How Genes Are Transmitted from Generation to Generation

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					How Genes Are Transmitted from
 Generation to Generation

          Chapter 4
Central Points

 Genes are transmitted from generation to
  generation

 Traits are inherited according to predictable
  rules

 Dominant, recessive, and X-linked traits follow
  these rules
Case A: A Family’s Dilemma

 Alan’s mother died from Huntington disease
  (HD)

 HD caused by a mutant gene and one copy of
  gene will cause the disease

 Neurologic symptoms develop between ages
  30–50, progress slowly, fatal in 10–20 years

 Genetic test available
4.1 How Are Genes Transmitted?

 Gregor Mendel: father of genetics

 Experiments with pea plants in 1800s

 Traits, distinguishing characteristics

 Specific patterns in the way traits were passed
  from parent to offspring
Mendel’s Experiments

 Some traits disappeared in the first generation of
  offspring (all tall)

 Reappeared in 3:1 ratio (tall:short)

 Dominant trait present in the first-generation
  offspring (tall)

 Recessive trait absent in first generation but
  reappeared in the next generation (short)
Traits Are Passed by Genes


 “Factors” or genes transmitted from parent to
  offspring

 Each parent carries a pair of genes for a trait but
  contributes only one gene to each offspring

 Separation of gene pair occurs during meiosis
Genes

 Alleles: variations of a gene

 Homozygous: identical alleles of a gene
  • TT or tt


 Heterozygous: nonidentical alleles
  • Tt
Different Plant Heights
Phenotype and Genotype

 Phenotype: what an organism looks like
  • tall or short

 Genotype: genetic makeup
  • TT, Tt, and tt


 Identical phenotypes may have different
  genotypes
  • TT or Tt have tall phenotype
Mendel’s Law of Segregation


 Two copies of each gene separate during
  meiosis

 One copy of each gene in the sperm or egg

 Each parent gives one copy of each gene
Mendel’s Law of Independent Assortment


 Members of a gene pair segregate into gametes
  independently of other gene pairs

 Gametes can have different combinations of
  parental genes
Sorting of Alleles
Animation: Segregation of alleles: pea
plants
Human Traits: Albinism

 Pigmentation dominant
  and lack of pigment
  recessive
  • AA, Aa: Pigmented
  • aa: Albino


 Both parents Aa, each
  child has 25% chance of
  being albino (3:1 ratio)
Segregation of the Albino Allele
Fig. 4-3a, p. 61
Fig. 4-3b, p. 61
Pedigree 1

 Shows all family members and identifies those
  affected with the genetic disorder
Pedigree 2
Pedigree Symbols
p. 62
p. 62
Proband

 Person who is the focus of the pedigree

 Indicated by an arrow and the letter P
Animation: Pedigree analysis - predicting
future generations
Animation: Observing Patterns in
Inherited Traits (Crossing Pea Plants)
Animation: Observing Patterns in
Genetic Traits (genetic terms)
Animation: Chromosomes and Human
Inheritance (pedigree diagrams)
4.2 Examining Human Pedigrees

 Determine trait has dominant or recessive
  inheritance pattern

 Predict genetic risk for:
   • Pregnancy outcome
   • Adult-onset disorder
   • In future offspring
Three Possible Patterns of Inheritance

 Autosomal recessive

 Autosomal dominant

 X-linked recessive

 Autosomal on chromosomes 1–22

 X-linked traits on the X chromosome
Autosomal Recessive

 Unaffected parents can have affected children

 All children of affected parents are affected

 Both parents Aa, risk of affected child is 25%

 ~Equal affected male and female

 Both parents must transmit the gene for a child
  to be affected
Consanguinity

 Individuals related to each other and indicated
  by double line between parents
Autosomal Recessive Pedigree
Autosomal Recessive Genetic Disorders
Albinism
 A = normal coloring; a = albinism

 Group of genetic conditions, lack of pigmentation
  (melanin) in the skin, hair, and/or eyes

 Normally, melanin in pigment granules inside
  melanocytes

 In albinism, melanocytes present but cannot make
  melanin

 Oculocutaneous albinism type I (OCA1)
Cystic Fibrosis (CF)
 C = normal; c = cystic fibrosis

 CF affects glands that produce mucus and
  digestive enzyme

 CF causes production of thick mucus in lungs
  blocks airways

 Develop obstructive lung diseases and infections

 Identified CF gene and protein (CFTR)
Animation: Segregation of alleles: cystic
fibrosis
Sickle Cell Anemia (SCA)

 S = normal red blood cells; s = sickle)

 High frequency in areas of West Africa,
  Mediterranean Sea, India

 Abnormal hemoglobin molecules aggregate to
  form rods

 Red blood cells, crescent- or sickle-shaped,
  fragile and break open
Normal and Sickled Cells
Autosomal Dominant (1)

 Requires one copy of the allele (Aa) rarely present
  in a homozygous condition (AA)

 aa: Unaffected individuals

 Affected individual has at least one affected parent

 Aa X aa: Each child has 50% chance of being
  affected
Autosomal Dominant (2)

 ~Equal numbers of affected males and females

 Two affected individuals may have unaffected
  children

 Generally, AA more severely affected, often die
  before birth or in childhood
Autosomal Dominant Pedigree
Autosomal Dominant Genetic Disorders
Animation: Chromosomes and Human
Inheritance (autosomal-dominant inheritance)
Animation: Chromosomes and Human
Inheritance (autosomal-recessive inheritance)
Neurofibromatosis (NF)
 N = Neurofibromatosis 1; n = normal

 Many different phenotypes

 Café-au-lait spots, or noncancerous tumors in
  the nervous system can be large and press on
  nerves

 Deformities of the face or other body parts
  (rarely)

 NF gene has a very high mutation rate
Neurofibromatosis
Huntington Disease (HD)
 H = Huntington disease; h = normal

 Causes damage in brain from accumulation of
  huntingtin protein

 Symptoms begin slowly (30–50 years old)

 Affected individuals may have already had
  children (50% chance with one Hh parent)

 Progressive neurological signs, no treatment,
  die within 10–25 years after symptoms
Brain Cells of a Person with HD
Adult-Onset Disorders

 Expressed later in life

 Present problems in pedigree analysis, genetic
  testing may be required

 Examples:
   • Huntington disease (HD)
   • Adult polycystic kidney disease (ADPKD)


 Both examples are autosomal dominant
Case A Questions

 Who should be tested?

 Who should know the results of the test?

 How should the test results be used?

 See the textbook for further questions on this
  case
4.3 X-Linked Recessive Traits

 Genes on X chromosome: X-linked

 Genes on Y chromosome: Y-linked

 For X-linked traits:
   • Females XX, XX*, or X*X*
   • Males XY or X*Y
   • Males cannot be homozygous or heterozygous,
     they are hemizygous for genes on X
   • Distinctive pattern of inheritance
X-Linked Recessive Inheritance

 Mother gives one X chromosome to offspring

 Father gives X to daughters and Y to sons

 Sons carry X from mother

 For recessive traits, X*X* and X*Y affected

 More males affected
Pedigrees: X-Linked Inheritance
X-Linked Recessive Genetic Disorders
Inheritance of X-Linked Disorder
Animation: Chromosomes and Human
Inheritance (X-linked inheritance)
Duchenne Muscular Dystrophy (DMD) (1)
 XM = normal; Xm = muscular dystrophy

 Most common form, affects ~1/3,500 males

 Infants appear healthy, symptoms age ~1–6 years

 Rapid, progressive muscle weakness

 Usually must use a wheelchair by age 12

 Death, age ~20 from respiratory infection or
  cardiac failure
Duchenne Muscular Dystrophy (DMD) (2)

 DMD gene on the end of X chromosome

 Encodes protein dystrophin that supports
  plasma membrane during contraction

 If dystrophin absent or defective, cells are torn
  apart

 Two forms: DMD, and less-serious Becker
  muscular dystrophy (BMD)
Cells of a Person with MD
Hemophilia
 XH = normal; Xh = hemophilia

 Lack of clotting: factor VIII in blood

 Affected individuals hemorrhage, often require
  hospitalization to treat bleeding

 Hemophilia A most common form of X-linked
  hemophilia

 Females affected if XhXh, both parents must
  carry the trait
Factor VIII

 1980s, half of all
  people with
  hemophilia became
  infected with HIV

 Recombinant DNA
  technology now used
  to make clotting
  factors free from
  contamination
Case B: The Franklins Find Out More

 Alan and siblings concerned about inheriting HD
  gene for themselves and future children

 Who should be tested and why?

 How will it affect health insurance coverage?

 See the textbook for further questions on this
  case

				
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