Genetics II Mendelian Genetics Complete dominance Nonmendelian Genetics

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Genetics II

Genetics II

Mendelian Genetics • Complete dominance

Mendelian Genetics • Complete dominance

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

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

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 O 2 concs kidney and bone damage

Epistasis One gene affects expression of another • Effects of one gene are modified

Epistasis One 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

Polygenic inheritance Additive effects of two or more genes on a phenotypic character •

Polygenic inheritance Additive 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

Polygenic Inheritance Problem Solving Petal length of a plant ranges from 4 mm to

Polygenic Inheritance Problem Solving Petal length of a plant ranges from 4 mm to 12 mm to 20 mm. Out of 770 plants, only 3 of them have 4 mm petals. 1. Give one genotype for a plant with 12 mm petals. 2. Give two possible genotypes for plants with 6 mm petals 3. What proportion of plants have 14 mm petals? 4. What is the phenotype of plants with 7 contributory alleles? 5. How many contributing alleles does a plant with 4 mm petals have?

Sex-related inheritance expression of traits is affected by the sex of the individual sex

Sex-related inheritance expression 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 1. Sex-linked inheritance 2. Sex-limited inheritance 3. Sex-influenced inheritance 44 + XY 44 + XX Parents 22 + Sperm 22 + Y X 22 + XY Ova 44 + Zygotes 44 + XX (offspring) XY (a) The X-Y system • 22 + XX 22 + X 76 + ZW 76 + ZZ (b) The X– 0 system (c) The Z–W system 32 (Diploid) (d) The haplo-diploid system 16 (Haploid)

Sex-linked inheritance • Gene linkage – genes located on the same chromosome are inherited

Sex-linked inheritance • 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 (1) Father affected with the disorder Xa, Mother is homozygous dominant (2) Carrier mates with a male of normal phenotype (3) Carrier mates with a male with the disorder

Examples of X-linked alleles Recessive alleles • Color-blindness • Duchenne muscular dystrophy • Hemophilia

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

Mammalian females • One of the two X chromosomes is randomly inactivated • If

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 Two cell populations in adult cat: Active X Early embryo: X chromosomes Cell division and X Inactive X chromosome Inactive X inactivation Allele for black fur Active X Orange fur Black fur

Examples of Y-linked (Holandric) Inheritance • Transmission of genes from father to son –

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

Sex-limited inheritance • involves autosomal genes that are expressed only in either males or

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 Genotype HH Hh hh Female Male hen-feathered hen-feathered rooster-feathered

Sex-influenced inheritance • dominant in one sex but recessive in the other • autosomal

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 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 Genotype b 1 b 1 b 1 b 2 b 2 b 2 Male (dominant) Female (recessive) bald not bald