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

– chromosomes that are the same in males and

females

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)

• Without testosterone, ovaries develop and

produce female sex hormones (estrogens)

Sexual Development in Humans

Fig. 12-2a, p. 186

diploid

germ cells

in female

diploid

germ cells

in male

meiosis, gamete

formation in both

female and male:

eggs sperm

X × Y

× X X

fertilization:

X X

X XX XX

Y XY XY

sex chromosome combinations

possible in the new individual

Karyotyping

Karyotype

• A micrograph of all metaphase chromosomes in a

cell, arranged in pairs by size, shape, and length

• Detects abnormal chromosome numbers and

some structural abnormalities

Construction of a karyotype

• Colchicine stops dividing cells at metaphase

• Chromosomes are separated, stained,

photographed, and digitally rearranged

Karyotyping

12.1 Key Concepts

Autosomes and Sex Chromosomes

All animals have pairs of autosomes –

chromosomes that are identical in length, shape,

and which genes they carry

Sexually reproducing species also have a pair of

sex chromosomes; the members of this pair

differ between males and females

12.2 Autosomal Inheritance Patterns

Many human traits can be traced to autosomal

dominant or recessive alleles that are inherited

in Mendelian patterns

Some of those alleles cause genetic disorders

Autosomal Dominant Inheritance

A dominant autosomal allele is expressed in

homozygotes and heterozygotes

• Tends to appear in every generation

• With one homozygous recessive and one

heterozygous parent, children have a 50%

chance of inheriting and displaying the trait

• Examples: achondroplasia, Huntington’s disease

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

• Example: galactosemia

Autosomal Inheritance

Galactosemia

Neurobiological Disorders

Most neurobiological disorders do not follow

simple patterns of Mendelian inheritance

• Depression, schizophrenia, bipolar disorders

Multiple genes and environmental factors

contribute to NBDs

12.3 Too Young to be Old

Progeria

• Genetic disorder that results in accelerated aging

• Caused by spontaneous mutations in autosomes

12.2-12.3 Key Concepts

Autosomal Inheritance

Many genes on autosomes are expressed in

Mendelian patterns of simple dominance

Some dominant or recessive alleles result in

genetic disorders

12.4 Examples of X-Linked Inheritance

X chromosome alleles give rise to phenotypes

that reflect Mendelian patterns of 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

• A female heterozygote has two X chromosomes

and may not show symptoms

Males transmit an X only to their daughters, not

to their sons

X-Linked Recessive Inheritance Patterns

Some X-Linked Recessive Disorders

Hemophilia A

• Bleeding caused by lack of blood-clotting protein

Red-green color blindness

• Inability to distinguish certain colors caused by

altered photoreceptors in the eyes

Duchenne muscular dystrophy

• Degeneration of muscles caused by lack of the

structural protein dystrophin

Hemophilia A in Descendents

of Queen Victoria of England

Red-Green Color Blindness

Fig. 12-9c, p. 191

Fig. 12-9d, p. 191

12.4 Key Concepts

Sex-Linked Inheritance

Some traits are affected by genes on the X

chromosome

Inheritance patterns of such traits differ in males

and females

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

Deletion: 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)

Does Chromosome Structure Evolve?

Changes in chromosome structure can reduce

fertility in heterozygotes; but accumulation of

multiple changes in homozygotes may result in

new species

Certain duplications may allow one copy of a

gene to mutate while the other carries out its

original function

Differences Among

Closely Related Organisms

Humans have 23 pairs

of chromosomes;

chimpanzees, gorillas,

and orangutans have

24

• Two chromosomes

fused end-to-end

Evolution of X and Y Chromosomes

from Homologous Autosomes

12.6 Heritable Changes in

the Chromosome Number

Occasionally, new individuals end up with the

wrong chromosome number

• Consequences range from minor to lethal

Aneuploidy

• Too many or too few copies of one chromosome

Polyploidy

• Three or more copies of each chromosome

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 gamete)

• Trisomy (n+1 gamete)

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

Can be detected through prenatal diagnosis

Trisomy 21

Fig. 12-13b, p. 194

n + 1

n + 1

n − 1

n − 1

chromosome

alignments at

metaphase I

NONDISJUNCTION

AT ANAPHASE I

alignments at

metaphase II

CHROMOSOME

NUMBER

IN GAMETES anaphase II

Down Syndrome and 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

12.5-12.6 Key Concepts: Changes in

Chromosome Structure or Number

On rare occasions, a chromosome may undergo

a large-scale, permanent change in its structure,

or the number of autosomes or sex

chromosomes may change

In humans, such changes usually result in a

genetic disorder

12.7 Human Genetic Analysis

Charting genetic connections with pedigrees

reveals inheritance patterns for certain alleles

Pedigree

• A standardized chart of genetic connections

• Used to determine the probability that future

offspring will be affected by a genetic abnormality

or disorder

Studying Inheritance in Humans

Genetic studies can

reveal inheritance

patterns or clues to

past events

• Example: A link

between a Y

chromosome and

Genghis Khan?

Defining Genetic Disorders

and Abnormalities

Genetic abnormality

• A rare or uncommon version of a trait; not

inherently life threatening

Genetic disorder

• An inherited condition that causes mild to severe

medical problems, characterized by a specific set

of symptoms (a syndrome)

Some Human Genetic Disorders

and Genetic Abnormalities

Table 12-1, p. 196

Stepped Art

Recurring Genetic Disorders

Mutations that cause genetic disorders are rare

and put their bearers at risk

Such mutations survive in populations for

several reasons

• Reintroduction by new mutations

• Recessive alleles are masked in heterozygotes

• Heterozygotes may have an advantage in a

specific environment

A Pedigree for Huntington’s Disease

A progressive degeneration of the nervous

system caused by an autosomal dominant allele

Constructing a Pedigree for Polydactyly

12.8 Prospects in Human Genetics

Genetic analysis can provide parents with

information about their future children

Genetic counseling

• Starts with parental genotypes, pedigrees, 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

• Involves risks to mother and fetus

Three types of prenatal diagnosis

• Amniocentesis

• Chorionic villus sampling (CVS)

• Fetoscopy

Amniocentesis

Fetoscopy

Preimplantation Diagnosis

Used in in-vitro fertilization

• An undifferentiated cell is removed from the early

embryo and examined before implantation

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

• Example: PKU can be managed with dietary

restrictions

Genetic Screening

Genetic screening (widespread, routine testing

for alleles associated with genetic disorders)

• Provides information on reproductive risks

• Identifies family members with a genetic disorder

• Used to screen newborns for certain disorders

• Used to estimate the prevalence of harmful

alleles in a population

12.7-12.8 Key Concepts

Human Genetic Analysis

Various analytical and diagnostic procedures

often reveal genetic disorders

What an individual, and society at large, should

do with the information raises ethical questions