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Understanding Human Genetics: Phenotype and Genotype Relationships

Explore human genetics concepts including phenotypes, genotypes, gamete formation, inheritance, Mendel’s principles, genetic disorders, and chromosome behavior. Discover how genes influence traits and how genetic variations occur.

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Understanding Human Genetics: Phenotype and Genotype Relationships

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  1. Human Genetics • Phenotype: observed physical and functional traits • Genotype: complete set of genes and alleles • Alleles: Different versions of homologous genes ex. B and b

  2. Human genetics • How are gametes made? • How does chromosome behavior affect inheritance of traits?

  3. Somatic cells are diploid. • Gametes are haploid, with only one set of chromosomes

  4. SPERMATOGENESIS b OOGENESIS a spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte polar body meiosis ll spermatids polar bodies (will be degraded) egg

  5. 1st law - segregation of alleles • Cells contain 2 copies (alleles) of each gene • Alleles separate during gamete formation (meiosis) • gametes carry only one copy of each gene

  6. Homozygous: BB and bb • Heterozygous: Bb Punnett squares show parental gametesand the genotypes of next generation Possible genotypes and their probabilities Figure 19.2

  7. Law of Independent Assortment • During gamete formation, genes for different traits separate independently into gametes • Why? random alignment of homologues at Meiosis I

  8. Chromosome behavior accounts for Mendel’s principles Figure 9.17

  9. Tetrad A B a b Crossing over A B a a B b A b • Genes on the same chromosome tend to be inherited together = linked genes • Crossing over produces gametes with recombinant chromosomes Gametes

  10. VARIATIONS ON MENDEL’S PRINCIPLES Incomplete dominance P GENERATION Whiterr Red RR • an offspring’s phenotype is intermediate between the phenotypes of its parents Gametes R r PinkRr F1 GENERATION 1/2 R 1/2 r 1/2 R 1/2 R Eggs Sperm RedRR 1/2 r 1/2 r PinkRr PinkrR F2 GENERATION Whiterr Figure 9.12A

  11. GENOTYPES: • Incomplete dominance in human hypercholesterolemia HH Homozygousfor ability to makeLDL receptors Hh Heterozygous hh Homozygousfor inability to makeLDL receptors PHENOTYPES: LDL LDLreceptor Cell Normal Mild disease Severe disease Figure 9.12B

  12. Many genes have more than two alleles in the population Ex. three alleles for ABO blood type in humans IA, IB, i

  13. Polygenic traits - A single trait may be influenced by many genes • Quantitative traits skin color, height, eye color Fraction of population Skin pigmentation

  14. Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A

  15. Family pedigrees are used to determine patterns of inheritance and individual genotypes Dd Joshua Lambert Dd Abigail Linnell D_ JohnEddy ? D_ HepzibahDaggett ? D_ Abigail Lambert ? dd JonathanLambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing Figure 9.8B

  16. Inherited Genetic Disorders Normal Dd Normal Dd PARENTS • Most mutations usually involve recessive alleles • Phenylketonuria, PKU • Tay-Sachs disease • Cystic fibrosis D D Eggs Sperm DD Normal d d Dd Normal (carrier) Dd Normal (carrier) OFFSPRING dd Deaf Figure 9.9A

  17. Examples: achondroplasia, Huntington’s disease • A few are caused by dominant alleles Figure 9.9B

  18. Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles • Ex: hemophilia, red-green color blindness These traits appear mostly in males. Why? • If a male receives a single X-linked recessive allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected Figure 9.23A

  19. Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease Figure 19.12

  20. A high incidence of hemophilia has plagued the royal families of Europe QueenVictoria Albert Alice Louis Alexandra CzarNicholas IIof Russia Alexis Figure 9.23B

  21. Variations on Mendel’s Principles • Codominance, multiple alleles • Pleiotropy • Polygenic traits • Sex-linked genes • Environmental effects

  22. Accidents during meiosis can alter chromosome number Nondisjunctionin meiosis I • Abnormal chromosome count is a result of nondisjunction • homologous pairs fail to separate during meiosis I Normalmeiosis II Gametes n + 1 n + 1 n – 1 n – 1 Number of chromosomes Figure 8.21A

  23. Or sister chromatids fail to separate during meiosis II Normalmeiosis I Nondisjunctionin meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

  24. An extra chromosome 21 causes Down syndrome • The chance of having a Down syndrome child goes up with maternal age Figure 8.20C

  25. Fetal testing can spot many inherited disorders early in pregnancy • Karyotyping and biochemical tests of fetal cells can help people make reproductive decisions • Fetal cells can be obtained through amniocentesis Amnioticfluidwithdrawn Centrifugation Amnioticfluid Fluid Fetalcells Fetus(14-20weeks) Biochemicaltests Placenta Severalweeks later Uterus Cervix Karyotyping Cell culture Figure 9.10A

  26. Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping Fetus(10-12weeks) Several hourslater Placenta Suction Karyotyping Fetal cells(from chorionic villi) Some biochemical tests Chorionic villi Figure 9.10B

  27. Examination of the fetus with ultrasound is another helpful technique PGD - Preimplantation Genetic Diagnosis genetic analysis of embryos from in vitro fertilization (IVF) before inserting into womb Figure 9.10C, D

  28. Genes and Behavior • Mechanism • Product from gene-specific proteins • Proteins have specific functions leading to phenotypes: hormones, enzymes, transport, neurotransmitters

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