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© 2014 Pearson Education, Inc. Human Biology Concepts and Current Issues Seventh Edition Michael D. Johnson Lecture Presentations by Robert J. Sullivan Marist College 19 Genetics and Inheritance
© 2014 Pearson Education, Inc. Introductory Genetics Terminology Genes: DNA sequences that contain instructions for building proteins Genetics: study of genes and their transmission from one generation to the next Genome: sum total of all of an organism’s DNA
© 2014 Pearson Education, Inc. Your Genotype Is the Genetic Basis of Your Phenotype Chromosomes: structures within the nucleus, composed of DNA and protein The genes are located on the chromosomes Humans have 23 pairs of chromosomes –22 pairs of autosomes –1 pair of sex chromosomes: determine gender –1 of each pair of autosomes and 1 sex chromosome is inherited from each parent
© 2014 Pearson Education, Inc. Your Genotype Is the Genetic Basis of Your Phenotype Homologous chromosomes –One member of each pair is inherited from each parent –Look alike (size, shape, banding pattern) –Not identical: may have different alleles of particular genes Alleles: alternative forms of a gene –Alleles arise from mutation Homozygous: two identical alleles at a particular locus Heterozygous: two different alleles at a particular locus
© 2014 Pearson Education, Inc. Figure 19.1 Pair of autosomes. Each autosome carries the same genes at the locus Gene locus (plural loci). The location of a specific pair of genes A pair of genes. Normally both genes have the same structure and function Alleles. Alternative versions of the same gene pair
© 2014 Pearson Education, Inc. Your Genotype Is the Genetic Basis of Your Phenotype Genotype: an individual’s complete set of alleles Phenotype: observable physical and functional traits –Examples –Hair color, eye color, skin color, blood type, disease susceptibility Phenotype is determined by inherited alleles and environmental factors
© 2014 Pearson Education, Inc. Genetic Inheritance Follows Certain Patterns Punnett square analysis –Predicts patterns of inheritance To set up a Punnett square: –Possible alleles of one parent are placed on one axis –Possible alleles of other parent are placed on the other axis –Possible combinations of parental alleles are written in the squares within the grid
© 2014 Pearson Education, Inc. Figure 19.2 Haploid sperm Male (diploid) Female (diploid) Haploid eggs Aa aa AA AAA A A AA A A a a a a a a a In a Punnett square, the possible combinations of male and female gametes are placed on two axes, and then the possible combinations of the offspring are plotted in the enclosed squares. This square shows that in a cross between two heterozygotes only half the offspring will be heterozygotes. A cross between two homozygotes produces offspring that are all the same genotype as each other, but not necessarily the same genotype as their parents. A cross between a homozygote and a heterozygote produces an equal number of offspring of each parent’s genotype.
© 2014 Pearson Education, Inc. Mendel Established the Basic Principles of Genetics Worked with pea plants in the 1850s in Austria Did multiple genetic experiments to develop basic rules of inheritance Law of segregation –Gametes carry only one allele of each gene Law of independent assortment –Genes for different traits are separated from each other independently during meiosis –Applies in most cases
© 2014 Pearson Education, Inc. Dominant Alleles Are Expressed Over Recessive Alleles Dominant allele –Masks or suppresses the expression of its complementary allele –Always expressed, even if heterozygous Recessive allele –Will not be expressed if paired with a dominant allele (heterozygous) –Will only be expressed if individual is homozygous for the recessive allele Dominant alleles are not always more common than recessive; sometimes they may be rare in a population
© 2014 Pearson Education, Inc. Figure 19.3 Mendel’s first cross between homozygous yellow-pea plants (YY) and homozygous green- pea plants (yy) yielded all yellow-pea plants. Mendel’s second cross between two of the offspring of his first cross yielded 75% yellow-pea and 25% green-pea plants. Green pea Yellow pea Y yellow peas y green peas Key: YY Yy yy Y Y YY y y y y
© 2014 Pearson Education, Inc. Figure 19.4 Straight hairline Widow’s peak Female Male Key: W widow’s peak w straight hairline Ww WW ww w w W W
© 2014 Pearson Education, Inc. Figure 19.5 Attached earlobes (Johnny Depp, left) and unattached earlobes (George Clooney, right).
© 2014 Pearson Education, Inc. Figure 19.6 A human infant with polydactyly. A polydactyl cat.
© 2014 Pearson Education, Inc. Two-Trait Crosses: Independent Assortment of Genes for Different Traits Outcome of two-trait crosses can be predicted by Punnett square analysis Law of independent assortment –The alleles of different genes are distributed to gametes independently during meiosis –This law applies only if the two genes in question are on different chromosomes
© 2014 Pearson Education, Inc. Figure 19.7 E free earlobes e attached earlobes Key: W widow’s peak w straight hairline Female Male Widow’s peak Free earlobes Straight hairline Attached earlobes A mating between a homozygous person with a widow’s peak and free earlobes (EEWW) and a homozygous person with a straight hairline and attached earlobes (eeww). All of the offspring will have the dominant widow’s peak and free earlobes phenotypes. A mating between two heterozygous people with widow’s peaks and free earlobes (EeWw). Because the alleles for the two traits assort independently, some of the offspring show one dominant and one recessive trait. ee ww Ee Ww ew EW EE WW Ee Ww EW Ew eW ew Ee Ww EE WW EE Ww Ee WW Ee Ww EE Ww EE ww Ee Ww Ee ww Ee WW Ee Ww ee WW ee Ww Ee Ww Ee ww ee Ww ee ww
© 2014 Pearson Education, Inc. Animation: One- and Two-Trait Crosses Right-click and select Play
© 2014 Pearson Education, Inc. Figure 19.8 E free earlobes e attached earlobes Key: W widow’s peak w straight hairline Female Ee E E e e EE ee Free earlobes Male Widow’s peak Attached earlobes Straight hairline Male Ww WW ww W W w w Free earlobes and widow’s peak? Free earlobes and straight hairline? Attached earlobes and widow’s peak? Attached earlobes and straight hairline? (3/4) (3/4) 9/16 (3/4) (1/4) 3/16 (1/4) (3/4) 3/16 (1/4) (1/4) 1/16 What percentage will have: 3/4 free earlobes 1/4 attached earlobes 3/4 widow’s peak 1/4 straight hairline
© 2014 Pearson Education, Inc. Incomplete Dominance: Heterozygotes Have an Intermediate Phenotype Examples –Hair –Straight hair: HH –Wavy hair: Hh –Curly hair: hh –Familial hypercholesterolemia –HH: Normal –Hh: blood cholesterol 2 –3 normal –hh: blood cholesterol 6 normal, heart attacks in childhood
© 2014 Pearson Education, Inc. Figure 19.9 hh curly hair HH straight hair Hh wavy hair Hh H H h h
© 2014 Pearson Education, Inc. Codominance: Both Gene Products Are Equally Expressed Examples –Genes for ABO blood types –A gene and B gene are codominant –An individual heterozygous for the A and B genes will be blood type AB, expressing both A and B antigens on red blood cells –Sickle-cell gene
© 2014 Pearson Education, Inc. Figure 19.10 Antigen AAntigen BAntigens A and BNeither A nor B antigens AA AO BB BO ABOO Possible genotypes Red blood cells Type A Type BType AB Type O
© 2014 Pearson Education, Inc. Codominance: Both Gene Products Are Equally Expressed Sickle-cell gene –Two different alleles of hemoglobin gene –Hb A : encodes normal hemoglobin –Hb S : encodes sickle cell hemoglobin –Sickle-cell anemia: Hb S Hb S (homozygous) –HbS will crystallize if O 2 is slightly decreased, resulting in bending of red blood cells into crescent shapes –Multi-organ damage may result –Sickle-cell trait: Hb A Hb S (heterozygous) –Affected individual makes both types of hemoglobin –Rarely symptomatic
© 2014 Pearson Education, Inc. Figure 19.11 Sickle-cell trait Female Key: Hb A normal hemoglobin Hb S sickle-cell allele Hb S Hb A Hb S Hb A Hb A Hb S Hb S Hb A Normal Male A Punnett square showing a mating between two individuals with the sickle-cell trait. A sickled red blood cell next to a normal red blood cell. Sickle-cell anemia
© 2014 Pearson Education, Inc. Animation: Codominance and Incomplete Dominance Right-click and select Play
© 2014 Pearson Education, Inc. Polygenic Inheritance: Phenotype Is Influenced By Many Genes Inheritance of phenotypic traits that depend on many genes Examples –Eye color, skin color –Height, body size and shape Polygenic traits are usually distributed within a population as a continuous range of values
© 2014 Pearson Education, Inc. Figure 19.12
© 2014 Pearson Education, Inc. Figure 19.13 Parents (medium height) AaBbCc AaBbCc aabbccAaBbccAaBbCcAABbCcAABBCC Median Shorter Taller Height Percent of population Bell-shaped curve
© 2014 Pearson Education, Inc. Both Genotype and the Environment Affect Phenotype Phenotype isn’t determined by genotype alone Environmental factors can profoundly influence phenotype –Example –Nutrition affects height, body size
© 2014 Pearson Education, Inc. Linked Alleles May or May Not Be Inherited Together Linked alleles: physically located on the same chromosome May be inherited together May be “shuffled” during crossing over during meiosis
© 2014 Pearson Education, Inc. Sex-Linked Inheritance: X and Y Chromosomes Carry Different Genes Sex chromosomes –23 rd pair of chromosomes –Not homologous –X and Y chromosomes carry different genes Males: have one X and one Y chromosome Females: have two X chromosomes Male –50% X-carrying gametes, 50% Y-carrying gametes –Male parent determines the gender of offspring
© 2014 Pearson Education, Inc. Sex-Linked Inheritance: X and Y Chromosomes Carry Different Genes Karyotype –A composite visual display of all of the chromosomes of an individual –Shows all 23 pair of chromosomes lined up side-by- side
© 2014 Pearson Education, Inc. Figure 19.14
© 2014 Pearson Education, Inc. Figure 19.15 Female XX Male XY XX XY XX X Y
© 2014 Pearson Education, Inc. Sex-Linked Inheritance Depends on Genes Located on Sex Chromosomes Sex-linked genes are located on sex chromosomes –Sex-linked or X-linked inheritance –Characteristics –More males than females express the disease –Passed to sons by mother –Father cannot pass the gene to sons, but daughters will be carriers Examples –Red-green color blindness –Hemophilia –Duchenne muscular dystrophy
© 2014 Pearson Education, Inc. Figure 19.16Slide 1 Female (normal) Male (normal) Carrier female Hemophiliac male A pedigree chart following the passage of hemophilia for five generations. Female carriers pass the hemophilia allele to half their daughters and the disease to half their sons. Males with the disease pass the hemophilia allele to all their daughters (if they survive long enough to have children), but never to their sons. Key: Generation 5 Generation 4 Generation 3 Generation 2 Generation 1 XhYXhY XhYXhY XhYXhY XhYXhY XHYXHY XHYXHYXHYXHY XHYXHYXHYXHY XHYXHYXHYXHY XHYXHY XHYXHY XHYXHY XHXhXHXh XHXhXHXh XHXhXHXh XHXhXHXh XHXhXHXh XHXhXHXh XHXhXHXh XHXHXHXH XHXHXHXH XHXHXHXH XHXHXHXH A Punnett square showing the possible outcomes of the mating in Generation 1. Female Male XHXhXHXh XHYXHY XHXHXHXH XHXhXHXh XHYXHY XHXH XHXH XhXh Y Normal femaleCarrier female Normal male Normal Carrier XhYXhY Hemophiliac male
© 2014 Pearson Education, Inc. Animation: Sex-Linked Traits Right-click and select Play
© 2014 Pearson Education, Inc. Sex-Influenced Traits Are Affected by Actions of Sex Genes Sex-influenced traits –Genes encoding these traits are located on the autosomes (not the sex chromosomes) –Expression of the trait is affected by presence of testosterone, estrogen Example –Baldness –Several genes influence hair patterns, but also influenced by the presence of estrogen or testosterone
© 2014 Pearson Education, Inc. Chromosomes May Be Altered in Number or Structure Nondisjunction during meiosis –Failure of homologous chromosomes or sister chromatids to separate –A gamete may end up with two copies of a chromosome, instead of just one –Examples –Down syndrome: trisomy 21 –Alterations of the number of sex chromosomes
© 2014 Pearson Education, Inc. Figure 19.17 Meiosis I Meiosis II Nondisjunction at meiosis I Nondisjunction at meiosis II Normal meiosis. Duplicated homologous chromosomes separate during meiosis I, sister chromatids separate during meiosis II. Nondisjunction during meiosis I. The duplicated homologous chromosomes fail to separate from each other. Nondisjunction during meiosis II. Sister chromatids fail to separate from each other.
© 2014 Pearson Education, Inc. Down Syndrome: Three Copies of Chromosome 21 Three copies of chromosome number 21 –Also referred to as trisomy 21 Distinct physical features Developmental disabilities 1/1000 live births in the United States Increased risk of trisomy with increasing maternal age Can be detected by fetal testing
© 2014 Pearson Education, Inc. Figure 19.18
© 2014 Pearson Education, Inc. Alterations in the Number of Sex Chromosomes Nondisjunction affecting sex chromosomes can produce a variety of combinations Jacob syndrome: XYY –Males, tall, otherwise fairly normal Klinefelter syndrome: XXY –Males, tall, sterile, mild mental impairment, some breast enlargement Turner syndrome: XO –Female, short, normal intelligence, sterile
© 2014 Pearson Education, Inc. Figure 19.19 Klinefelter syndrome (XXY). Turner syndrome (XO).
© 2014 Pearson Education, Inc. Table 19.1
© 2014 Pearson Education, Inc. Deletions and Translocations Alter Chromosome Structure Deletions –Piece of a chromosome breaks off –Example: Cri-du-chat syndrome Translocations –Piece of chromosome breaks off and attaches to a different chromosome
© 2014 Pearson Education, Inc. Many Inherited Genetic Disorders Involve Recessive Alleles Many genetic disorders involve recessive alleles –To develop these diseases, one recessive allele is inherited from each parent, who most often are themselves heterozygous (carriers) –Phenylketonuria (PKU) –Lack enzyme to metabolize phenylalanine –May cause mental retardation –Treatment: limit phenylalanine in diet –Tay-Sachs disease –Lack enzyme to metabolize a brain lipid –Leads to brain dysfunction and death by age four
© 2014 Pearson Education, Inc. Huntington Disease Is Caused by Dominant-Lethal Allele Caused by lethal dominant allele Always expressed in heterozygote –Not expressed until midlife Always lethal Has persisted in the human population –Isn’t expressed until midlife so affected individuals have often had children prior to onset of symptoms –Each child of an affected individual has a 50% chance of inheriting the lethal gene
© 2014 Pearson Education, Inc. Genes Code for Proteins, Not for Specific Behaviors Genes: encode specific proteins Proteins have specific functions leading to phenotypes Protein functions –Hormones –Enzymes –Structural –Neurotransmitters
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