Human Inheritance

1. Human Inheritance Chapter 9

2. Overview Human inheritance patterns: • Autosomal • Sex-linked Sex determination systems

3. Human Chromosomes Humans: male & female, 2n 23 pairs of homologous chromosomes in cells Each pair is structurally identical except sex chromosomes (Female XX, male XY) Autosomes are same in both sexes

4. Human X & Y chromosomes differ in appearance & genes Have small region that allows to act like homologues during meiosis Inheritance of sex chromosomes in certain combos determines gender

5. Karyotyping Individual’s metaphase chromosomes organized by length, shape, centromere location, etc. Can detect abnormalities in chromosome structure or altered chromosome # by comparing individual’s karyotype against species standard

6. Most traits come from autosomal dominant / recessive alleles inherited in simple Mendelian patterns Some of these alleles cause genetic disorders

7. Genetic Abnormality Rare / uncommon version of trait Not life-threatening e.g. polydactyly

8. Genetic Disorder Heritable condition Mild to severe medical repercussions Characterized by set of symptoms = syndrome

9. Autosomal Dominant Inheritance Dominant allele Trait usually appears each generation because allele is expressed in homozygous dominants & heterozygotes Remember: phenotypic ratio 3:1 e.g. Huntington’s disease, lactose intolerance

10. Huntington’s Disease Degeneration of neurons in brain Affects 1/20,000 – 1/1,000,000 people Results in uncontrolled movements, emotional problems, loss of brain function Symptoms include mood swings, difficulty making decisions & retaining info No cure

11. If 1 parent is heterozygous & other is homozygous recessive, offspring has 50% chance of being heterozygous a a A a

12. Some dominant alleles that cause severe problems persist in populations because: • Expression of allele doesn’t affect reproduction • Affected individuals reproduce before symptoms are evident • Spontaneous mutations

13. Autosomal Recessive Inheritance Recessive allele Must be homozygous recessive to express trait If heterozygous for the trait = carrier e.g. cystic fibrosis, sickle cell anemia

14. Cystic Fibrosis Production of very thick, sticky mucus Affects lungs & digestive system (clogs lungs & hampers pancreas from breaking down & absorbing food) ~30,000 people in US are affected Average lifespan = 35-40 years

15. Sickle Cell Anemia Body produces abnormally-shaped RBCs = break down prematurely & cause anemia Affects 1/500 African-Americans If only 1 allele = sickle cell trait • 1/12 African-Americans have trait • Resistance to malaria

16. If both parents are carriers (heterozygous), offspring has 50% chance of being carrier (heterozygous) & 25% chance of being affected (homozygous recessive) A a A a

17. Sex Determination in Humans Every normal female egg has 1 X chromosome ½ of sperm cells have X, ½ have Y Sperm that fertilizes egg determines gender

18. The SRY Gene 1 of 255 Y chromosome genes Master gene for male sex determination When expressed in XY embryos, initiates testes formation Testes produce testosterone (controls expression of male 2 sexual traits)

19. XX embryo = no Y, no SRY,  testosterone = ovaries form (make estrogens & other sex hormones that control expression of female 2 sexual traits)

20. The X Chromosome 1141 genes: Some associated with sexual traits e.g. distribution of body hair & fat Most of genes associated with non-sexual traits expressed in both males & females (because males get 1 X chromosome)

21. X-Linked Inheritance Thomas Hunt Morgan & Drosophila Determined that genes for non-sexual traits are located on X chromosome

22. X-Linked Inheritance Males show their only allele Males inherit only from mother Fathers pass their only allele to all daughters

23. X chromosome alleles result in phenotypes that follow simple Mendelian inheritance Many recessive alleles cause genetic disorders e.g. hemophilia A, red-green colour blindness

24. Hemophilia A Bleeding disorder (caused by lack of clotting factor) Occurs primarily in males (1/10,000) Severity varies

25. Red-Green Colour Blindness Impairment or loss of function in light-sensitive cone cells in eyes Little or no perception of reds, greens, yellows Affects ~10% of males

26. Punnett Squares for X-Linked Crosses XA Xa XA Y Set up in much the same way as regular Punnett Squares, but use X & Y to represent sex chromosomes with superscript letters to represent the alleles carried on those chromosomes

27. Unaffected female & affected male Female offspring: All carriers Male offspring: All normal

28. Carrier female & normal male Female offspring: 0.5 carrier 0.5 normal Male offspring: 0.5 normal 0.5 affected

29. Female carrier & affected male Female offspring: 0.5 carrier 0.5 unaffected Male offspring: 0.5 normal 0.5 affected

30. More males than females affected Heterozygous females have dominant allele on other X that masks recessive allele’s effects Males only have 1 X chromosome (no 2nd X chromosome to counteract effects of recessive allele)

31. Unaffected male Affected male Unaffected female Carrier female Females are the bridge between generations of affected males

32. Pedigrees Genetic connections among individuals Info from several generations collected Can predict probability of trait being expressed as well as trace trait origins backwards

33. Basic procedure is to create a family tree & apply Mendelian genetics Can’t assume that an individual has a trait or is a carrier without evidence

34. A pedigree for a dominant trait I I I I I I A pedigree for a recessive trait I II ? ? ? ? III ? ? ? IV How to read pedigrees I, II, III = generations = male = female = parents = offspring = shows trait or = does not show trait or = known carrier (heterozygote) for recessive trait or ? ? or Common Symbols Used in Pedigrees Notice you can use parents to determine children’s genotypes or children to determine parents’. = cannot determine genotype from pedigree

35. Looking at this pedigree, is the trait caused by a dominant or recessive allele? How do you know? Can you tell anything about the genotypes of these individuals?

36. Y-Linked Inheritance Genes can only be passed from father to son No effect from mother No effect on daughters e.g. hairy ear (pinna) syndrome

37. An Example In cats, coat colour is determined by an X-linked gene. The black allele causes black coat colour while the other allele, orange, causes orange colour, but in heterozygotes the cats are tortoiseshell (patches of black & orange). This is an example of what type of inheritance? What kind of offspring would you expect from a black female & an orange male?

38. Heritable Changes in Chromosome # Chance events occur before or after cell division that result in wrong chromosome # Consequences can be minor or lethal

39. Most changes in chromosome number occur because of non-disjunction = 1 pair of chromosomes do not separate during mitosis or meiosis

40. (a) Aneuploidy Normal # ± 1 chromosome Usually fatal Basis of most miscarriages Chances of non-disjunction  with  maternal age

41. e.g. Down Syndrome (Trisomy 21) Child inherits extra copy of chromosome 21 (2n for all other chromosomes) 1/900 births

42. Distinct physical characteristics: Weak muscle tone, small mouth that can’t accommodate tongue, uniquely-shaped eyelids Varying degrees of mental retardation Often ↓ immune response, heart malformations

43. (b) Polyploidy Cells have ≥ 3 of each type of chromosome (e.g. 3n, 4n, etc.) Many angiosperms, insects, fish, animals are actually polyploid Responsible for evolution via speciation

44. e.g. polyploidy in plants Fertilized diploid egg duplicates chromosomes but fails to divide = tetraploid (4n) Produce diploid gametes that can fuse with other diploid gametes = 4n offspring Can self-fertilize or interbreed with other 4n individuals of same species

45. If breed with 2n individual from original species, offspring is triploid (sterile because meiosis fails) • 4n & 2n of original species can’t interbreed successfully = new species can form in 1 generation

46. Polyploidy is common in plants because they can reproduce asexually If a 4n animal was produced, it would have to mate with a 2n individual ↓ All 3n offspring would be sterile ↓ = no speciation occurs

47. Changes in # of Sex Chromosomes Non-disjunction causes most of changes in # of X & Y chromosomes Relatively frequent: Often results in learning disabilities & speech problems

48. Female Sex Chromosome Abnormalities Turner Syndrome Trisomy X

49. (a) Turner Syndrome 1 X chromosome; no corresponding X or Y = XO Affects 1/2500-1/10,000 newborn females (75% because of non-disjunction from father) 98% of embryos spontaneously abort

50. Generally, XO females are 4’8” but well-proportioned ↓ sex hormone production & non-functional ovaries (2˚ sexual traits do not develop properly)