Unit 10 : Mendelian Genetics and Heredity

1. What the heck do pea plants have to do with genetics? Unit 10: Mendelian Genetics and Heredity

2. Mendelian Genetics • Genetics is the study of heredity. • It explains how certain characteristics are passed on from parents to children.

3. Gregor Mendel • Monk that lived in the 1800s. • Known as the father of genetics because he discovered most of what we know about genetics today. • He studied pea plants to develop his theories.

4. Genetic Basics • Genes – sections of DNA on chromosomes that code for a specific protein. • Genotype – the genetic makeup of an organism; represented by two letters (TT, Tt, or tt) • Phenotype – what the organism physically looks like (Tall, short)

5. Genetic Basics • Trait – a specific characteristic that is different in various organisms • Ex: hair color, eye color, height, etc. • Alleles – different forms of the same trait • Ex: if the trait is hair color, then the alleles could be blond, brunette, red, etc.

6. Genetic Basics • Dominant allele – always displays the trait; represented with capital letter (T) • Recessive allele – displays the trait if there are two; represented with lowercase letter (t)

7. Genetic Basics • Homozygous dominant – two dominant genes (TT); dominant allele is expressed • Homozygous recessive – two recessive genes (tt); recessive allele is expressed • Heterozygous – one dominant gene, one recessive gene (Tt); dominant allele is expressed

8. In a monohybrid cross, one trait is being studied at a time. • The first cross is always referred to as the F1 generation, and crosses a homozygous dominant with a homozygous recessive.

9. Example: Cross a homozygous dominant tall pea plant (TT) with a homozygous recessive short pea plant (tt)

10. Monohybrid cross F1 T T t t

11. Results • There is 1 phenotype: tall plants • There is 1 genotype: Tt • All offspring are heterozygous T T t t

12. Monohybrid Cross • The second cross is referred to as the F2 generation. • It is a cross between two of the offspring from the F1 generation (two heterozygous genotypes)

13. Monohybrid Cross F2 T t T t

14. Results T t • There are 2 different phenotypes: tall and short • There are 3 different genotypes: TT, Tt, and tt • The ratio of phenotypes is 3:1 (3 tall, and 1 short) • The ratio of genotypes is 1:2:1 (1 TT, 2 Tt, and 1 tt) T t

15. Punnett squares Pp x Pp PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Aaaaah, phenotype & genotypecan have different ratios % genotype % phenotype F1 generation (hybrids) PP Pp Pp pp 1:2:1 3:1

16. Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas 100% purple-flower peas F1 generation (hybrids) 100% 75% purple-flower peas 25% white-flower peas 3:1 F2 generation X P Where did the whiteflowers go? Whiteflowers cameback! self-pollinate

17. Dihybrid Crosses • In a dihybrid cross, two traits are being studied at the same time.

18. Dihybrid Crosses • According to Mendel’s Law of Independent Assortment, genes for different traits can segregate without affecting each other. • So when completing a dihybrid cross we must allow for all the different variations that are possible.

19. Example: Perform a dihybrid cross looking at the traits for plant height (tall or short) and plant color (green or yellow).

20. Because the traits can segregate independently we must used all the alleles possible in our test cross : • TG (tall and green) • Tg (tall and yellow) • tG (short and green) • tg (short and yellow)

21. TG TgtGtg TG Tg tG tg

22. TG TgtGtg TG Tg tG tg

23. Results: For the 16 offspring there are 4 possible phenotypes: • 9 tall and green plants • 3 tall and yellow plants • 3 short and green plants • 1 short and yellow plant • The ratio for heterozygous dihybrid crosses is always 9:3:3:1

24. F1 generation (hybrids) yellow, round peas 100% F2 generation Dihybrid cross P true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas

25. 9/16 yellow round YyRr YyRr 3/16 green round YR YR yr YR yR Yr yr Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross or YyRr x YyRr YR Yr yR yr  YYRR YYRr YyRR YyRr BINGO! YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

26. Mendel’s Laws Law of Dominance – The dominant trait masks the effect of the recessive trait • Ex: Brown eyes are dominant over blue eyes

27. Mendel’s Laws Law of Segregation – Alleles can segregate and recombine • Ex: Parents with brown eyes can produce a baby with blue eyes

28. P P P p PP Pp pp p p Mendel’s 1st law of heredity • Law of segregation • during meiosis, alleles segregate • homologous chromosomes separate • each allele for a trait is packaged into a separate gamete

29. Mendel’s Laws Law of Independent Assortment – traits can segregate and recombine independently of other traits • Ex: A tall pea plant can be either yellow or green

30. yellow green round wrinkled Mendel’s 2nd law of heredity Can you thinkof an exceptionto this? • Law of independent assortment • different loci (genes) separate into gametes independently • non-homologous chromosomes align independently • classes of gametes produced in equal amounts • YR = Yr = yR = yr • only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1

31. Beyond Mendelian Genetics • Not all patterns of inheritance obey the principles of Mendelian Genetics. • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

32. Incomplete dominance In some cases, the alleles will blend when the offspring is heterozygous. • Ex: a white snapdragon crossed with a red snapdragon produces a pink snapdragon.

33. Incomplete dominance • Red snapdragon: RR • White snapdragon: rr • Pink snapdragon: Rr R R r r

34. 100% pink flowers F1 generation (hybrids) 100% 25% red 50% pink 25% white 1:2:1 F2 generation Incomplete dominance X true-breeding red flowers true-breeding white flowers P It’s likeflipping 2 pennies! self-pollinate

35. Codominance sometimes you will see an equal expression of both alleles. • Ex 1: a person with blood type AB (both the A allele (IA) and B allele (IB) are expressed)

36. Codominance • Ex 2: heterozygous checkerboard chickens have both white and black feathers • Ex 3: roan horses have both red and white hairs on their bodies

37. Codominance • A blood: IA IA or IA i • B blood: IB IB or IBi • O blood: ii • AB blood: IA IB IAi IB i

38. Multiple alleles – • many genes have more than just two alleles that exist in the population. • Ex 1: rabbit coats have four alleles: full color (C), chinchilla (Cch), Himalayan (Ch), and albino (c).

39. Multiple Alleles • Ex 2: Humans can have 3 different alleles for blood type: IA, IB , and i

40. Polygenic/Polygenetic traits • in some cases, a trait results from the interaction of many genes together. • You can usually tell which traits are polygenic because there is a wide range of different phenotypes. • Ex: height, skin color, and hair color.

41. Sex-Linked Traits • Humans contain 23 pairs of chromosomes. • 22 of the 23 pairs are called autosomes (they code for many different traits) autosomalchromosomes sexchromosomes

42. The other remaining pair contains the sex chromosomes (X or Y) • A female has two X chromosomes (XX) • A male has one X and one Y chromosome (XY)

43. Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Duchenne muscular dystrophy Becker muscular dystrophy Chronic granulomatous disease Retinitis pigmentosa-3 Adrenal hypoplasia Glycerol kinase deficiency Norrie disease Retinitis pigmentosa-2 Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Androgen insensitivity Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease PRPS-related gout Lowe syndrome Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Lesch-Nyhan syndrome HPRT-related gout Immunodeficiency, X-linked, with hyper IgM Lymphoproliferative syndrome Hunter syndrome Hemophilia B Hemophilia A G6PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked Albinism-deafness syndrome Fragile-X syndrome Sex-Linkedtraits • Some traits are carried on the sex chromosomes and are called sex-linked traits • Ex: color-blindness and hemophilia • Most sex-linked traits are found on the X chromosome ( X-Linked)

44. Sex-Linked Traits X X X Y

45. Sex-Linked Traits X X X Y

46. Sex-Linked Traits X X X Y

47. Sex-Linked Traits X X X Y

48. Sex-Linked Traits X X X Y

49. Sex-linked Traits • Since males have one X chromosome and one Y chromosome, they are more likely to express a sex-linked trait. • This happens because his one and only X chromosome is defective. • He doesn’t have another X chromosome to mask the effect of the bad X.

50. Sex-linked Traits • If a female has one defective X chromosome, and one normal X chromosome, she won’t express the sex-linked trait. • For a female to express the trait, she would have to inherit two defective X chromosomes.