Transcript Document
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