Human Inheritance

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Human Inheritance

• Chapter 14

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Human Genetic Analysis

• Inheritance patterns in humans are typically
  studied by tracking observable traits that crop
  up in families over many generations
• Data is organized in pedigree charts

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Human Genetic Analysis

• Pedigree chart symbols

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Human Genetic Analysis
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Human genetic analysis

• Some easily observed human traits follow
  Mendelian inheritance patterns
• Controlled by a single gene
• Two alleles, one dominant and one recessive

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Human Genetic Analysis

• Mid-digital hair
• Dominant condition (MM, Mm)
• Presence of hair between the two top joints of
  your fingers
• Even the slightest amount of hair indicates the
  dominant phenotype
• Recessive condition (mm)
• Complete absence of hair

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Human Genetic Analysis

• Tongue rolling
• Dominant condition (TT, Tt)
• Ability to roll ones tongue
• Recessive condition (tt)
• Inability to roll the tongue

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Human Genetic Analysis

• Widows peak
• Dominant condition (WW, Ww)
• A distinct downward point in the frontal hairline
  
• Recessive condition (ww)
• A continuous hairline

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Human Genetic Analysis

• Earlobe attachment
• Dominant condition (EE, Ee)
• Detached or free earlobes
• Recessive condition (ee)
• Earlobes attached directly to the head

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Human Genetic Analysis

• Hitchhikers thumb
• Dominant condition (HH, Hh)
• Cannot extend the thumbs backward to
  approximately 45
• Recessive condition (hh)
• Can bend the thumbs at least 45, if not further

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Autosomal Inheritance Patterns

• Dominant Patterns
• Dominant alleles are expressed in both
  homozygotes and heterozygotes
• The trait specified tends to appear in every
  generation

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Autosomal Inheritance Patterns

• Dominant Patterns
• Examples
• Achondroplasia
• The allele interferes with formation of the
  embryonic skeleton
• Adults average about 44 and have short limbs
• Huntingtons disease
• Nervous system slowly deteriorates
• Involuntary muscle movements increase

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Autosomal Inheritance Patterns

• Recessive Patterns
• Expressed only in homozygous people
• Traits may skip generations
• Heterozygotes are carriers of the allele, but do
  not express the trait

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Autosomal Inheritance Patterns

• Recessive Patterns
• Examples
• Albinism
• Lack of melanin
• Tay-Sachs disease
• Malfunction of a lysosomal enzyme that breaks
  down gangliosides
• Lipids accumulate to toxic levels in nerve cells

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X-linked Inheritance Patterns

• The X and Y chromosomes carry different genes
• Recessive alleles on the X chromosome create a
  unique pattern of inheritance

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X-linked Inheritance Patterns

• More males affected
• Each male receives one X and one Y chromosome
• Two possible genotypes XAY, XaY
• Females receive two X chromosomes
• Three possible genotypes XAXA, XAXa , XaXa
• Thus heterozygous males are affected while
  heterozygous females are not

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X-linked Inheritance Patterns

• Affected fathers cannot pass X-linked recessive
  alleles to a son
• All children who inherit their fathers X
  chromosome are female

XA
XA
Xa
XAXa
XAXa
Daughters
Y
XAY
XAY
Sons
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X-linked Inheritance Patterns

• Affected fathers cannot pass X-linked recessive
  alleles to a son
• Heterozygous females are the bridge between an
  affected male and his affected grandson

Heterozygote daughter
XA
XA
XA
Xa
Xa
XAXa
XAXa
XA
XAXA
XAXa
Affected father
Y
XAY
XAY
Y
XAY
XaY
Affected grandson
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X-linked Inheritance Patterns

• Examples
• Red-Green color blindness
• Hemophilia A
• Duchenne Muscular Dystrophy

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Fig. 14.8, p. 209
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X-linked Inheritance Patterns

• X-linked dominant alleles that cause disorders
  are rarer because they tend to be lethal in male
  embryos
• X-linked hypophosphatemic rickets
• Rett syndrome
• Aicardi syndrome

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Heritable Changes in Chromosome Structure

• Changes in chromosome structure
• Are rare
• Usually cause drastic health effects
• Responsible for 1/2 of miscarriages
• Genetic disorders
• Sometimes evolutionarily important
• Occur spontaneously or induced by exposure to
  certain chemicals or radiation

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Heritable Changes in Chromosome Structure

• Changes in chromosome structure
• Duplication
• Happens during prophase I of meiosis
• Crossing over occurs unequally between homologs
• One chromosome will have a deleted segment
• The other will have the duplication
• Huntingtons

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Heritable Changes in Chromosome Structure

• Changes in chromosome structure
• Deletion
• The loss of some portion of a chromosome
• Cause serious disorders and are often lethal in
  mammals
• Cri-du-chat
• Deletion in chromosome 5
• Causes mental impairment and an abnormally shaped
  larynx

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Heritable Changes in Chromosome Structure

• Changes in chromosome structure
• Inversion
• A segment of DNA is flips in the reverse
  direction
• Usually no genes are lost
• Causes infertility
• Inverted segments cause homologs to mispair
  during meiosis

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Heritable Changes in Chromosome Structure

• Changes in chromosome structure
• Translocation
• A piece of one chromosome may break and attach
• to a different chromosome or
• to a different part of the same chromosome
• Burkitts lymphoma reciprocal translocation
  between chromosomes 8 and 14
• Can cause infertility (mis-pairing during meiosis)

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Heritable Changes in Chromosome Structure

• Chromosome changes in Evolution
• Large-scale changes in chromosomes can lead to
  speciation
• Most are usually lethal or cause genetic
  disorders and infertility
• In a very few instances chromosome changes can be
  adaptive
• Multiple globin chain genes possibly arose due to
  duplications
• An individual homozygous for an inversion could
  become the founder of a new species

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Heritable Changes in Chromosome Number

• During meiosis homologs may fail to separate
• Referred to as nondisjunction
• Affects the chromosome number at fertilization
• Normal gamete (n) nondisjunction gamete (n 1)
  zygote (2n 1)
• Trisomic zygote will have three of one type of
  chromsome and two of every other type
• Normal gamete (n) nondisjunction gamete (n - 1)
  zygote (2n -1)
• monosomic zygote will have one of one type of
  chromsome and two of every other type

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Stepped Art
Fig. 14.12, p. 212
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Heritable Changes in Chromosome Number

• Trisomy and monosomy
• Trisomy (polyploid) is common in some plants,
  insects, and fish
• Usually fatal in humans
• Trisomy 21 individuals will survive infancy
• Down syndrome

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Fig. 14.13, p. 213
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Heritable Changes in Chromosome Number

• Changes in Sex Chromosome Number
• Turner syndrome (XO)
• Inherit an unstable Y from the dad
• Female, short
• XXX syndrome
• Usually does not result in physical or medical
  problems
• Klinefelter syndrome (XXY)
• Overweight, tall, normal intelligence, estrogen gt
  testosterone
• XYY syndrome
• Taller than average, mild mental impairment
• NOT predisposed to a life of crime

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Summary

• Human genetic analysis
• Pedigree charts
• Autosomal inheritance patterns
• Dominant and recessive
• X-linked inheritance patterns
• More common in males
• Chromosome structure changes
• Duplications
• Deletions
• Inversions
• Translocations
• Chromosome number changes
• Trisomy
• Sex chromosome numbers

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Fig. 14.17, p. 217
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p. 217