Chapter 9 Patterns of Inheritance

9.1 Menacing Mucus Cystic fibrosis, the most common fatal genetic disorder in the US, is caused by a deletion in the CFTR gene The CF allele persists at high frequency despite devastating effects Only those homozygous for the CF allele have the disorder

Victims of Cystic Fibrosis Figure 9.1 A few of the many victims of cystic fibrosis. At least one young person dies every day in the United States from complications of this disease.

ANIMATED FIGURE: Crossing garden pea plants To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

9.2 Tracking Traits Mid-1800s: Genes and chromosomes were unknown; Gregor Mendel’s experiments with pea plants established principles of inheritance

Breeding Garden Peas carpel anther A) The flowers of garden pea plants have reproductive parts called anthers and carpels. Pollen grains that form in anthers produce male gametes; female gametes form in carpels. Figure 9.2 Animated! A breeding experiment with garden pea plants. 6

Breeding Garden Peas B) Experimenters can control the transfer of hereditary material from one pea plant to another by snipping off a flower’s anthers (to prevent the flower from self-fertilizing), and then brushing pollen from another flower onto its carpel. In this example, pollen from a plant that has purple flowers is brushed onto the carpel of a white-flowered plant. Figure 9.2 Animated! A breeding experiment with garden pea plants.

Breeding Garden Peas C) Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant. Figure 9.2 Animated! A breeding experiment with garden pea plants.

Breeding Garden Peas D) Every plant that arises from the cross has purple flowers. Predictable patterns such as this are evidence of how inheritance works. Figure 9.2 Animated! A breeding experiment with garden pea plants.

Inheritance in Modern Terms Organisms breed true for a trait because they carry identical alleles of genes governing that trait Homozygous Having identical alleles of a gene Heterozygous Having two different alleles of a gene

Inheritance in Modern Terms The particular set of alleles an individual carries is the individual’s genotype Gene expression results in phenotype – an individual’s observable traits An allele is dominant when its effect masks that of a recessive allele paired with it

Genotype gives rise to phenotype PP (homozygous for dominant allele P) pp (homozygous for recessive allele p) Figure 9.3 Genotype gives rise to phenotype. In this example, the dominant allele P specifies purple flowers; the recessive allele p, white flowers. Pp (heterozygous for alleles P and p) 12

9.3 Mendelian Inheritance Patterns A cross (mating) between heterozygous individuals can reveal dominance relationships among the alleles under study Monohybrid cross Cross in which individuals with different alleles of a gene are crossed Dominant trait will have a 3:1 phenotype ratio

Segregation of Genes When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate Each gamete that forms carries only one of the two genes of a pair

Segregation of Genes DNA replication meiosis I meiosis II gametes (P) 1 2 DNA replication meiosis I meiosis II gametes (P) gametes (p) Figure 9.4 Segregation of genes on homologous chromosomes into gametes. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries one of the two members of each gene pair. For clarity, only one set of chromosomes is illustrated. 1 All gametes made by a parent homozygous for a dominant allele carry that allele. 2 All gametes made by a parent homozygous for a recessive allele carry that allele. 3 If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. All offspring of this cross will be heterozygous. zygote (Pp) 3 15

DNA replication meiosis I gametes (p) meiosis II gametes (P) 1 2 gametes (p) meiosis II gametes (P) zygote (Pp) 3 Figure 9.4 Segregation of genes on homologous chromosomes into gametes. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries one of the two members of each gene pair. For clarity, only one set of chromosomes is illustrated. 1 All gametes made by a parent homozygous for a dominant allele carry that allele. 2 All gametes made by a parent homozygous for a recessive allele carry that allele. 3 If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. All offspring of this cross will be heterozygous. Stepped Art Figure 9-4 p153

Punnett Squares Punnett squares are used to calculate the probability of the genotype and phenotype of offspring of crosses In a testcross, an individual with a dominant trait (but an unknown genotype) is crossed with an individual known to be homozygous for the recessive allele The pattern of traits among offspring can reveal whether the tested individual is heterozygous or homozygous

Punnett Squares: A Monohybrid Cross Figure 9.5 Making a Punnett square. Parental gametes are listed in circles on the top and left sides of a grid. Each square is filled with the combination of alleles that would result if the gametes in the corresponding row and column met up.

Monohybrid cross: First generation parent plant homozygous for purple flowers parent plant homozygous for white flowers Pp hybrid Figure 9.6 Animated! A monohybrid cross. two types of gametes A) All of the F1 (first generation) offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous (Pp). These offspring make two types of gametes: P and p.

Monohybrid cross: Second generation Figure 9.6 Animated! A monohybrid cross. B) A cross between two of the identically heterozygous F1 offspring is a monohybrid cross. In this example, the phenotype ratio among the F2 (second generation) offspring is 3:1 (three purple to one white).

Dihybrid Crosses Mendel’s dihybrid crosses showed inheritance of one trait did not affect inheritance of other traits Dihybrid cross Experiment in which individuals with different alleles of two genes are crossed (9:3:3:1 ratio) Independent assortment A gene tends to be distributed independently of how other genes are distributed

Dihybrid Cross: First generation parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems 1 Figure 9.7 Animated! A dihybrid cross between plants that differ in flower color and plant height. In this example, P and p are dominant and recessive alleles for flower color; T and t are dominant and recessive alleles for height 2 PpTt dihybrid 3 four types of gametes

Dihybrid Cross: Second generation Figure 9.7 Animated! A dihybrid cross between plants that differ in flower color and plant height. In this example, P and p are dominant and recessive alleles for flower color; T and t are dominant and recessive alleles for height 4

The Contribution of Crossovers Two genes located close together on the same chromosome tend to be inherited together When two genes on the same chromosome are far apart, crossing over occurs more frequently between them; they tend to assort independently

A Human Example: Skin Color Variations in skin color depend on the kinds and amounts of melanins produced More than 100 gene products affect production and deposition of melanins Independent assortment of these genes produces a wide variety of phenotypes

Variation in Skin Color Figure 9.8 Variation in human skin color (left) begins with differences in alleles.

ANIMATED FIGURE: Dihybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATED FIGURE: Independent assortment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATED FIGURE: Monohybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATED FIGURE: Test Cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

9.4 Beyond Simple Dominance Codominant Refers to two alleles that are both fully expressed in heterozygous individuals Incomplete dominance Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

Codominance: Blood Type Figure 9.9 Combinations of alleles that are the basis of blood type. AA or AO BB or BO Genotypes: AB OO Phenotypes (blood type): A AB B O

Incomplete Dominance homozygous parent (RR) homozygous parent (rr) Figure 9.10 Animated! Incomplete dominance in heterozygous (pink) snapdragons. One allele (R) results in the production of a red pigment; the other (r) results in no pigment. homozygous parent (RR) homozygous parent (rr) heterozygous offspring (Rr) X A) Cross a red-flowered with a white-flowered snap-dragon, and all of the offspring will have pink flowers.

Incomplete Dominance Figure 9.10 Animated! Incomplete dominance in heterozygous (pink) snapdragons. One allele (R) results in the production of a red pigment; the other (r) results in no pigment. B) If two of the pink-flowered snapdragons are crossed, the phenotypes of their offspring will occur in a 1:2:1 ratio.

Epistasis Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis Epistasis Effect in which a trait is influenced by the products of multiple genes Example: Labrador retriever coat color

Epistasis Figure 9.11 Animated! Epistasis in dogs. Interactions among products of two gene pairs affect coat color in Labrador retrievers. All dogs with an E and a B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur.

Pleiotropy Products of pleiotropic genes influence two or more traits Mutations in pleiotropic genes are associated with sickle cell anemia, cystic fibrosis, and Marfan syndrome Figure 9.12 Marfan syndrome. Basketball star Haris Charalambous died suddenly in 2006 when his aorta burst during warmup exercises. He was 21. Charalambous was very tall and lanky, with long arms and legs —traits that are valued in professional athletes such as basketball players. These traits are also associated with About 1 in 5,000 people are affected by Marfan syndrome worldwide. Like many of them, Charalambous did not realize he had the syndrome.

INTERACTION: Incomplete dominance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATION: Chicken combs To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATION: Dog color To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

9.5 Complex Variations in Traits Mutations, interactions among genes, and environmental conditions can affect one or more steps in a metabolic pathway, and contribute to variation in phenotypes Example: Seasonal changes affect production of pigments that color the skin and fur of many animals Example: Water flea phenotypes depend on whether the aquatic insects that prey on them are present Example: Genetically identical yarrow plants grow to different heights at different altitudes

Snowshoe Hare in Summer and Winter Figure 9.13 Examples of environmental effects on phenotype. A) The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left ); in winter, it is white (right ). The variation offers seasonally appropriate camouflage from predators.

Water flea, with and without predators Figure 9.13 Examples of environmental effects on phenotype. B) The body form of the water flea on the top develops in environments with few predators. A longer tail spine and a pointy head (bottom) develop in response to chemicals emitted by predatory insects.  

Elevation (meters above sea level) Environmental Effects on Plant Phenotypes 60 Height (centimeters) Figure 9.13 Examples of environmental effects on phenotype. 3060 1400 30 Elevation (meters above sea level) C) The height of a mature yarrow plant depends on the elevation at which it grows.

Continuous Variation Continuous variation A range of small increments of phenotype in a trait that is influenced by the products of multiple genes The more genes and other factors that influence a trait, the more continuous the distribution of phenotype Bell curve Curve that results when range of variation in a continuous trait is plotted against frequency in a population

Continuous Variation in Eye Color Figure 9.14 Human iris color, a trait that varies continuously.

Continuous Variation in Height Figure 9.15 Animated! Example of continuous variation. A) Male biology students at the University of Florida were divided into categories of one-inch increments in height and counted.

Continuous Variation (Bell Curve) 20 15 10 Number of individuals 5 Figure 9.15 Animated! Example of continuous variation. 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 B) Graphing the resulting data produces a bell-shaped curve, an indication that height varies continuously. 

9.5 Human Genetic Analysis Inheritance patterns in humans are studied by following inherited genetic disorders in a family through generations and graphing results as a pedigree chart Pedigree analyses shows whether a trait is associated with a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome

Pedigree: Polydactyly Figure 9.16 Animated! An example of a pedigree.

Types of Genetic Variation Single genes on autosomes or sex chromosomes govern more than 6,000 genetic abnormalities and disorders Genetic abnormality An uncommon version of a heritable trait that does not result in medical problems Genetic disorder A heritable condition that results in a syndrome of mild or severe medical problems

ANIMATION: Coat color in the Himalayan rabbit To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATION: Height Graph To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

9.6 Human Genetic Disorders Some dominant or recessive alleles on autosomes or the X chromosome are associated with genetic abnormalities or disorders An autosomal dominant allele is expressed in homozygotes and heterozygotes An autosomal recessive allele is expressed only in homozygotes

Some Autosomal Dominant Traits

Autosomal Dominant Inheritance normal mother affected father X meiosis and gamete formation Figure 9.17 Animated! Autosomal dominant inheritance. A) A dominant allele on an autosome (red ) is fully expressed in heterozygous people affected child normal child disorder-causing allele (dominant) 56

disorder-causing allele (dominant) affected father normal mother meiosis and gamete formation affected child normal child Figure 9.17 Animated! Autosomal dominant inheritance. Stepped Art Figure 9-17a p163

ANIMATED FIGURE: Pedigree diagrams To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Some Autosomal Recessive Traits

Autosomal Recessive Inheritance carrier mother carrier father X A) Only people homozygous for a recessive allele on an autosome have the trait associated with the allele.  In this example, both parents are carriers (red). Each of their children has a 25 percent chance of inheriting two alleles, and being affected by the trait.  meiosis and gamete formation Figure 9.18 Animated! Autosomal recessive inheritance. affected child carrier child normal child B) The albino phenotype is associated with autosomal recessive alleles that cause a deficiency in melanin.   disorder-causing allele (recessive) 60

meiosis and gamete formation disorder-causing allele (recessive) carrier father carrier mother meiosis and gamete formation Figure 9.18 Animated! Autosomal recessive inheritance. normal child affected child carrier child Stepped Art Figure 9-18a p163

Victim of Tay–Sachs disease Figure 9.19 Conner Hopf, who was diagnosed with Tay–Sachs disease at age 7–1/2 months. He died at 22 months.

X-Linked Recessive Disorders Alleles on the X chromosome are inherited and expressed differently in males and females Males cannot transmit a recessive X-linked allele to their sons Females pass X-linked alleles to male offspring Example: red-green color blindness

Some X-Linked Recessive Disorders

X-Linked Recessive Inheritance carrier mother normal father X meiosis and gamete formation A) In this example of X-linked inheritance, the mother carries a recessive allele on one of her two X chromosomes (red ). Figure 9.20 Animated! X-linked recessive inheritance. normal daughter or son carrier daughter affected son recessive allele on X chromosome 65

meiosis and gamete formation recessive allele on X chromosome normal father carrier mother meiosis and gamete formation Figure 9.20 Animated! X-linked recessive inheritance. affected son normal daughter or son carrier daughter Stepped Art Figure 9-20a p165

Red–green color blindness Figure 9.20 Animated! X-linked recessive inheritance. B) A view of color blindness. The image on the left shows how a person with red–green color blindness sees the image on the right. The perception of blues and yellows is normal; red and green appear similar. 

Red–green color blindness You may have one form of red–green color blindness if you see a 7 instead of a 29 in this circle. C) Part of a standardized test for color blindness. A set of 38 of these circles is commonly used to diagnose deficiencies in color perception.  Figure 9.20 Animated! X-linked recessive inheritance. You may have another form of red–green color blindness if you see a 3 instead of an 8 in this circle.

INTERACTION: Autosomal-dominant inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

INTERACTION: Autosomal-recessive inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

INTERACTION: X-linked inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

VIDEO: Genetics, Sociology, and Breast Cancer

9.8 Changes in Chromosome Number Many flowering plants, and some insects, fishes and other animals are polyploid – having three or more of each type of chromosome characteristic of the species Chromosome number can change permanently, usually resulting from nondisjunction – the failure of chromosomes to separate normally during meiosis or mitosis

Nondisjunction During Meiosis Metaphase I Anaphase I Telophase I Metaphase II Figure 9.21 Animated! An example of nondisjunction during meiosis. Of the two pairs of homologous chromosomes shown here, one fails to separate during anaphase I. A zygote that forms from any of the resulting gametes will have an abnormal chromosome number. Anaphase II Telophase II

Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Figure 9.21 Animated! An example of nondisjunction during meiosis. Of the two pairs of homologous chromosomes shown here, one fails to separate during anaphase I. A zygote that forms from any of the resulting gametes will have an abnormal chromosome number. Anaphase II Telophase II Stepped Art Figure 9-21 p166

Aneuploidy Aneuploidy A chromosome abnormality in which a cell has too many or too few copies of a particular chromosome (trisomy, monosomy) The most common aneuploidy in humans, trisomy 21, causes Down syndrome

Some Disorders Caused by Aneuploidy

Autosomal Change and Down Syndrome Trisomy 21 (Down syndrome) The only autosomal trisomy that allows humans to survive to adulthood Affected individuals tend to have certain physical features and impairments Nondisjunction leading to trisomy 21 increases with age of the mother

Down Syndrome Figure 9.22 Down syndrome, genotype and phenotype.

Change in Sex Chromosome Number Usually associated with learning difficulties, speech delays, and motor skill impairment Female sex chromosome abnormalities: Turner syndrome (XO) XXX syndrome Male sex chromosome abnormalities: Klinefelter syndrome (XXY) XYY syndrome

9.9 Genetic Screening Geneticists estimate the chance that a couple’s offspring will inherit a genetic abnormality or disorder Potential parents who may be at risk of transmitting a harmful allele to offspring have screening or treatment options

Prenatal Diagnosis Obstetric sonography may reveal defects associated with a genetic disorder Other tests performed before birth carry risks of miscarriage or injury to fetus Amniocentesis Chorionic villi sampling (CVS) Fetoscopy

Three ways of imaging a fetus C) Fetoscopy Figure 9.23 Three ways of imaging a human fetus. A) Conventional ultrasound B) 4D ultrasound

The amniotic sac amniotic sac chorion Figure 9.24 An 8-week-old fetus. With amniocentesis, fetal cells shed into the fluid inside the amniotic sac are tested for genetic disorders. Chorionic villus sampling tests cells of the chorion, which helps form the placenta. chorion

Preimplantation Diagnosis A single cell taken from an embryo produced by in vitro fertilization is tested before implantation

9.10 Menacing Mucus (revisited) The cystic fibrosis (CF) allele is very common in some populations The CF allele is lethal in homozygotes, but offers heterozygotes some protection against bacterial diseases such as typhoid fever

Digging Into Data: Cystic Fibrosis and Typhoid Fever Figure 9.25 Effect of the CF mutation on uptake of three different strains of Salmonella typhi bacteria by epithelial cells.