Heredity Part 2

1. Heredity Part 2

2. Genetics • The study of the mechanism of heredity • Nuclei of all human cells (except gametes) contain 46 chromosomes • Sex chromosomes determine the genetic sex (XX = female, XY = male) • Karyotype – the diploid chromosomal complement displayed in homologous pairs • Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal

3. 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 • Dominant – an allele masks or suppresses the expression of its partner • Recessive – the allele that is masked or suppressed

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

5. 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

6. 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 • 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

7. Independent Assortment Figure 29.2

8. 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

9. Crossover Figure 29.3.1

10. Crossover and Genetic Recombination Figure 29.3.2

11. 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

12. 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 • Example: probability of different offspring from mating two heterozygous parents T = tongue roller and t = cannot roll tongue

13. Dominant-Recessive Inheritance Figure 29.4

14. 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

16. Incomplete Dominance • Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive • Sickling gene 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 Hb is made)

17. Incomplete dominance in snapdragon color

18. Incomplete dominance in human hypercholesterolemia

19. 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

20. ABO Blood Groups(codominant) Table 29.2

21. Multiple alleles for the ABO blood groups

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

24. 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, metabolic rate,inteligence.

25. 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

26. Polygenic Inheritance of Skin Color Figure 29.5

27. A model for polygenic inheritance of skin color

28. Sickle cell disease, multiple effects of a single human gene

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

30. Thank you

31. 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 • 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

32. 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

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

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

35. Pedigree Analysis Figure 29.6

36. 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

37. Fetal Testing Figure 29.7

38. 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

39. What makes a dominant gene dominant? • Usually, the masking effect is done by virtue of the fact that the recessive gene has a loss of some function that the dominant gene has. For example, in the case of ABO blood types, the O type is recessive because it does not produce any antigens or antibodies, whereas A and B types (which are co-dominant) do. Or, in the case of eye color, there is a complete loss of pigment in blue-eyed people, therefore to express the phenotype, both copies of the gene (after all, humans are diploid) must have that same loss of function.

40. What makes an allele recessive (the opposite of dominant) is if it DOESN'T code for something, or if it codes for an ALTERED something. In the case of blue eyes, there is no gene that says "make melanin!", so your eyes don't get dark, they stay light blue or gray. In the case of green eyes you have a gene that says "make some really weird melanin!" and so your eyes get a green pigment instead of a normal dark pigment.