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