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