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Genetics and Inheritance

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1 Genetics and Inheritance
19 Genetics and Inheritance 1

2 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

3 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

4 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

5 Pair of autosomes. Each autosome carries the same genes at the locus
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 Figure 19.1 Autosomes. Alleles. Alternative versions of the same gene pair 5

6 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

7 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

8 Figure 19.2 Punnett squares.
Female (diploid) Aa AA AA Aa Haploid sperm A a Haploid eggs A A A A A a A AA Aa a Aa Aa A AA AA a Aa aa Aa aa AA aa a Aa aa a Aa Aa A AA AA a Aa aa Male (diploid) 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. Figure 19.2 Punnett squares. 8

9 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

10 Dominant Alleles Are Expressed Over Recessive Alleles
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

11 Figure 19.3 Key: Y  yellow peas y  green peas Yellow pea YY Yy Y Y Y y Green pea y Yy Yy Y YY Yy yy Yy y Yy Yy y Yy yy Figure 19.3 Mendel’s one-trait crosses in pea plants. 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. 11

12 Female Key: Widow’s peak W  widow’s peak Ww w  straight hairline W w
Figure 19.4 Female Key: Widow’s peak W  widow’s peak Ww w  straight hairline W w Male W WW Ww Ww Straight hairline w Ww ww Figure 19.4 Hairline patterns in humans. 12

13 Figure 19.5 Figure 19.5 Harmless dominant/recessive traits and alleles. Attached earlobes (Johnny Depp, left) and unattached earlobes (George Clooney, right). 13

14 A human infant with polydactyly. A polydactyl cat.
Figure 19.6 Figure 19.6 Dominant alleles are not always common in a population. A human infant with polydactyly. A polydactyl cat. 14

15 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

16 Figure 19.7 Key: Female E  free earlobes Ee Ww e  attached earlobes W  widow’s peak w  straight hairline EW Ew eW ew Female Widow’s peak EE WW EE Ww Ee WW Ee Ww EW Free earlobes EE WW Male EE Ww EE ww Ee Ww Ee ww Ew EW EW Ee Ww Male Straight hairline Ee Ww Ee Ww Ee WW Ee Ww ee WW ee Ww ew eW ee ww Attached earlobes Figure 19.7 A two-trait cross showing independent assortment of alleles for different traits. ew Ee Ww Ee Ww Ee Ww Ee ww ee Ww ee ww ew 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. 16

17 Animation: One- and Two-Trait Crosses Right-click and select Play

18 3/4 free earlobes 1/4 attached earlobes
Figure 19.8 Key: E  free earlobes e  attached earlobes W  widow’s peak w  straight hairline Female Female Widow’s peak Free earlobes Ee Ww E e W w Male Male E EE Ee W WW Ww Ee Ww Straight hairline Figure 19.8 Another method of calculating phenotypic ratios from two-trait crosses. e Ee ee Attached earlobes w Ww ww 3/4 free earlobes 1/4 attached earlobes 3/4 widow’s peak 1/4 straight hairline What percentage will have: Free earlobes and widow’s peak? (3/4)  (3/4)  9/16 Free earlobes and straight hairline? (3/4)  (1/4)  3/16 Attached earlobes and widow’s peak? (1/4)  (3/4)  3/16 Attached earlobes and straight hairline? (1/4)  (1/4)  1/16 18

19 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

20 hh curly hair HH straight hair h h H Hh Hh H Hh Hh Hh wavy hair
Figure 19.9 hh curly hair HH straight hair h h H Hh Hh H Hh Hh Figure 19.9 Incomplete dominance. Hh wavy hair 20

21 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

22 Neither A nor B antigens
Figure 19.10 Type A Type B Type AB Type O Antigen A Antigen B Antigens A and B Neither A nor B antigens Red blood cells Possible genotypes AA AO BB BO Figure Blood type: An example of codominance. AB OO 22

23 Codominance: Both Gene Products Are Equally Expressed
Sickle-cell gene Two different alleles of hemoglobin gene HbA: encodes normal hemoglobin HbS: encodes sickle cell hemoglobin Sickle-cell anemia: HbS HbS (homozygous) HbS will crystallize if O2 is slightly decreased, resulting in bending of red blood cells into crescent shapes Multi-organ damage may result Sickle-cell trait: HbA HbS (heterozygous) Affected individual makes both types of hemoglobin Rarely symptomatic

24 A sickled red blood cell next to a normal red blood cell.
Figure 19.11 Female Key: HbA  normal hemoglobin Sickle-cell trait HbA HbS HbS  sickle-cell allele HbA HbS Normal Male HbA HbA HbA HbS HbA HbA HbS Sickle-cell anemia HbA HbS HbS HbS HbS Figure Sickle-cell anemia: an example of codominance. 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. 24

25 Animation: Codominance and Incomplete Dominance Right-click and select Play

26 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

27 Figure 19.12 Figure Polygenic inheritance. 27

28 Parents (medium height)
Figure 19.13 Parents (medium height) AaBbCc  AaBbCc aabbcc AaBbcc AaBbCc AABbCc AABBCC Median Percent of population Bell-shaped curve Figure Continuous range of variation in traits governed by polygenic inheritance. Shorter Taller Height 28

29 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

30 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

31 Sex-Linked Inheritance: X and Y Chromosomes Carry Different Genes
Sex chromosomes 23rd 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

32 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

33 Figure 19.14 Figure The human karyotype. 33

34 Female XX X X X XX XX Male XY Y XY XY Figure 19.15
Figure Sex determination in humans. Y XY XY 34

35 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

36 Figure 19.16 Slide 1 Generation 1 XHY XHXh Generation 2 XhY XHY XHXH XHXh XHY Generation 3 XHY XHXh XHY XhY XHXH Female XH Xh Carrier Generation 4 XHY XHXh XHY XhY XHXH XH Xh Generation 5 XHXh XHY XHY XHXh Male XH XH XH Xh Figure A typical inheritance pattern for hemophilia, an X-linked recessive disease. XH XH Y Key: Normal female Carrier female Normal XH Y Xh Y XHXH Female (normal) XHXh Carrier female Y XHY Male (normal) XhY Hemophiliac male Hemophiliac male Normal 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. A Punnett square showing the possible outcomes of the mating in Generation 1.

37 Animation: Sex-Linked Traits Right-click and select Play

38 Sex-Influenced Traits Are Affected by Actions of Sex Genes
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

39 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

40 Nondisjunction at meiosis I Nondisjunction at meiosis II
Figure 19.17 Meiosis I Nondisjunction at meiosis I Meiosis II Nondisjunction at meiosis II Figure Nondisjunction during meiosis. 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. 40

41 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

42 Figure 19.18 Figure A person with Down syndrome. 42

43 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

44 Klinefelter syndrome (XXY). Turner syndrome (XO).
Figure 19.19 Figure Alterations of sex chromosome number. Klinefelter syndrome (XXY). Turner syndrome (XO). 44

45 Table 19.1 Table 19.1 Some common alterations of chromosome number. 45

46 Deletions and Translocations Alter Chromosome Structure
Piece of a chromosome breaks off Example: Cri-du-chat syndrome Translocations Piece of chromosome breaks off and attaches to a different chromosome

47 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

48 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

49 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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