Chapter 12 Chromosomes & Human Inheritance OCC BIO-114

In this chapter… you will learn how biologists use their knowledge of DNA & chromosome behavior to study how traits are inherited & expressed. you will learn how biologists use their knowledge of DNA & chromosome behavior to study how traits are inherited & expressed.

Sex-determining Chromosomes and Linkage sex chromosomes sex chromosomes autosomes autosomes

Sex Chromosomes It’s all about the “X” and the “Y” It’s all about the “X” and the “Y”

Sex determination and Inheritance of Sex-linked genes: Mammals/ many insects= male/female same number of chromosomes -one pair= sex chromosomes Mammals/ many insects= male/female same number of chromosomes -one pair= sex chromosomes different appearance and genetic composition different appearance and genetic composition XX= female XX= female XY= male XY= male Humans: Humans: Autosomes = # 1-22 Autosomes = # 1-22 Sex chromosome = #23 Sex chromosome = #23 sex chromosome carried by sperm determines sex of offspring ( X or Y sperm) sex chromosome carried by sperm determines sex of offspring ( X or Y sperm)

Sex-Linkage In 1910, Thomas Hunt Morgan - Studied inheritance patterns of fruit fly, Drosophila melanogaster, discovered presence of a white eye in certain individuals. In 1910, Thomas Hunt Morgan - Studied inheritance patterns of fruit fly, Drosophila melanogaster, discovered presence of a white eye in certain individuals. Since this was a distinctive feature, Morgan decided to study the inheritance pattern for this recessive eye color. Since this was a distinctive feature, Morgan decided to study the inheritance pattern for this recessive eye color.

Sex-Linkage Morgan made several crosses using a white-eyed male, expecting the standard Mendelian results. He did not get them. Morgan made several crosses using a white-eyed male, expecting the standard Mendelian results. He did not get them. While the ratio of 3:1 was obtained, all of the white-eyed second generation offspring were male flies. While the ratio of 3:1 was obtained, all of the white-eyed second generation offspring were male flies. All females had red eyes (and 25% of the males also had red eyes). All females had red eyes (and 25% of the males also had red eyes).

Sex-Linkage Morgan did a series of reciprocal crosses of white-eye males with red-eye females and red-eye males with white-eye females. Morgan did a series of reciprocal crosses of white-eye males with red-eye females and red-eye males with white-eye females. He concluded that the gene for eye color was located on the X chromosome. He concluded that the gene for eye color was located on the X chromosome. Males passed the trait their daughters (on their solitary X chromosome) & mothers passed the trait sons. Males passed the trait their daughters (on their solitary X chromosome) & mothers passed the trait sons. White eyed females could also pass the white eye allele to their daughters, but if the father fly had red eyes, the eye color of the daughters would be red, while the eye color of the sons of white-eyed females would always be white. White eyed females could also pass the white eye allele to their daughters, but if the father fly had red eyes, the eye color of the daughters would be red, while the eye color of the sons of white-eyed females would always be white.

Sex-Linkage Morgan concluded that eye color was related to sex, & that the sex-determining chromosomes also had genes that were unrelated to gender determination. Morgan concluded that eye color was related to sex, & that the sex-determining chromosomes also had genes that were unrelated to gender determination. Prior to Morgan's discovery, no one knew that genes unrelated to gender were also located on these chromosomes. Prior to Morgan's discovery, no one knew that genes unrelated to gender were also located on these chromosomes.

Sex-Linkage These other traits are said to be sex-linked because they are inherited along with the sex of the individual. These other traits are said to be sex-linked because they are inherited along with the sex of the individual. Because the X and Y chromosome are not exactly matching, the X chromosome can have genes that are not located on the Y chromosome, and vice-versa. Because the X and Y chromosome are not exactly matching, the X chromosome can have genes that are not located on the Y chromosome, and vice-versa. Some of these genes are unrelated to the sexual characteristics, but are inherited with the sex- determination. This is referred to as sex- linkage. Some of these genes are unrelated to the sexual characteristics, but are inherited with the sex- determination. This is referred to as sex- linkage.

sex-linked (X-linked) genes on one sex chromosome; not on other genes on one sex chromosome; not on other Y= carries relatively few genes X= many genes; some not specifically related to female traits Y= carries relatively few genes X= many genes; some not specifically related to female traits Ex: genes for: color vision; blood clotting; structural proteins blood clotting; structural proteins female XX= homozygous or hybrid (dominant/recessive) female XX= homozygous or hybrid (dominant/recessive) male XY= fully expresses all alleles on single X (whether dominant or recessive) male XY= fully expresses all alleles on single X (whether dominant or recessive) Ex: color blindness; hemophilia; muscular dystrophy

Some human sex-linked traits are Hemophilia (X) Hemophilia (X) Hairy ear rims (Y) Hairy ear rims (Y) Red-green color blindness (X) (fig.12.9) Red-green color blindness (X) (fig.12.9) Duchene muscular dystrophy (X) Duchene muscular dystrophy (X)

X-Linked Only found on X chromosome Only found on X chromosome Colorblindness – red/green, most common in males (8%) Colorblindness – red/green, most common in males (8%) Hemophilia – blood clotting, affects males Hemophilia – blood clotting, affects males Duchene Muscular Dystrophy – weakens & destroys muscle tissue Duchene Muscular Dystrophy – weakens & destroys muscle tissue *Not all X-Linked traits are diseases = Only a few of hundereds of genes on X chrom. Others code for normal functioning proteins

The Effect of Recombination on Gene Linkage and Inheritance Meiosis results in exchange of bits & pieces of DNA between homologous pairs of chromosomes at the chiasmata during prophase I of meiosis. Meiosis results in exchange of bits & pieces of DNA between homologous pairs of chromosomes at the chiasmata during prophase I of meiosis. This process of recombination results in gametes (or meiotic products) that are not identical; some of the linkage groups have been changed by the crossing-over. This process of recombination results in gametes (or meiotic products) that are not identical; some of the linkage groups have been changed by the crossing-over. As a result of recombination, new allele combinations are formed, and we have more genetic variation. As a result of recombination, new allele combinations are formed, and we have more genetic variation.

crossing over segments of homologous chromosomes are exchanged w/ each other at site called chiasmata during meiosis I segments of homologous chromosomes are exchanged w/ each other at site called chiasmata during meiosis I forms new gene combinations on both homologous chromosomes forms new gene combinations on both homologous chromosomes gene combinations for daughter cell different from parent cell gene combinations for daughter cell different from parent cell

Crossing Over and Recombination

Cell Mutations Germ-cell mutation – gametes, passed on to offspring Germ-cell mutation – gametes, passed on to offspring Somatic mutations – body cells, affect organism. Not passed on to offspring. i.e. leukemia Somatic mutations – body cells, affect organism. Not passed on to offspring. i.e. leukemia Lethal mutations – cause death. Are they beneficial??? Lethal mutations – cause death. Are they beneficial???

Chromosome mutations(12.5) Deletion – piece of chrom breaks off & is lost (end); due to virus, radiation, chemicals, or envir factors. Most are lethal Deletion – piece of chrom breaks off & is lost (end); due to virus, radiation, chemicals, or envir factors. Most are lethal Ex:ABCDEFG becomes ABCFG, Cri-du-chat syndrome (p.192) Ex:ABCDEFG becomes ABCFG, Cri-du-chat syndrome (p.192) Inversion – breaks off & reattaches in reverse Inversion – breaks off & reattaches in reverse Ex: ABCDEFG becomes ABGFEDC Ex: ABCDEFG becomes ABGFEDC Translocation – breaks off & reattaches to different (non-homologous chrom) Translocation – breaks off & reattaches to different (non-homologous chrom) Ex: ABCDEFG becomes ABCDLMNOP Ex: ABCDEFG becomes ABCDLMNOP

Changes in Chrom # Nondisjunction – chrom fail to separate during Anaphase (gamete formation) (p.192) Nondisjunction – chrom fail to separate during Anaphase (gamete formation) (p.192) Aneuploidy – 1 extra or less chrom Aneuploidy – 1 extra or less chrom Monosomic-1 less, Turner syndrome (XO) Monosomic-1 less, Turner syndrome (XO) Trisomic-1 more Trisomic-1 more Polyploidy – 3 or more of each type of chrom; lethal for humans Polyploidy – 3 or more of each type of chrom; lethal for humans

Inheritance of Recessive Alleles Any alteration of a gene, called a mutation, has the potential to inhibit the formation of a needed enzyme. With diploid organisms, however, a mutation most likely affects just one of the homologues, and the second can still code for the appropriate enzyme with little or no phenotypic effect on the individual. Any alteration of a gene, called a mutation, has the potential to inhibit the formation of a needed enzyme. With diploid organisms, however, a mutation most likely affects just one of the homologues, and the second can still code for the appropriate enzyme with little or no phenotypic effect on the individual. Gene alterations that affect health are called genetic disorders (Table 12.1, p.196). Gene alterations that affect health are called genetic disorders (Table 12.1, p.196). Those that are just "abnormal" but do not affect physiological health, are called genetic abnormalities i.e. 6 toes Those that are just "abnormal" but do not affect physiological health, are called genetic abnormalities i.e. 6 toes When the genetic alteration causes a host of symptoms, it may be called a syndrome. When the genetic alteration causes a host of symptoms, it may be called a syndrome. Disease –illness caused by infections, not by inheritance Disease –illness caused by infections, not by inheritance

Single Allele Traits Controlled by a single allele of a gene. Controlled by a single allele of a gene. >200 human traits governed by single dominant allele. >200 human traits governed by single dominant allele. Ex: Huntington’s Disease (HD) caused by dominant allele located on an autosome = autosomal dominant pattern of inheritance Ex: Huntington’s Disease (HD) caused by dominant allele located on an autosome = autosomal dominant pattern of inheritance Read description of HD on p.196, Table 12.1 Read description of HD on p.196, Table 12.1

HD continued Geneticists discovered a genetic marker for HD allele. Geneticists discovered a genetic marker for HD allele. Genetic marker is a short section of DNA known to have a close association w/ a particular gene located nearby. Easy to I.D. the HD allele. Genetic marker is a short section of DNA known to have a close association w/ a particular gene located nearby. Easy to I.D. the HD allele. If marker is present = 96% chance of dev HD If marker is present = 96% chance of dev HD Parents can be tested for marker before conceiving a child. Parents can be tested for marker before conceiving a child.

Other Single-Allele Traits (Table 12.1) Homozygous recessive (must have 2 copies of recessive allele): Homozygous recessive (must have 2 copies of recessive allele): Cystic Fibrosis Cystic Fibrosis Sickle Cell Anemia Sickle Cell Anemia *these are recessive alleles located on autosomes = Autosomal Recessive pattern of inhertiance pattern of inhertiance

polygenic inheritance Trait that is controlled by 2 or more genes = many different variations Trait that is controlled by 2 or more genes = many different variations Ex: Skin Color-influenced by additive effects of 3 to 6 genes. Each gene results in certain amount of melanin (brownish-black pigment) Ex: Skin Color-influenced by additive effects of 3 to 6 genes. Each gene results in certain amount of melanin (brownish-black pigment) More melanin=darker More melanin=darker Ex: eye color. Light blue eyes=very little melanin Ex: eye color. Light blue eyes=very little melanin Human height – polygenic, but influenced by environmental factors, such as disease & nutrition Human height – polygenic, but influenced by environmental factors, such as disease & nutrition

Disorders due to Nondisjunction Occurs during meiosis causes gametes to lack a chromosome or have an extra one (fig 12-10, p. 231) Occurs during meiosis causes gametes to lack a chromosome or have an extra one (fig 12-10, p. 231) A zygote w/ 45 chrom. = monosomy A zygote w/ 45 chrom. = monosomy 47 chrom. = trisomy 47 chrom. = trisomy Often lethal Often lethal Trisomy 21 = Down Syndrome – mild to severe mental retardation… Trisomy 21 = Down Syndrome – mild to severe mental retardation…

Nondisjunction cont. Males w/ 1 extra X chrom. = Klinefelter’s syndrome (XXY). Some feminine characteristics, mentally retarded & infertile Males w/ 1 extra X chrom. = Klinefelter’s syndrome (XXY). Some feminine characteristics, mentally retarded & infertile Turner’s Syndrome – Have a single x chrom. (XO) = female appearance, but do not mature sexually & remain infertile. What happens if zygote only receives a Y chrom.? Turner’s Syndrome – Have a single x chrom. (XO) = female appearance, but do not mature sexually & remain infertile. What happens if zygote only receives a Y chrom.?

Detection (fig.12.21, 12.22) Genetic Screening = Karyotype (picture of person’s chrom. Genetic Screening = Karyotype (picture of person’s chrom. Amniocentesis – procedure removes amniotic fluid from fetus & tested (14 th -16 th week of pregnancy) Amniocentesis – procedure removes amniotic fluid from fetus & tested (14 th -16 th week of pregnancy) Chorionic Villi Sampling – Removes fetal cells from chorion fluid (between mothers uterus & fetus) Chorionic Villi Sampling – Removes fetal cells from chorion fluid (between mothers uterus & fetus) Ultrasound – Sound waves to observe fetus Ultrasound – Sound waves to observe fetus

Detection Cont. In U.S. 1 out of 10,000 babies is afflicted w/ phenylketonuria (PKU) – body cannot metabolize amino acid phenylalanine = brain damage. In U.S. 1 out of 10,000 babies is afflicted w/ phenylketonuria (PKU) – body cannot metabolize amino acid phenylalanine = brain damage. Genetic Counseling – medical guidance for couples at risk Genetic Counseling – medical guidance for couples at risk

12-10 Human Genetics Humans have up to 20 times as many genes as Drosophila, & our 23 pairs of chromosomes & are made up of about 100,000 genes. Humans have up to 20 times as many genes as Drosophila, & our 23 pairs of chromosomes & are made up of about 100,000 genes. Geneticists focus on disease-causing genes b/c of concern for human pop. Geneticists focus on disease-causing genes b/c of concern for human pop.

Pedigree Analysis Pedigree – a family record that shows how a trait is inherited over several generations (fig 12-19). Pedigree – a family record that shows how a trait is inherited over several generations (fig 12-19). Certain phenotypes are usually repeated in predictable patterns from 1 generation to the next = patterns of inheritance. Certain phenotypes are usually repeated in predictable patterns from 1 generation to the next = patterns of inheritance. Carriers – individuals who have 1 copy of a recessive allele, but can pass it along to their offspring. Carriers – individuals who have 1 copy of a recessive allele, but can pass it along to their offspring.

How to read a Pedigree Human pedigrees Human pedigrees Before we consider human Mendelian inheritance it is convenient to consider the symbols used to draw pedigrees. Before we consider human Mendelian inheritance it is convenient to consider the symbols used to draw pedigrees. Generations are numberered from the top of the pedigree in uppercase Roman numerals, I, II, III etc. Individuals in each generation are numbered from the left in arab numberals as subscripts, III1, III2, III3 etc. Generations are numberered from the top of the pedigree in uppercase Roman numerals, I, II, III etc. Individuals in each generation are numbered from the left in arab numberals as subscripts, III1, III2, III3 etc.

Hemophilia pedigree of the European Royal Families

Practice Problems Glencoe Glencoe Glencoe

References 1/lectures/pdfs/HumanInheritance101.pdf 1/lectures/pdfs/HumanInheritance101.pdf