Chromosomes and Human Inheritance Chapter 12. Impacts, Issues: Strange Genes, Tortured Minds ...
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Transcript of Chromosomes and Human Inheritance Chapter 12. Impacts, Issues: Strange Genes, Tortured Minds ...
Impacts, Issues:Strange Genes, Tortured Minds
Exceptional creativity often accompanies neurobiological disorders such as schizophrenia, autism, chronic depression, and bipolar disorder• Examples: Lincoln, Woolf, and Picasso
12.1 Human Chromosomes
In humans, two sex chromosomes are the basis of sex – human males have XY sex chromosomes, females have XX
All other human chromosomes are autosomes
Sex Determination in Humans
Sex of a child is determined by the father• Eggs have an X chromosome; sperm have X or Y
Sex Determination in Humans
The SRY gene on the Y chromosome is the master gene for male sex determination• Triggers formation of testes, which produce the
male sex hormone (testosterone)
Karyotyping
Karyotype• A micrograph of all metaphase chromosomes in a
cell, arranged in pairs by size, shape, and length• Detects abnormal chromosome numbers
12.2 Autosomal Inheritance Patterns
Many human traits can be traced to autosomal dominant or recessive alleles that are inherited
Some of those alleles cause genetic disorders
Autosomal Dominant Inheritance
A dominant allele is expressed in homozygotes and heterozygotes• With one homozygous recessive and one
heterozygous parent, children have a 50% chance of inheriting and displaying the trait
Autosomal Recessive Inheritance
Autosomal recessive alleles are expressed only in homozygotes; heterozygotes are carriers and do not have the trait• A child of two carriers has a 25% chance of
expressing the trait
12.4 Examples of X-Linked Inheritance
Mutated alleles on the X chromosome cause or contribute to over 300 genetic disorders
X-Linked Inheritance Patterns
More males than females have X-linked recessive genetic disorders• Males have only one X chromosome and can
express a single recessive allele
Some X-Linked Recessive Disorders
Hemophilia A• Bleeding caused by lack of blood-clotting protein
Duchenne muscular dystrophy• Degeneration of muscles caused by lack of the
structural protein dystrophin
12.5 Heritable Changes in Chromosome Structure
On rare occasions, a chromosome’s structure changes; such changes are usually harmful or lethal, rarely neutral or beneficial
A segment of a chromosome may be duplicated, deleted, inverted, or translocated
Duplication
DNA sequences are repeated two or more times; may be caused by unequal crossovers in prophase I
Deletion
Loss of some portion of a chromosome; usually causes serious or lethal disorders• Example: Cri-du-chat
Inversion
Part of the sequence of DNA becomes oriented in the reverse direction, with no molecular loss
Nondisjunction
Changes in chromosome number can be caused by nondisjunction, when a pair of chromosomes fails to separate properly during mitosis or meiosis
Affects the chromosome number at fertilization• Monosomy (n-1)• Trisomy (n+1)
Autosomal Change and Down Syndrome
Only trisomy 21 (Down syndrome) allows survival to adulthood• Characteristics include physical appearance,
mental impairment, and heart defects
Incidence of nondisjunction increases with maternal age
Change in Sex Chromosome Number
Changes in sex chromosome number may impair learning or motor skills, or be undetected
Female sex chromosome abnormalities• Turner syndrome (XO)• XXX syndrome (three or more X chromosomes)
Male sex chromosome abnormalities• Klinefelter syndrome (XXY)• XYY syndrome
Turner Syndrome
XO (one unpaired X chromosome)• Usually caused by
nondisjunction in the father
• Results in females with undeveloped ovaries
Defining Genetic Disorders and Abnormalities
Genetic disorder• An inherited condition that causes mild to severe
medical problems, characterized by a specific set of symptoms (a syndrome)
12.8 Prospects in Human Genetics
Genetic analysis can provide parents with information about their future children
Genetic counseling• Starts with parental genotypes, and genetic
testing for known disorders• Information is used to predict the probability of
having a child with a genetic disorder
Prenatal Diagnosis
Tests done on an embryo or fetus before birth to screen for sex or genetic problems
Three types of prenatal diagnosis• Amniocentesis • Chorionic villus sampling (CVS)