* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Ch. 9 Presentation - Faculty Website Listing
Epigenetics of human development wikipedia , lookup
Polymorphism (biology) wikipedia , lookup
Y chromosome wikipedia , lookup
Public health genomics wikipedia , lookup
Human genetic variation wikipedia , lookup
Transgenerational epigenetic inheritance wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Behavioural genetics wikipedia , lookup
Medical genetics wikipedia , lookup
History of genetic engineering wikipedia , lookup
Population genetics wikipedia , lookup
X-inactivation wikipedia , lookup
Genomic imprinting wikipedia , lookup
Genome (book) wikipedia , lookup
Genetic drift wikipedia , lookup
Designer baby wikipedia , lookup
Quantitative trait locus wikipedia , lookup
Hardy–Weinberg principle wikipedia , lookup
Chapter 9 Patterns of Inheritance MENDEL’S LAWS © 2012 Pearson Education, Inc. 9.1 The science of genetics has ancient roots The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was – suggested in the 19th century by scientists studying plants but – later rejected because it did not explain how traits that disappear in one generation can reappear in later generations. © 2012 Pearson Education, Inc. 9.2 Experimental genetics began in an abbey garden Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel – began the field of genetics in the 1860s, – deduced the principles of genetics by breeding garden peas, and – relied upon a background of mathematics, physics, and chemistry. © 2012 Pearson Education, Inc. 9.2 Experimental genetics began in an abbey garden In 1866, Mendel – correctly argued that parents pass on to their offspring discrete “heritable factors” and – stressed that the heritable factors (today called genes), retain their individuality generation after generation. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white flowers, is a trait. © 2012 Pearson Education, Inc. Figure 9.2A 9.2 Experimental genetics began in an abbey garden True-breeding varieties result when self-fertilization produces offspring all identical to the parent. The offspring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. True-breeding parental plants are the P generation. Hybrid offspring are the F1 generation. A cross of F1 plants produces an F2 generation. © 2012 Pearson Education, Inc. Figure 9.2B Petal Carpel Stamen Figure 9.2C_s1 White 1 Removal of stamens Stamens Carpel Parents (P) 2 Transfer Purple of pollen Figure 9.2C_s2 White 1 Removal of stamens Stamens Carpel Parents (P) 2 Transfer Purple of pollen 3 Carpel matures into pea pod Figure 9.2C_s3 White 1 Removal of stamens Stamens Carpel Parents (P) 2 Transfer Purple of pollen 3 Carpel matures into pea pod 4 Seeds from pod planted Offspring (F1) Figure 9.2D Traits Character Dominant Recessive Purple White Axial Terminal Yellow Green Round Wrinkled Inflated Constricted Green Yellow Tall Dwarf Flower color Flower position Seed color Seed shape Pod shape Pod color Stem length 9.3 Mendel’s law of segregation describes the inheritance of a single character A cross between two individuals differing in a single character is a monohybrid cross. Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers. – The F1 generation produced all plants with purple flowers. – A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers. © 2012 Pearson Education, Inc. Figure 9.3A_s1 The Experiment P generation (true-breeding parents) Purple flowers White flowers Figure 9.3A_s2 The Experiment P generation (true-breeding parents) Purple flowers F1 generation White flowers All plants have purple flowers Figure 9.3A_s3 The Experiment P generation (true-breeding parents) Purple flowers F1 generation White flowers All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 generation 3 4 1 of plants of plants 4 have purple flowers have white flowers 9.3 Mendel’s law of segregation describes the inheritance of a single character The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis. Mendel needed to explain why – white color seemed to disappear in the F1 generation and – white color reappeared in one quarter of the F2 offspring. Mendel observed the same patterns of inheritance for six other pea plant characters. © 2012 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. – A homozygous genotype has identical alleles. – A heterozygous genotype has two different alleles. © 2012 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character 3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. – The phenotype is the appearance or expression of a trait. – The genotype is the genetic makeup of a trait. – The same phenotype may be determined by more than one genotype. © 2012 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character 4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation. – The F1 hybrids all have a Pp genotype. – A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. © 2012 Pearson Education, Inc. Figure 9.3B_s1 The Explanation P generation Genetic makeup (alleles) White flowers Purple flowers PP pp Gametes All P All p Figure 9.3B_s2 The Explanation P generation Genetic makeup (alleles) White flowers Purple flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Gametes 1 2 P 1 2 p Figure 9.3B_s3 The Explanation P generation Genetic makeup (alleles) White flowers Purple flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Gametes 1 2 P Alleles segregate 1 2 p Fertilization Sperm from F1 plant F2 generation P Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp p P PP Pp Eggs from F1 plant p Pp pp 9.4 Homologous chromosomes bear the alleles for each character A locus (plural, loci) is the specific location of a gene along a chromosome. For a pair of homologous chromosomes, alleles of a gene reside at the same locus. – Homozygous individuals have the same allele on both homologues. – Heterozygous individuals have a different allele on each homologue. © 2012 Pearson Education, Inc. Figure 9.4 Gene loci P a B P a b Dominant allele Homologous chromosomes Genotype: PP Homozygous for the dominant allele aa Homozygous for the recessive allele Recessive allele Bb Heterozygous, with one dominant and one recessive allele 9.5 The law of independent assortment is revealed by tracking two characters at once A dihybrid cross is a mating of parental varieties that differ in two characters. Mendel performed the following dihybrid cross with the following results: – P generation: round yellow seeds wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation: – 9/16 had round yellow seeds – 3/16 had wrinkled yellow seeds – 3/16 had round green seeds – 1/16 had wrinkled green seeds © 2012 Pearson Education, Inc. Figure 9.5A P generation RRYY Gametes RY F1 generation rryy ry Sperm RrYy 1 4 RY 1 4 rY 1 4 Ry 1 4 ry Sperm 1 2 1 2 F2 generation RY 1 2 ry RY Eggs 1 2 ry 1 4 RY 1 4 rY Eggs 1 4 1 4 The hypothesis of dependent assortment Data did not support; hypothesis refuted RRYY RrYY RRYy RrYy RrYY RrYy rrYy rrYY Ry RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry 9 16 Yellow round 3 16 Green round 3 16 Yellow wrinkled 1 16 Green wrinkled The hypothesis of independent assortment Actual results; hypothesis supported 9.5 The law of independent assortment is revealed by tracking two characters at once Mendel needed to explain why the F2 offspring – had new nonparental combinations of traits and – a 9:3:3:1 phenotypic ratio. Mendel – suggested that the inheritance of one character has no effect on the inheritance of another, – suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and – called this the law of independent assortment. © 2012 Pearson Education, Inc. 9.5 The law of independent assortment is revealed by tracking two characters at once The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: – black versus chocolate color, – normal vision versus progressive retinal atrophy. © 2012 Pearson Education, Inc. Figure 9.5B Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Blind Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision 1 Chocolate coat, blind (PRA) 9.6 Geneticists can use the testcross to determine unknown genotypes A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual. A testcross can show whether the unknown genotype includes a recessive allele. Mendel used testcrosses to verify that he had truebreeding genotypes. The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. © 2012 Pearson Education, Inc. Figure 9.6 What is the genotype of the black dog? Testcross Genotypes B_? bb Two possibilities for the black dog: BB Gametes B b Offspring Bb or Bb All black b B b Bb bb 1 black : 1 chocolate 9.7 Mendel’s laws reflect the rules of probability Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected – the segregation of allele pairs during gamete formation and – the re-forming of pairs at fertilization. The probability scale ranges from 0 to 1. An event that is – certain has a probability of 1 and – certain not to occur has a probability of 0. © 2012 Pearson Education, Inc. 9.7 Mendel’s laws reflect the rules of probability The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event. The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition. © 2012 Pearson Education, Inc. Figure 9.7 F1 genotypes Bb male Bb female Formation of eggs Formation of sperm 1 2 1 2 B b Sperm 1 1 (2 2 ) 1 2 F2 genotypes B b b 1 4 B 1 4 b B 1 4 Eggs 1 2 B B b b 1 4 9.8 Genetic traits in humans can be tracked through family pedigrees The inheritance of some human traits follows Mendel’s laws. A pedigree – shows the inheritance of a trait in a family through multiple generations, – demonstrates dominant or recessive inheritance, and – can also be used to deduce genotypes of family members. © 2012 Pearson Education, Inc. Figure 9.8 Straight hairline Widow’s peak Figure 9.8 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele Mating 1ST GENERATION Al Beth Charles Children Evelyn Frank Gary Debbie 2ND GENERATION Henry Straight hairline Isabel Widow’s peak 3RD GENERATION Kristin Juliana Lori Figure 9.8 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele 1ST GENERATION Al Beth Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry hh hh Hh Straight hairline A parent with a widow’s peak who has a child with a straight hairline must be Hh. Isabel Juliana Hh hh 3RD GENERATION Kristin hh Kristin has a straight Lori hairline but her parents do not, so straight hairline must be homozygous recessive (hh). Widow’s peak Figure 9.8 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele 1ST GENERATION Al Beth Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry Isabel Juliana HH hh hh Hh Hh hh or Hh Straight hairline Widow’s peak 3RD GENERATION Not all genotypes can be determined. Lori could be Kristin HH or Hh and there is no Lori way to know (unless she hh has some children and the HH or pedigree is extended). Hh 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene Because a trait is dominant does not mean that it is – “normal” or – more common than a recessive trait. Wild-type traits are – those most often seen in nature and – not necessarily specified by dominant alleles. © 2015 Pearson Education, Inc. Figure 9.9a-0 Dominant Traits Recessive Traits Freckles No freckles Normal pigmentation Albinism Key Wild-type (more common) trait Mutant (less common) trait 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene The genetic disorders listed in Table 9.9 are known to be inherited as dominant or recessive traits controlled by a single gene. These human disorders therefore show simple inheritance patterns like the traits Mendel studied in pea plants. The genes discussed in this module are all located on autosomes. © 2015 Pearson Education, Inc. Table 9.9 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene Thousands of human genetic disorders—ranging in severity from relatively mild, such as albinism, to invariably fatal, such as cystic fibrosis—are inherited as recessive traits. Most people who have recessive disorders are born to normal parents who – are both heterozygotes, carriers of the recessive allele for the disorder, but – are phenotypically normal. © 2015 Pearson Education, Inc. Figure 9.9b Normal Aa Parents Normal Aa A Sperm a A Offspring AA Normal Aa Normal (carrier) Aa Normal (carrier) aa Albinism Eggs a 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is – recessive and – carried by about 1 in 31 Americans, mostly caucasians. Dominant human disorders include – achondroplasia, resulting in dwarfism, and – Huntington’s disease, a degenerative disorder of the nervous system. © 2012 Pearson Education, Inc. Figure 9.9B Achondroplasia 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene Until relatively recently, the onset of symptoms was the only way to know if a person had inherited the Huntington’s allele. A genetic test is now available that can detect the presence of the Huntington’s allele in an individual’s genome. This is one of several genetic tests currently available. © 2015 Pearson Education, Inc. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy New technologies offer ways to obtain genetic information – before conception, – during pregnancy, and – after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. © 2012 Pearson Education, Inc. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy Several technologies can be used for detecting genetic conditions in a fetus. – Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits – karyotyping and – biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. – Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. © 2012 Pearson Education, Inc. Figure 9.10A Amniocentesis Amniotic fluid extracted Ultrasound transducer Fetus Chorionic Villus Sampling (CVS) Tissue extracted from the Ultrasound chorionic villi transducer Fetus Placenta Chorionic villi Placenta Uterus Cervix Cervix Uterus Centrifugation Amniotic fluid Fetal cells Several hours Cultured cells Several weeks Several weeks Karyotyping Biochemical and genetics tests Fetal cells Several hours Several hours 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy Blood tests on the mother at 15–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. Newborn screening can detect diseases that can be prevented by special care and precautions. © 2012 Pearson Education, Inc. Figure 9.10B Ultrasound of a fetus 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy New technologies raise ethical considerations that include – the confidentiality and potential use of results of genetic testing, – time and financial costs, and – determining what, if anything, should be done as a result of the testing. © 2012 Pearson Education, Inc. VARIATIONS ON MENDEL’S LAWS © 2012 Pearson Education, Inc. 9.11 Incomplete dominance results in intermediate phenotypes Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance. For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which – neither allele is dominant over the other and – expression of both alleles occurs. © 2012 Pearson Education, Inc. Figure 9.11A P generation Red RR White rr Gametes R r F1 generation Pink hybrid Rr Gametes 1 2 R 1 2 r Sperm 1 1 R 2 2 r F2 generation 1 R 2 RR rR 1 r 2 Rr rr Eggs 9.11 Incomplete dominance results in intermediate phenotypes Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation. One example of incomplete dominance in humans is hypercholesterolemia, in which – dangerously high levels of cholesterol occur in the blood and – heterozygotes have intermediately high cholesterol levels. © 2012 Pearson Education, Inc. Figure 9.11B HH Homozygous for ability to make LDL receptors Genotypes Hh Heterozygous hh Homozygous for inability to make LDL receptors Phenotypes LDL LDL receptor Cell Normal Mild disease Severe disease 9.12 Many genes have more than two alleles in the population Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population. Human ABO blood group phenotypes involve three alleles for a single gene. The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance. © 2012 Pearson Education, Inc. 9.12 Many genes have more than two alleles in the population In codominance, – neither allele is dominant over the other and – expression of both alleles is observed as a distinct phenotype in the heterozygous individual. – AB blood type is an example of codominance. © 2012 Pearson Education, Inc. Figure 9.12 Blood Group (Phenotype) Genotypes Carbohydrates Present on Red Blood Cells Carbohydrate A A IAIA or I Ai Carbohydrate B B IBIB or IBi AB IAIB Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB Anti-B Anti-A Carbohydrate A and Carbohydrate B None Anti-A O ii Neither Anti-B No reaction Clumping reaction 9.13 A single gene may affect many phenotypic characters Pleiotropy occurs when one gene influences many characteristics. Sickle-cell disease is a human example of pleiotropy. This disease – affects the type of hemoglobin produced and the shape of red blood cells and – causes anemia and organ damage. – Sickle-cell and nonsickle alleles are codominant. – Carriers of sickle-cell disease are resistant to malaria. © 2012 Pearson Education, Inc. Figure 9.13A Sickle cell anemia Figure 9.13b An individual homozygous for the sickle-cell allele Produces sickle-cell (abnormal) hemoglobin The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Damage to organs Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis) Other effects Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 9.14 A single character may be influenced by many genes Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes. Human skin color is an example of polygenic inheritance. © 2012 Pearson Education, Inc. Figure 9.14 P generation aabbcc AABBCC (very light) (very dark) F1 generation AaBbCc AaBbCc Sperm 1 8 F2 generation 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 Fraction of population Eggs 1 8 1 8 1 8 1 8 1 8 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Skin color 9.15 The environment affects many characters Many characters result from a combination of heredity and the environment. For example, – skin color is affected by exposure to sunlight, – susceptibility to diseases, such as cancer, has hereditary and environmental components, and – identical twins show some differences. Only genetic influences are inherited. © 2012 Pearson Education, Inc. Figure 9.15B THE CHROMOSOMAL BASIS OF INHERITANCE © 2012 Pearson Education, Inc. 9.16 Chromosome behavior accounts for Mendel’s laws The chromosome theory of inheritance states that – genes occupy specific loci (positions) on chromosomes and – chromosomes undergo segregation and independent assortment during meiosis. © 2012 Pearson Education, Inc. 9.16 Chromosome behavior accounts for Mendel’s laws Mendel’s laws correlate with chromosome separation in meiosis. – The law of segregation depends on separation of homologous chromosomes in anaphase I. – The law of independent assortment depends on alternative orientations of chromosomes in metaphase I. © 2012 Pearson Education, Inc. Figure 9.16_s1 F1 generation R r y All yellow round seeds (RrYy) Y R Y r y Metaphase I of meiosis r R Y y Figure 9.16_s2 F1 generation R r y All yellow round seeds (RrYy) Y r R Y R y Metaphase I of meiosis r R Y y r r R Y y Anaphase I Y y Metaphase II R r r R Y y Y y Figure 9.16_s3 F1 generation R r All yellow round seeds (RrYy) y Y r R Y R y Metaphase I of meiosis r R Y y r r R Y y Anaphase I Y y Metaphase II R r r R Y y Y y Gametes Y Y R R 1 4 RY y y r r 1 4 Y Y r r ry F2 generation 9 Fertilization :3 :3 :1 1 4 rY y y R R 1 4 Ry Figure 9.16_4 Sperm 1 1 1 1 4 RY 4 rY 4 Ry 4 ry 1 4 RY RRYY RrYY RRYy RrYy 1 4 rY Eggs 1 Ry 4 1 ry 4 RrYY rrYY RrYy rrYy RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy 9 16 Yellow round 3 16 Green round 3 16 Yellow wrinkled 1 16 Green wrinkled 9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which – are located close together on the same chromosome and – tend to be inherited together. © 2012 Pearson Education, Inc. Figure 9.17 The Experiment Purple flower PpLl Phenotypes PpLl Long pollen Observed offspring 284 21 21 55 Purple long Purple round Red long Red round Prediction (9:3:3:1) 215 71 71 24 The Explanation: Linked Genes PL Parental diploid cell PpLl pl Meiosis Most gametes pl PL Fertilization Sperm PL Most offspring pl PL PL PL pl pl pl PL pl PL Eggs pl 3 purple long : 1 red round Not accounted for: purple round and red long 9.18 Crossing over produces new combinations of alleles Crossing over between homologous chromosomes produces new combinations of alleles in gametes. Linked alleles can be separated by crossing over, forming recombinant gametes. The percentage of recombinants is the recombination frequency. © 2012 Pearson Education, Inc. Figure 9.18A p L p l PL Parental gametes pl p L Tetrad Crossing over (pair of homologous chromosomes) P l Recombinant gametes Figure 9.18C The Explanation The Experiment Gray body, long wings (wild type) Black body, vestigial wings GgLl ggll Female Male GgLl Female GL gl gl gl ggll Male Crossing over GL Gl g l gl gL Eggs Sperm Offspring Gray long Black vestigial Gray vestigial Black long Offspring GL g l G l g L g l g l g l g l Parental 965 944 Parental phenotypes Recombination frequency 206 185 Recombinant phenotypes 391 recombinants 0.17 or 17% 2,300 total offspring Recombinant 9.19 Geneticists use crossover data to map genes When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur. Recombination frequencies can thus be used to map the relative position of genes on chromosomes. © 2012 Pearson Education, Inc. Figure 9.19A Section of chromosome carrying linked genes g c l 17% 9% 9.5% Recombination frequencies Figure 9.19B Mutant phenotypes Short aristae Black body (g) Cinnabar Vestigial wings eyes (l) (c) Red Long aristae Gray Normal eyes (appendages body wings (C) on head) (G) (L) Wild-type phenotypes Brown eyes Red eyes SEX CHROMOSOMES AND SEX-LINKED GENES © 2012 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species Many animals have a pair of sex chromosomes, – designated X and Y, – that determine an individual’s sex. In mammals, – males have XY sex chromosomes, – females have XX sex chromosomes, – the Y chromosome has genes for the development of testes, and – an absence of the Y allows ovaries to develop. © 2012 Pearson Education, Inc. Figure 9.20A Human sex chromosomes X Y Figure 9.20b Parents (diploid) Gametes (haploid) Offspring (diploid) 22 + X Male Female 44 + XY 44 + XX 22 + Y Sperm 22 + X 44 + XX Female Egg 44 + XY Male 9.20 Chromosomes determine sex in many species Grasshoppers, roaches, and some other insects have an X-O system, in which – O stands for the absence of a sex chromosome, – females are XX, and – males are XO. In certain fishes, butterflies, and birds, – the sex chromosomes are Z and W, – males are ZZ, and – females are ZW. © 2012 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species Some organisms lack sex chromosomes altogether. In bees, sex is determined by chromosome number. – Females are diploid. – Males are haploid. © 2012 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species In some animals, environmental temperature determines the sex. – For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female. – Global climate change may therefore impact the sex ratio of such species. © 2012 Pearson Education, Inc. 9.21 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes. The X chromosome carries many genes unrelated to sex. The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait. © 2012 Pearson Education, Inc. Figure 9.21A Fruit fly eye color determined by sexlinked gene Figure 9.21B Female Male XRXR XrY Sperm Eggs XR Xr Y XRXr XRY R red-eye allele r white-eye allele Figure 9.21C Female Male XRXr XRY Sperm Y xR XR XRXR XRY XrXR XrY Eggs Xr R red-eye allele r white-eye allele Figure 9.21D Female Male XRXr XrY Sperm Xr Y XR XRXr XRY Xr XrXr XrY Eggs R red-eye allele r white-eye allele 9.22 CONNECTION: Human sex-linked disorders affect mostly males Most sex-linked human disorders are – due to recessive alleles and – seen mostly in males. A male receiving a single X-linked recessive allele from his mother will have the disorder. A female must receive the allele from both parents to be affected. © 2012 Pearson Education, Inc. 9.22 CONNECTION: Human sex-linked disorders affect mostly males Recessive and sex-linked human disorders include – hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting, – red-green color blindness, a malfunction of lightsensitive cells in the eyes, and – Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination. © 2012 Pearson Education, Inc.