Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 CHAPTER 16 SIMPLE PATTERNS OF INHERITANCE Prepared by Brenda Leady, University of Toledo

2 Gregor Johann Mendel 1822–1884 Entered monastery and became a priest Historic studies on pea plants Paper ignored at the time Independently rediscovered years later

3 Garden Pea, Pisum sativum Several advantageous properties  Many readily available characters or traits  Normally self-fertilizing True-breeding lines exhibit the same traits  Ease of making crosses with large flowers Cross-fertilization or hybridization

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7 Single-factor cross  Experimenter follows the variants of only 1 trait P generation  True-breeding parents F 1 generation  Offspring of P cross  Monohybrids – if parents differ in 1 trait F 2 generation  F 1 self-fertilizes

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9 3 important ideas 1. Dominant and recessive traits  Dominant is displayed trait  Recessive trait masked by dominant trait 2. Genes and alleles Particulate mechanism of inheritance His “unit factors” are genes Every individual has 2 genes for a trait Gene has two variant forms or alleles

10 3. Segregation of alleles  Approximately 3:1 ratio  Two copies of a gene carried by an F 1 plant segregate (separate) from each other, so that each sperm or egg carries only one allele  Mendel’s Law of Segregation 2 copies of a gene segregate from each other during the transmission from parent to offspring

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12 Genotype and phenotype Genotype  Genetic composition of individual  TT or tt – homozygous  Tt – heterozygous Phenotype  Characteristics that are the result of gene expression  TT and Tt are tall  Tt is dwarf

13 Step 5. Determine relative proportions of genotypes and phenotypes. TT tt ♀ ♂ Male gametes Female gametes TT Tt tt Genotype ratio TT:Tt:tt 1:2:1 Phenotype ratio Tall: dwarf 3:1

14 Testcross A dwarf pea plant must be tt A tall pea plant could be either TT or Tt Cross unknown individual to a homozygous recessive individual If some offspring are dwarf, unknown individual must have been Tt If all offspring are tall, the unknown individual was TT

15 Chromosomes and segregation Mendel’s law of segregation can be explained by the pairing and segregation of homologous chromosomes during meiosis Locus – physical location of a gene on a chromosome

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18 Chromosomes and independent assortment Law of independent assortment can also be explained by the behavior of chromosomes during meiosis Random alignment of chromosome pairs during meiosis I leads to the independent assortment of alleles found on different chromosomes

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20 Table 16.1a

21 Table 16.1b

22 Sample size Accuracy of prediction depends on the number of events observed or sample size Random sampling error – deviation between observed and expected outcome Larger samples have smaller sampling errors Humans have small families and observed data may be very different from expected outcome

23 Pedigree analysis Inherited trait is analyzed over the course of a few generations in one family Cystic fibrosis (CF) example  Approximately 3% of Americans of European descent are heterozygous carriers of the recessive CF allele and phenotypically normal  Individuals who are homozygous exhibit disease symptoms

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25 Many of the alleles causing human genetic disease are recessive like CF Some are dominant like Huntington disease Huntington disease has an autosomal inheritance pattern  Gene on one of 22 pairs of autosomes  Genes also found on sex chromosomes

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27 Sex chromosomes Found in many but not all species with 2 sexes Several mechanisms for sex determination  X-Y system – males are XY and females XX  X-O system – females are XX and males X or XO  Z-W system – male is ZZ and female ZW

28 Not all chromosomal mechanisms involve sex chromosomes  Bees are haplo-diploid – male is haploid and female is diploid Other mechanisms also exist  Sex is controlled by environment (temperature) in some reptiles and fish Plants  Some have a single type of plant making male and female gametophytes  Others have sexually distinct plants making male or female gametophytes only

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30 X-linked In humans, X chromosome is larger and carries more genes than the Y chromosome Genes found on the X but not the Y are X- linked genes  Sex linked genes are found on one sex chromosome but not the other Males are hemizygous for X-linked genes  Hemophilia A example

31 Hemophilia A caused by recessive X- linked gene Encodes defective clotting protein

Morgan’s Experiments Showed a Correlation Between a Genetic Trait and the Inheritance of a Sex Chromosome in Drosophila X-linked gene the first to be located on a specific chromosome Found a true-breeding line with white-eyed males rather than the normal red eyes Results suggested a connection between alleles for eye color and sex Lower than expected number of white-eyed flies explained by a decreased survival rate 1933, Morgan first geneticist to receive Nobel prize

34 Mendelian inheritance Inheritance pattern of genes that segregate and assort independently Simple Mendelian inheritance – one trait is completely dominant over the other X-linked inheritance – pairs of dominant and recessive alleles found on the X chromosome

35 Wild-type allele  Prevalent allele in a population  Encodes a protein made in the proper amount and functioning normally Mutant alleles  Altered by mutation  Tend to be rare in natural populations  Defective in its ability to express a functional protein

36 In simple dominance, the recessive allele does not affect the phenotype of the heterozygote A single copy of the dominant allele is sufficient to mask the recessive allele Purple pigment, P  One P allele makes enough functional protein to provide a normal phenotype  In other cases, the heterozygote may make more than 50% of the normal amount of protein – up-regulated

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Single-Gene Mutations Cause Many Inherited Diseases and Have Pleiotropic Effects Several examples of human genetic diseases in which a recessive allele fails to produce a specific active protein Over 7,000 human disorders caused by mutations in a single gene Majority are recessive but some are dominant Pleiotropy- mutation in a single gene has multiple effects

Cystic Fibrosis (CF) example Normal CF allele codes for protein that regulates chloride ion balance Mutation diminishes function of transporter Thick mucus in lungs due to water imbalance due to ion balance People with CF have very salty sweat because salt cannot be recycled back into body Males with CF are infertile because Cl - transporter needed for proper development of vas deferens in embryo

41 Incomplete dominance  Heterozygote has intermediate phenotype  Neither allele is dominant  Pink four-o’clocks 50% of normal protein not enough to give red color  Phenylketonuria (PKU) Heterozygotes appear phenotypically normal but heterozygotes have double the normal phenylalanine levels

42 Multiple alleles  3 or more variants in a population  Phenotype depends on which 2 alleles are inherited  ABO blood types in humans Type AB is codominance- expressing both alleles equally

43 Sex-influenced inheritance  Allele is dominant in one sex but recessive in the other  Pattern baldness Baldness allele dominant in men but not women Only a woman homozygous for baldness allele would be bald  Not X-linked

44 Role of environment Norm of reaction – effects of environmental variation on a phenotype Genetically identical plants grow to different heights in different temperatures People with PKU can develop normally if given a diet free of phenylalanine  If their diet contains phenylalanine, they develop mental retardation, underdeveloped teeth and foul-smelling urine

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