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Non-Mendelian Genetics • Complete dominance • Law of segregation • Law of independent assortment • One gene one trait Mendelian Genetics • Codominance • Incomplete dominance • Multiple alleles • Pleiotropy • Epistasis • Polygenic inheritance • Sex-related inheritance Non-mendelian Genetics Pleiotropy (pleion, “more”) • One gene, many phenotypic effects 40% of cats with white fur and blue eyes are deaf Phenylketonuria (lack of enzyme that breaks down Phe) Sickle cell anemia (misshapen RBCs at low O2 concs) http://3.bp.blogspot.com/bUvXE8h_wuE/TycwHFuFqrI/AAAAAAAAA4 E/fWfyDR0ETNU/s1600/Phenylketonuria+(P KU).jpg Two genes affect coat color • Effects of one gene are modified by one or several other genes B black, b brown, E pigment deposited, e no pigment • Examples: – – – – Coat color in mammals Color of wheat grains Fruit color in squash Bombay phenotype in ABO blood groups in man Epistasis One gene affects expression of another Bombay phenotype • Quantitative characters vary in a population along a continuum or gradation • Due to the presence of contributory or dominant (ABC) and non-contributory or recessive alleles (abc) • Expression can be affected by environmental factors Polygenic inheritance Additive effects of two or more genes on a phenotypic character 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 (very light) (very dark) Aa Bb Cc x Aa Bb Cc (medium brown) brown Petal length of a plant ranges from 4mm to 12mm to 20mm. Out of 770 plants, only 3 of them have 4mm petals. 1. How many genes affect petal length? 2. How many phenotypes are present for petal length? 3. Give one genotype for a plant with 12 mm petals. 4. Give two possible genotypes for plants with 6 mm petals 5. What proportion of plants have 14 mm petals? 6. What is the phenotype of plants with 7 contributory alleles? 7. How many contributing alleles does a plant with 8 mm petals have? Polygenic Inheritance Problem Solving • • sex is an inherited phenotypic character determined by the presence/absence of certain chromosomes SRY (sex-determining region of Y) in humans 1. Sex-linked inheritance 2. Sex-limited inheritance 3. Sex-influenced inheritance Sex-related inheritance expression of traits is affected by the sex of the individual • 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 Sex-linked inheritance Recessive alleles • Color-blindness • Duchenne muscular dystrophy • Hemophilia • Testicular feminization Dominant traits • Hypophosphatemia Examples of X-linked traits Transmission of genes from father to son • Testis-determining factor (TDF/SRY gene) • Hypertrichosis of the ears Examples of Y-linked (Holandric) 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 • Examples: • milk production • cryptorchidism • feathers in domestic fowl Genotype Female Male HH hen-feathered hen-feathered Hh hen-feathered hen-feathered hh hen-feathered rooster-feathered Sex-limited 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 are unaffected and express the trait regardless of the hormone produced Sex-influenced inheritance • pattern baldness • baldness allele is dominant in males but recessive in females • a heterozygous male is bald, but a heterozygous female is not Sex-influenced inheritance Genotype Female recessive Male dominant b1b1 bald bald b1b2 not bald bald b2b2 not bald not bald