Chapter 14 Chromosomes and Human Inheritance

Albia Dugger • Miami Dade College

Chapter 14Chromosomes and Human Inheritance

14.1 Shades of Skin

• Variations in skin color may have evolved as a balance between vitamin D production and UV protection

• More than 100 gene products are involved in the synthesis of melanin, and the formation and deposition of melanosomes

• Mutations in some of these genes may have contributed to regional variations in human skin color

Variation in Human Skin Color

14.1 Human Chromosomes

• Geneticists study inheritance patterns in humans by tracking genetic disorders and abnormalities through families

• Charting genetic connections with pedigrees reveals inheritance patterns of certain traits

Pedigrees

• Inheritance patterns in humans are typically studied by tracking observable traits in families over generations

• A standardized chart of genetic connections (pedigree) is used to determine the probability that future offspring will be affected by a genetic abnormality or disorder

• Pedigree analyses also reveals whether a trait is associated with a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome

Standard Symbols Used in Pedigrees

male

female

marriage/mating

offspring

individual showing trait being studied

sex not specified

generation

Polydactyly

A Pedigree for Polydactyly

A Pedigree for Huntington’s Disease

Genetic Abnormalities and Disorders

• A genetic abnormality is an uncommon version of a trait that is not inherently life-threatening,

• A genetic disorder causes medical problems that may be severe

• A genetic disorder is often characterized by a specific set of symptoms (a syndrome)

Types of Genetic Variation

• Single genes on autosomes or sex chromosomes govern more than 6,000 genetic abnormalities

• Most human traits, including skin color, are polygenic (influenced by multiple genes) and some have epigenetic contributions or causes

Patterns of Inheritance

• Based on variations in single genes (Mendelian patterns)• Autosomal dominant• Autosomal recessive• X-linked recessive• X-linked dominant

• Based on variations in whole chromosomes• Changes in chromosome number• Changes in chromosome structure

Table 14-1 p221

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

Take-Home Message: How do we study inheritance patterns in humans?

• Inheritance patterns in humans are often studied by tracking traits through generations of families

• A genetic abnormality is a rare version of an inherited trait; a genetic disorder is an inherited condition that causes medical problems

• Some human genetic traits are governed by a single gene and inherited in a Mendelian fashion; many others are influenced by multiple genes and epigenetics

ANIMATED FIGURE: Pedigree diagrams

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14.3 Autosomal Inheritance Patterns

• An allele is inherited in an autosomal dominant pattern if the trait it specifies appears in heterozygous people

• An allele is inherited in an autosomal recessive pattern if the trait it specifies appears only in homozygous people

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• Hutchinson–Gilford progeria

disorder-causing allele (dominant)

affected father

normal mother

meiosis and gamete formation

Stepped Art

affected child

normal child

Figure 14-3a p222

Figure 14-3b p222

Figure 14-3c p222

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

• Examples: • Albinism• Tay-Sachs didease

disorder-causing allele (recessive)

carrier fathercarrier mother


normal child

affected child

carrier child

Stepped Art

Figure 14-4a p223

Figure 14-4b p223

INTERACTION: Autosomal-dominant inheritance



INTERACTION: Autosomal-recessive inheritance



Take-Home Message: How do we know a trait is associated with an allele on an autosome?

• With an autosomal dominant inheritance pattern, persons heterozygous for an allele have the associated trait; the trait appears in every generation

• With an autosomal recessive inheritance pattern, only persons who are homozygous for an allele have the associated trait, which can skip generations

12.4 Examples of X-Linked Inheritance

• Traits associated with recessive alleles on the X chromosome appear more frequently in men than in women

• A man cannot pass an X chromosome allele to a son

• Mutated alleles on the X chromosome cause or contribute to over 300 genetic disorders

X-Linked Recessive Pattern

• 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

recessive allele on X chromosome

normal fathercarrier mother


affected son

normal daughter or son

carrier daughter

Stepped Art

Figure 14-6a1 p224

Some X-Linked Recessive Disorders

• 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• Bleeding caused by lack of blood-clotting protein

Figure 14-6b1 p224

Figure 14-6c1 p224

You may have one form of red–green color blindness if you see a 7 in this circle instead of a 29.

You may have another form of red–green color blindness if you see a 3 instead of an 8 in this circle.

INTERACTION: X-linked inheritance



Hemophilia A in Descendents of Queen Victoria of England

Take-Home Message: Is a trait associated with an allele on an X chromosome?

• Men who have an X-linked recessive allele have the trait associated with the allele; heterozygous women do not, they have a normal allele on their second X chromosome – the trait appears more often in men

• Men transmit an X-linked allele to their daughters, but not to their sons

14.5 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 that are repeated two or more times

• Duplication may be caused by unequal crossovers in prophase

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

• If a chromosome breaks, the broken part may get attached to a different chromosome, or to a different part of the same one

• Most translocations are reciprocal, or balanced, which means that two chromosomes exchange broken parts

• A reciprocal translocation between chromosomes 8 and 14 is the usual cause of Burkitt’s lymphoma

Translocation

D With a translocation, a broken piece of a chromosome gets reattached in the wrong place. This example shows a reciprocal translocation, in which two chromosomes exchange chunks.

Chromosome Changes in Evolution

• 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

• Example: X and Y chromosomes were once homologous autosomes in reptile-like ancestors of mammals

Evolution of X and Y Chromosomes from Homologous Autosomes

Figure 14-9a p227

Ancestral reptiles >350 mya

(autosome pair)

Figure 14-9b p227

Ancestral reptiles 350 mya

SRY

Y X

Figure 14-9c p227

Monotremes 320–240 mya

area that cannot cross

over

Y X

Figure 14-9d p227

Y X

Humans 50–30 mya

Monkeys 130–80 mya

Marsupials 170–130 mya

Y X Y X

Differences Among Closely Related Organisms

• Humans have 23 pairs of chromosomes; chimpanzees, gorillas, and orangutans have 24

• Two chromosomes fused end-to-end

chimpanzeehuman

telomere sequence

Take-Home Message:Does chromosome structure change?

• A segment of a chromosome may be duplicated, deleted, inverted, or translocated

• Such a change is usually harmful or lethal, but may be conserved in the rare circumstance that it has a neutral or beneficial effect

ANIMATION: Deletion



ANIMATION: Duplication



ANIMATION: Translocation



14.6 Heritable Changes in Chromosome Number

• Occasionally, abnormal events occur before or during meiosis, and new individuals end up with the wrong chromosome number

• Consequences range from minor to lethal changes in form and function

Polyploidy and Aneuploidy

• Many flowering plant species, and some insects, fishes, and other animals, are polyploid – they have three or more complete sets of chromosomes

• Trisomy and monosomy are examples of aneuploidy, in which an individual’s cells have too many or too few copies of a 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

Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II

Stepped Art

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: Genotype

Trisomy 21: Phenotype

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

Take-Home Message: What are the effects of chromosome number changes?

• Nondisjunction can change the number of autosomes or sex chromosomes in gametes; such changes usually cause genetic disorders in offspring

• Sex chromosome abnormalities are usually associated with some degree of learning difficulty and motor skill impairment

ANIMATION: Sources of genotype variation



14.7 Genetic Screening

• Our understanding of human inheritance can provide prospective parents with information about the health of their future children

Detecting Genetic Disorders

• Surgery, prescription drugs, hormone replacement therapy, and dietary controls can minimize and in some cases eliminate the symptoms of a genetic disorder

• Some disorders can be detected early enough to start countermeasures before symptoms develop

• Example: Most hospitals in the United States now screen newborns for mutations that cause phenylketonuria (PKU)

Family Planning

• Couples may choose to know if their future children face a high risk of inheriting a severe genetic disorder

• Genetic analysis starts with parental karyotypes, pedigrees, and genetic testing for known disorders

• Information is used to predict the probability of having a child with a genetic disorder

Prenatal Diagnosis

• In prenatal diagnosis, an embryo or fetus is tested before birth to screen for sex or genetic problems

• Noninvasive techniques include obstetric sonography

• Invasive procedures in which samples of tissue or blood are taken involve risks to mother and fetus• Fetoscopy• Amniocentesis • Chorionic villus sampling (CVS)

Ultrasound

Conventional ultrasound 4D ultrasound

Fetoscopy

Fetoscopy

Figure 14-14 p230

amniotic sac

chorion

Preimplantation Diagnosis

• Couples at high risk of having a child with a genetic disorder may choose in vitro fertilization for preimplantation diagnosis

• An undifferentiated cell is removed from the early embryo and its genes are examined before implantation

• If the embryo has no detectable genetic defects, it is inserted into the mother’s uterus to continue developing

Embryo After 3 Mitotic Divisions

Take-Home Message: How do we use what we know about human inheritance?

• Genetic testing can provide prospective parents with information about the health of their future children

Chapter 14 Chromosomes and Human Inheritance

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