetics II

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  1. Genetics II

  2. Mendelian Genetics • Complete dominance

  3. Non-mendelian Genetics • Codominance • Incomplete dominance • Multiple alleles • Pleiotropy • Epistasis • Polygenic inheritance • Sex-related inheritance

  4. Pleiotropy (pleion, “more”) • Ability of a gene to affect an organism in multiple ways • Examples: • 40% of cats with white fur and blue eyes are deaf • Phenylketonuria in humans (PKU) • Lack of an enzyme that converts Phe  Tyr • Mental retardation, reduced hair and skin pigmentation, eczema • Sickle-cell anemia • Problem with hemoglobin molecules  misshapen RBC’s at low O2 concs  kidney and bone damage

  5. EpistasisOne gene affects expression of another • Effects of one gene are modified by one or several other genes called modifier genes • Gene whose phenotype is expressed  epistatic • Examples: • Coat color in mammals • Color of wheat grains • Fruit color in squash • Bombay phenotype in ABO blood groups in man

  6. Polygenic inheritanceAdditive effects of two or more genes on a phenotypic character • Quantitative characters vary in a population along a continuum or gradation • Due to the presence of contributory (ABC) and non-contributory alleles (abc) • Expression can be affected by environmental factors • Example: • Skin pigmentation controlled by at least three separately inherited genes • Each dark skin allele contributes a unit of ‘darkness’ to the individual • AABBCC  very dark • aabbcc  very light

  7. Polygenic Inheritance Problem Solving Petal length of a plant ranges from 4mm to 12mm to 20mm. Out of 770 plants, only 3 of them have 4mm petals. • Give one genotype for a plant with 12 mm petals. • Give two possible genotypes for plants with 6 mm petals • What proportion of plants have 14 mm petals? • What is the phenotype of plants with 7 contributory alleles? • How many contributing alleles does a plant with 4mm petals have?

  8. 22 + XX 22 + X 44 + XY 44 + XX Parents (b) The X–0 system 22 + XY 76 + ZZ 22 + Y 22 + X 76 + ZW Sperm Ova (c) The Z–W system 44 + XX Zygotes (offspring) 44 + XY 16 (Haploid) 32 (Diploid) (a) The X-Y system (d) The haplo-diploid system Sex-related inheritanceexpression of traits is affected by the sex of the individual • sex is an inherited phenotypic character determined by the presence/absence of certain chromosomes • SRY (sex-determining region of Y) in humans • Sex-linked inheritance • Sex-limited inheritance • Sex-influenced inheritance

  9. Sex-linked inheritance (1) Father affected with the disorder Xa, Mother is homozygous dominant • Gene linkage – genes located on the same chromosome are inherited together • Sex-linkage • Sex chromosomes contain genes for many characters unrelated to sex • X-linked/Y-linked gene (2) Carrier mates with a male of normal phenotype (3) Carrier mates with a male with the disorder

  10. Examples of X-linked alleles Recessive alleles • Color-blindness • Duchenne muscular dystrophy • Hemophilia • Testicular feminization Dominant traits • Hypophosphatemia

  11. Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division and X chromosome inactivation Inactive X Inactive X Black fur Allele for black fur Active X Mammalian females • One of the two X chromosomes is randomly inactivated • If female is heterozygous for a gene located on the X chromosome, she is a mosaic

  12. Examples of Y-linked (Holandric) Inheritance • Transmission of genes from father to son • Testis-determining factor (TDF/SRY gene) • hypertrichosis

  13. Sex-limited inheritance • involves autosomal genes that are expressed only in either males or females • resulting in a part or function of the body that is present in one sex but not the other • e.g. milk production cryptorchidism feathers in domestic fowl GenotypeFemaleMale HH hen-feathered hen-feathered Hh hen-feathered hen-feathered hh hen-feathered rooster-feathered

  14. Sex-influenced inheritance • dominant in one sex but recessive in the other • autosomal • difference in expression due to the hormonal difference between the sexes • in heterozygotes, the expression of the trait is affected by sex hormones • homozygotes unaffected and express the trait regardless of the hormone produced • e.g. pattern baldness • gene for hair growth pattern has 2 alleles: one that produces hair all over the head and another that causes pattern baldness • the baldness allele is dominant in males but recessive in females • a heterozygous male is bald, but a heterozygous female is not • GenotypeMale (dominant)Female (recessive) • b1b1 bald bald • b1b2 bald not bald • b2b2 not bald • not bald

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