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Chapter 30

Chapter 30. Heredity. 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

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Chapter 30

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  1. Chapter 30 Heredity

  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 • 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

  6. Segregation and Independent Assortment Figure 30.2

  7. Crossover • Homologous chromosomes synapse in meiosis I • One chromosome segment exchanges positions with its homologous maternal counterpart • Crossing over occurs, forming a chiasma • Genetic information is exchanged between homologous chromosomes • Two recombinant chromosomes are formed

  8. Crossover Figure 30.3.1

  9. Crossover Figure 30.3.2

  10. 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

  11. 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

  12. Dominant-Recessive Inheritance Figure 30.4

  13. Dominant and Recessive Traits • 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 own to their offspring

  14. Incomplete Dominance • Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive • Sickling 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)

  15. 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

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

  17. 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

  18. 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 Figure 30.5

  19. 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

  20. 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

  21. Genomic Imprinting Figure 30.6

  22. 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

  23. 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)

  24. Genetic Screening, Counseling, and Therapy Figure 30.7

  25. Carrier Recognition • Identification for 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

  26. 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

  27. Fetal Testing Figure 30.9

  28. 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

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