Genetics and Inheritance

1. Genetics and Inheritance 0 19

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. Figure 19.1 Pair of autosomes. Eachautosome carries the samegenes at the locus Gene locus (plural loci). The location of aspecific pair of genes A pair of genes. Normally both genes havethe same structure and function Alleles. Alternative versions of the samegene pair

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 Female(diploid) Aa Aa AA AA Haploidsperm a Haploideggs A A A A A A a Aa Aa Aa Aa AA aa A AA AA a A a Aa aa aa AA AA Aa Aa Aa Aa AA aa a aa a a A Male(diploid) A cross between a homozygoteand a heterozygote producesan equal number of offspringof each parent’s genotype. In a Punnett square, thepossible combinations of maleand female gametes areplaced on two axes, and thenthe possible combinations ofthe offspring are plotted in theenclosed squares. This squareshows that in a cross between twoheterozygotes only half theoffspring will be heterozygotes. A cross between two homozygotesproduces offspring that are all the samegenotype as each other, but not necessarilythe same genotype as their parents.

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 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

11. Figure 19.3 Key: Y  yellow peas y  green peas Yy YY Yellow pea y Y Y Y Green pea Yy Yy y Yy YY Y yy Yy Yy yy Yy Yy y y Mendel’s second crossbetween two of theoffspring of his first crossyielded 75% yellow-peaand 25% green-pea plants. Mendel’s first crossbetween homozygousyellow-pea plants (YY)and homozygous green-pea plants (yy) yieldedall yellow-pea plants.

12. Figure 19.4 Female Key: Widow’speak W  widow’s peak Ww w  straight hairline W w Male WW Ww W Ww Straighthairline w Ww ww

13. Figure 19.5 Attached earlobes (Johnny Depp, left) and unattachedearlobes (George Clooney, right).

14. Figure 19.6 A human infant with polydactyly. A polydactyl cat.

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 Female Key: E  free earlobes EeWw e  attached earlobes W  widow’s peak w  straight hairline eW ew Ew EW Female EeWw EEWW EEWw EeWW Widow’s peak EW EEWW Free earlobes Male EEWw EeWw EEww Eeww Ew EW EW EeWw Male EeWw EeWw EeWw eeWW eeWw EeWW Straighthairline ew eW eeww Attachedearlobes EeWw Eeww eeWw eeww EeWw EeWw ew ew A mating between a homozygous person with awidow’s peak and free earlobes (EEWW) and ahomozygous person with a straight hairline andattached earlobes (eeww). All of the offspring willhave the dominant widow’s peak and free earlobesphenotypes. A mating between two heterozygous people with widow’speaks and free earlobes (EeWw). Because the alleles for thetwo traits assort independently, some of the offspring show onedominant and one recessive trait.

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

18. Figure 19.8 Key: E  free earlobes e  attached earlobes W  widow’s peak w  straight hairline Female Female Widow’s peak Ww Free earlobes Ee W w E e Male Male W WW Ww E EE Ee Ww Ee Straighthairline e Ww ww Attachedearlobes w Ee ee 3/4 widow’s peak1/4 straight hairline 3/4 free earlobes1/4 attached earlobes 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 (1/4)  (1/4)  1/16 Attached earlobes and straight hairline?

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

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. Figure 19.10 Type B Type AB Type A Type O Antigen A Antigen B Antigens A and B Neither A norB antigens Red bloodcells Possiblegenotypes BBBO AAAO AB OO

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. Figure 19.11 Female Key: HbA normal hemoglobin Sickle-celltrait HbAHbS HbS sickle-cell allele HbS HbA Normal Male HbAHbA HbAHbS HbA HbAHbS Sickle-cellanemia HbSHbS HbAHbS HbS A Punnett square showing a matingbetween two individuals with thesickle-cell trait. A sickled red blood cell next to a normalred blood cell.

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

28. Figure 19.13 Parents (medium height) AaBbCc  AaBbCc aabbcc AaBbcc AaBbCc AABbCc AABBCC Median Percent of population Bell-shapedcurve Shorter Taller Height

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

34. Figure 19.15 FemaleXX X X XX XX X MaleXY XY XY Y

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. Slide 1 Generation 1 XHXh XHY Figure 19.16 XHXH XHXh XHY XHY Generation 2 XhY XHXh XHXH XHY XHY XhY Generation 3 Female XHXh Carrier XHXh XHXH XHY XHY XhY Generation 4 Xh XH Male XHXH XHXh XHXh XHXh XHY XHY Generation 5 XH XHY Normal female Carrier female Key: XHY XhY Normal Female (normal) Carrier female XHXH XHXh Y Hemophiliacmale Male (normal) Hemophiliac male XHY XhY Normal male A Punnett square showing the possibleoutcomes of the mating in Generation 1. A pedigree chart following the passage of hemophiliafor five generations. Female carriers pass the hemophiliaallele to half their daughters and the disease to half their sons.Males with the disease pass the hemophilia allele to all theirdaughters (if they survive long enough to have children), butnever to their sons.

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

38. 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

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. Figure 19.17 MeiosisI Nondisjunctionat meiosis I MeiosisII Nondisjunctionat meiosis II Normal meiosis. Duplicatedhomologous chromosomes separateduring meiosis I, sister chromatidsseparate during meiosis II. Nondisjunction during meiosis II.Sister chromatids fail to separatefrom each other. Nondisjunction during meiosis I.The duplicated homologouschromosomes fail to separate fromeach other.

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

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. Figure 19.19 Klinefeltersyndrome (XXY). Turnersyndrome (XO).

45. Table 19.1

46. 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

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