Heredity

1. Heredity Unit 3 Chapter 29a

2. Heredity • Who we are is guided by the gene-bearing chromosomes we receive from our parents in egg and sperm. • Segments of DNA called genes are blueprints for proteins, many which are enzymes, that dictate the synthesis of all of our body’s molecules.

3. Heredity • Genes are expressed in our hair color, sex, blood type and so on • However, these genes are influenced by other genes and by environmental influences

4. Genetics • Genetics is the study of the mechanism of heredity • Genes = give birth to • Nuclei of all human cells (except gametes) contain 46 chromosomes (or 23 pair) • Sex chromosomes determine the genetic sex (XX = female, XY = male)

5. Genetics • Karyotype– the diploid chromosomal complement displayed in homologous pairs – a picture of our genome • Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal

6. Alleles • Alleles - Matched genes at the same locus on homologous chromosomes • Homozygous – two alleles controlling a single trait are the same • Heterozygous – the two alleles for a trait are different

7. Alleles • Dominant – an allele masks or suppresses the expression of its partner – represented by a capital letter • Recessive – the allele that is masked or suppressed – represented by a lower case letter

8. Alleles & Genotype • AA = both alleles dominate – homozygous dominant • Aa = one dominant allele and one recessive allele = heterozygous • aa = both alleles recessive – homozygous recessive

9. Genotype and Phenotype • Genotype – the genetic makeup • Phenotype – the way one’s genotype is expressed

10. Segregation and Independent Assortment • Chromosomes are randomly distributed to daughter cells • Members of the allele pair for each trait are segregated during meiosis • Alleles on different pairs of homologous chromosomes are distributed independently

11. Segregation and Independent Assortment • The number of different types of gametes can be calculated by this formula: 2n, where n is the number of homologous pairs

12. Segregation and Independent Assortment • In a man’s testes, the number of gamete types that can be produced based on independent assortment is 223, which equals 8.5 million possibilities

13. Independent Assortment Figure 29.2

14. Crossover • Homologous chromosomes synapse in meiosis I • One chromosome segment exchanges positions with its homologous counterpart • Genetic information is exchanged between homologous chromosomes • Two recombinant chromosomes are formed

15. Crossover Figure 29.3

16. Crossover Figure 29.3

17. Random Fertilization • A single egg is fertilized by a single sperm in a random manner • Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million  8.5 million) zygote possibilities

18. Dominant-Recessive Inheritance • Reflects the interaction of dominant and recessive alleles • Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype

19. Dominant-Recessive Inheritance • Example: probability of different offspring from mating two heterozygous parents T = tongue roller and t = cannot roll tongue

20. Figure 29.4

21. Dominant-Recessive Inheritance • Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease • Examples of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease • Carriers – heterozygotes who do not express a trait but can pass it on to their offspring

22. Now try some Punnet Square problems on you own!

23. Quiz next time! Study guide check Pages 713-718 (6 points)

24. Heredity Unit 3 Chapter 29b

25. Incomplete Dominance • Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive

26. Incomplete Dominance • Sicklinggene is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s) SS = normal Hb is made Ss = sickle-cell trait (both aberrant and normal Hb is made) ss = sickle-cell anemia (only aberrant Hbis made)

27. Multiple-Allele Inheritance • Genes that exhibit more than two alternate alleles • ABO blood grouping is an example • Three alleles (IA, IB,i) determine the ABO blood type in humans • IA and IB are codominant (both are expressed if present), and i is recessive

28. ABO Blood Groups Table 29.2

29. Sex-Linked Inheritance • Inherited traits determined by genes on the sex chromosomes • X chromosomes bear over 2500 genes; Y chromosomes carry about 15 genes

30. Sex-Linked Inheritance • X-linked genes are: • Found only on the X chromosome • Typically passed from mothers to sons • Never masked or damped in males since there is no Y counterpart

31. Polygene Inheritance • Depends on several different gene pairs at different loci acting in tandem • Results in continuous phenotypic variation between two extremes • Examples: skin color, eye color, and height

32. Polygenic Inheritance of Skin Color • Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc) • The first generation offspring each have three “units” of darkness (intermediate pigmentation) • The second generation offspring have a wide variation in possible pigmentations

33. Polygenic Inheritance of Skin Color Figure 29.5

34. Environmental Influence on Gene Expression • Phenocopies – environmentally produced phenotypes that mimic mutations

35. Environmental Influence on Gene Expression • Environmental factors can influence genetic expression after birth • Poor nutrition can effect brain growth, body development, and height • Childhood hormonal deficits can lead to abnormal skeletal growth

36. Genomic Imprinting • The same allele can have different effects depending upon the source parent • Deletions in chromosome 15 result in: • Prader-Willi syndrome if inherited from the father • Angelman syndrome if inherited from the mother

37. Genomic Imprinting • During gametogenesis, certain genes are methylated and tagged as either maternal or paternal • Developing embryos “read” these tags and express one version or the other

38. Extrachromosomal (Mitochondrial) Inheritance • Some genes are in the mitochondria • All mitochondrial genes are transmitted by the mother • Unusual muscle disorders and neurological problems have been linked to these genes

39. Heredity Unit 3 Chapter 29c

40. Genetic Screening, Counseling, and Therapy • Newborn infants are screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and PKU

41. Genetic Screening, Counseling, and Therapy • Genetic screening alerts new parents that treatment may be necessary for the well-being of their infant • Example: a woman pregnant for the first time at age 35 may want to know if her baby has trisomy-21 (Down syndrome)

42. Carrier Recognition • Identification of the heterozygote state for a given trait • Two major avenues are used to identify carriers: pedigrees and blood tests

43. Carrier Recognition • Pedigrees trace a particular genetic trait through several generations; helps to predict the future • Blood tests and DNA probes can detect the presence of unexpressed recessive genes • Sickling, Tay-Sachs, and cystic fibrosis genes can be identified by such tests

44. Pedigree Analysis Figure 29.6

45. Fetal Testing • Is used when there is a known risk of a genetic disorder • Amniocentesis – amniotic fluid is withdrawn after the 14th week and sloughed fetal cells are examined for genetic abnormalities • Chorionic villi sampling (CVS) – chorionic villi are sampled and karyotyped for genetic abnormalities

46. Fetal Testing Figure 29.7

47. Human Gene Therapy • Genetic engineering has the potential to replace a defective gene • Defective cells can be infected with a genetically engineered virus containing a functional gene • The patient’s cells can be directly injected with “corrected” DNA

48. Quiz next time! Study guide check Pages 719 – 726 (8 pts)