Chapter 12 Inheritance Patterns

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Chapter 12Inheritance Patterns Human Genetics

• MATES Biology

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In this chapter

• you will learn how biologists use their knowledge
  of DNA chromosome behavior to study how traits
  are inherited expressed.

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12-1 pp.221-226Sex-determining Chromosomes and
Linkage

• One of the earliest discoveries about gene
  linkage related to another significant thing
  about chromosomes and species, especially animal
  species.
• By the early1900's it was known that males and
  females of most species have one pair of
  "not-exactly-matching" homologous chromosomes,
  which determined the gender of the individual.
• These chromosomes were called the sex
  chromosomes. (The truly matching chromosomes are
  the autosomes.)

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Sex-determining Chromosomes and Linkage

• Its all about the X and the Y

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Sex-determining Chromosomes and Linkage

• Some species have the reverse pattern of sex
  chromosomes (male XX and female XY), and some
  species have one gender (female) with a pair of
  chromosomes and one gender (male) with a single
  unmatched chromosome.
• In all cases the gender with the dissimilar
  pattern will determine the gender of the
  offspring.

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Sex determination and Inheritance of Sex-linked
genes

• Mammals/ many insects male/female same number of
  chromosomes-one pair sex chromosomes
• different appearance and genetic composition
• XX female
• XY male
• Humans
• Autosomes 1-22
• Sex chromosome 23
• sex chromosome carried by sperm determines sex of
  offspring ( X or Y sperm)

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Sex-Linkage

• In 1910, Thomas Hunt Morgan - Studied inheritance
  patterns of fruit fly, Drosophila melanogaster,
  discovered presence of a white eye in certain
  individuals.
• Since this was a distinctive feature, Morgan
  decided to study the inheritance pattern for this
  recessive eye color.

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Sex-Linkage

• Morgan made several crosses using a white-eyed
  male, expecting the standard Mendelian results.
  He did not get them.
• While the ratio of 31 was obtained, all of the
  white-eyed second generation offspring were male
  flies.
• All females had red eyes (and 25 of the males
  also had red eyes).

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Sex-Linkage

• Morgan did a series of reciprocal crosses of
  white-eye males with red-eye females and red-eye
  males with white-eye females.
• He concluded that the gene for eye color was
  located on the X chromosome.
• Males passed the trait their daughters (on their
  solitary X chromosome) mothers passed the trait
  sons.
• White eyed females could also pass the white eye
  allele to their daughters, but if the father fly
  had red eyes, the eye color of the daughters
  would be red, while the eye color of the sons of
  white-eyed females would always be white.

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Sex-Linkage

• Morgan concluded that eye color was related to
  sex, that the sex-determining chromosomes also
  had genes that were unrelated to gender
  determination.
• Prior to Morgan's discovery, no one knew that
  genes unrelated to gender were also located on
  these chromosomes.

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Sex-Linkage

• These other traits are said to be sex-linked
  because they are inherited along with the sex of
  the individual.
• Because the X and Y chromosome are not exactly
  matching, the X chromosome can have genes that
  are not located on the Y chromosome, and
  vice-versa.
• Some of these genes are unrelated to the sexual
  characteristics, but are inherited with the
  sex-determination. This is referred to as
  sex-linkage.

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sex-linked (X-linked)

• genes on one sex chromosome not on other
• Y carries relatively few genesX many genes
  some not specifically related to female traits
• Ex genes for
• color vision
• blood clotting structural proteins
• female XX homozygous or hybrid
  (dominant/recessive)
• male XY fully expresses all alleles on single X
  (whether dominant or recessive)
• Ex color blindness hemophilia muscular
  dystrophy

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Some human sex-linked traits are

• Hemophilia (X)
• Hairy ear rims (Y)
• Red-green color blindness (X)
• Duchene muscular dystrophy (X)

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Variations on Mendelian theme

• presence of multiple alleles in populations of
  organisms for each gene-traits often controlled
  by more than 1 gene
• incomplete dominance inheritance pattern
• Ex snap dragon flowers
• RR (red) x R1R1 (white)
• F1 pink flowers
• occurs when both copies of a functional allele
  are necessary in order to produce enough protein
  to give rise to the dominant phenotype
• R red pigment code
• R1 defective enzyme code
• RR1 _ amount red pigment produced pink color

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The Effect of Recombination on Gene Linkage and
InheritanceCrossing over

• Meiosis results in exchange of bits pieces of
  DNA between homologous pairs of chromosomes at
  the chiasmata during prophase I of meiosis.
• This process of recombination results in gametes
  (or meiotic products) that are not identical
  some of the linkage groups have been changed by
  the crossing-over.
• As a result of recombination, new allele
  combinations are formed, and we have more genetic
  variation.

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crossing over

• segments of homologous chromosomes are exchanged
  w/ each other at site called chiasmata during
  meiosis I
• forms new gene combinations on both homologous
  chromosomes
• gene combinations for daughter cell different
  from parent cell

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Crossing Over and Recombination
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genetic recombination

• generation of new combinations of alleles by
  exchange of DNA between homologous chromosomes
  during crossing over

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Chromosome Mapping

• The farther apart the genes are on chromosome,
  the likelier they are to be separated by
  crossovers.
• Results of crossing over appear in offspring as
  new combinations of traits.
• The greater the of offspring that show new
  comb. of traits, the farther apart the 2 genes
  are on chrom.

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Chromosome Map

• A diagram that shows the linear sequence of genes
  on a chromosome.
• Sturtevant used Drosophila
• Fig 12-5 p.224
• 1 map unit 1

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Mutations

• Germ-cell mutation gametes, passed on to
  offspring
• Somatic mutations body cells, affect organism.
  Not passed on to offspring. i.e. leukemia
• Lethal mutations cause death. Are they
  beneficial???

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Chromosome mutations(fig 12-6)

• Deletion piece of chrom breaks off is lost
  (end) due to virus, radiation, chemicals, or
  envir factors. Most are lethal
• ExABCDEFG becomes ABCFG, Cri-du-chat syndrome
• Inversion breaks off reattaches in reverse
• Ex ABCDEFG becomes ABGFEDC
• Translocation breaks off reattaches to
  different (non-homologous chrom)
• Ex ABCDEFG becomes ABCDLMNOP

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Changes in Chrom

• Nondisjunction chrom fail to separate during
  Anaphase (gamete formation)
• Aneuploidy 1 extra or less chrom
• Monosomic-1 less
• Trisomic-1 more
• Polyploidy 3 or more of each type of chrom
  lethal for humans

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Gene Mutations (fig 12-7)

• Point Mutation substitution, addition, or
  removal of a single nucleotide.
• Substitution-one nucleotide replaced w/ a diff.
  onenew codon (fig.12-7)
• Point mutation - adenine replaced w/ thymine in
  single DNA codon defective form of protein
  hemoglobin sickle cell anemia
• Frame shift mutation (fig.12-8) occurs anytime
  of nucleotides inserted or deleted is not a
  multiple of 3.

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12-2 Human Genetics

• Humans have up to 20 times as many genes as
  Drosophila, our 23 pairs of chromosomes are
  made up of about 100,000 genes.
• Geneticists focus on disease-causing genes b/c of
  concern for human pop.

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Pedigree Analysis

• Pedigree a family record that shows how a trait
  is inherited over several generations (fig 12-9,
  p.227).
• Certain phenotypes are usually repeated in
  predictable patterns from 1 generation to the
  next patterns of inheritance.
• Carriers individuals who have 1 copy of a
  recessive allele, but can pass it along to their
  offspring.

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How to read a Pedigree

• Human pedigrees
• Before we consider human Mendelian inheritance it
  is convenient to consider the symbols used to
  draw pedigrees.
• Generations are numberered from the top of the
  pedigree in uppercase Roman numerals, I, II, III
  etc. Individuals in each generation are numbered
  from the left in arab numberals as subscripts,
  III1 , III2, III3 etc.

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Hemophilia pedigree of the European Royal
Families
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Inheritance of Recessive Alleles

• Any alteration of a gene, called a mutation, has
  the potential to inhibit the formation of a
  needed enzyme. With diploid organisms, however, a
  mutation most likely affects just one of the
  homologues, and the second can still code for the
  appropriate enzyme with little or no phenotypic
  effect on the individual.
• Gene alterations that affect health are called
  genetic disorders (Table 21-1, p. 228).
• Those that are just "abnormal" but do not affect
  physiological health, are called genetic
  abnormalities i.e. 6 toes
• When the genetic alteration causes a host of
  symptoms, it may be called a syndrome.
• Disease illness caused by infections, not by
  inheritance

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Single Allele Traits

• Controlled by a single allele of a gene.
• gt200 human traits governed by single dominant
  allele.
• Ex Huntingtons Disease (HD) caused by dominant
  allele located on an autosome autosomal
  dominant pattern of inhertiance
• Read decription of HD on p.230, Table 12-3

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HD continued

• Geneticists discovered a genetic marker for HD
  allele.
• Genetic marker is a short section of DNA known to
  have a close association w/ a particular gene
  located nearby. Easy to I.D. the HD allele.
• If marker is present 96 chance of dev HD
• Parents can be tested for marker before
  conceiving a child.

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Other Single-Allele Traits (Table 12-3)

• Homozygous recessive (must have 2 copies of
  recessive allele)
• Cystic Fibrosis
• Sickle Cell Anemia
• these are recessive alleles located on autosomes
  Autosomal Dominant
• pattern of inhertiance

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polygenic inheritance

• inheritance pattern in which the interaction of
  two or more functionally similar genes contribute
  to a single phenotype
• Ex human skin color eye color

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Multiple Allele Traits

• Controlled by 3 or more alleles of same gene that
  code for a single trait.
• Humans, ABO blood groups controlled bt 3 alleles
  IA, AB, i.
• Each genotype consists of 2 of these alleles.
• IA AB are codominant (both expressed when
  together), both are dominant to I allele.
• Table 12-2, p. 229

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polygenic inheritance

• Trait that is controlled by 2 or more genes
  many different variations
• Ex Skin Color-influenced by additive effects of
  3 to 6 genes. Each gene results in certain amount
  of melanin (brownish-black pigment)
• More melanindarker
• Ex eye color. Light blue eyesvery little
  melanin
• Human height polygenic, but influenced by
  environmental factors, such as disease nutrition

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Pleiotropy

• Genes with More Than One Effect - Pleiotropy
  (Pleio means "more")
• The phenotype conferred by the gene can result in
  many additional alterations in the individual,
  many of which seem unrelated to each other. In
  reality, most genes are probably pleiotropic, but
  some are dramatically so.
• Some Examples
• Albino condition (No Pigment)
• Eye and skin sensitivity to light in many animals
• Frizzle feathers in chickens -- affects feather
  shape
• Feathers can't insulate properly
• Metabolic problems relating to inability to
  thermoregulate.
• Abnormal hemoglobin molecule -- affects shape of
  hemoglobin Protein

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X-Linked

• Only found on X chromosome
• Colorblindness red/green, most common in males
  (8)
• Hemophilia blood clotting, affects males
• Duchene Muscular Dystrophy weakens destroys
  muscle tissue
• Not all X-Linked traits are diseases Only a
  few of hundereds of genes on X chrom. Others code
  for normal functioning proteins

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Sex-Influenced Traits

• Influenced by presence of male or female sex
  hormones.
• Alleles that code for most sex-infl traits
  located on autosomes.
• Males Females have different phenotypes, but
  same genotype.
• Ex pattern baldness controlled by allele B,
  dominant in males 7 recessive in females
• Men women who are BB lose hair
• Allele B normal, nonbald phenotype, so BB
  woman will not lose hair, but BB man will.
• Testosterone in men reacts w/ BB

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Disorders due to Nondisjunction

• Occurs during meiosis causes gametes to lack a
  chromosome or have an extra one (fig 12-10, p.
  231)
• A zygote w/ 45 chrom. monosomy
• 47 chrom. trisomy
• Often lethal
• Trisomy 21 Down Syndrome mild to severe
  mental retardation

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Nondisjunction cont.

• Males w/ 1 extra X chrom. Klinefelters
  syndrome (XXY). Some feminine characteristics,
  mentally retarded infertile
• Turners Syndrome Have a single x chrom. (XO)
  female appearance, but do not amture sexually
  remain infertile. What happens if zygote only
  receives a Y chrom.?

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Detection (fig 12-11, p.232)

• Genetic Screening Karyotype (picture of
  persons chrom.
• Amniocentesis procedure removes amniotic fluid
  from fetus tested (14th-16th week of pregnancy)
• Chorionic Villi Sampling Removes fetal cells
  from chorion fluid (between mothers uterus
  fetus)
• Ultrasound Sound waves to observe fetus

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Detection Cont.

• In U.S. 1 out of 10,000 babies is afflicted w/
  phenylketonuria (PKU) body cannot metabolize
  amino acid phenylalanine brain damage.
• Genetic Counseling medical guidance for couples
  at risk

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References

• http//www.scidiv.bcc.ctc.edu/rkr/Biology101/lectu
  res/pdfs/HumanInheritance101.pdf