Chromosomes and Human Inheritance Chapter 12

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

Fig. 12-4a, p. 188

Animation: Autosomal dominant inheritance

Animation: Autosomal recessive inheritance

12.3 Too Young to be Old  Progeria Genetic disorder that results in accelerated aging

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

X-Linked Recessive Inheritance Patterns

Animation: X-linked inheritance

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

Translocation  Typically, two broken chromosomes exchange parts (reciprocal translocation)

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)

Nondisjunction

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)

After Preimplantation Diagnosis  When a severe problem is diagnosed, some parents choose an induced abortion  In some cases; surgery, prescription drugs, hormone replacement therapy, or dietary controls can minimize or eliminate symptoms of a genetic disorder