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Human Genetics

Human Genetics. Inheritance Patterns for Autosomal and Sex-Linked Genes Variations in Genetic Patterns. Loci, Genes and Alleles. Locus = area on chromosome where gene is located (plural = loci) Gene = sequence of DNA that codes for a protein, gives rise to physical trait

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Human Genetics

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  1. Human Genetics Inheritance Patterns for Autosomal and Sex-Linked Genes Variations in Genetic Patterns

  2. Loci, Genes and Alleles • Locus = area on chromosome where gene is located (plural = loci) • Gene = sequence of DNA that codes for a protein, gives rise to physical trait • Alleles = alternative forms of a gene • Dominant allele masks other alleles • Recessive allele is masked Locus 1 Locus 2 Locus 3

  3. T t Chromosomesduplicate T T t t Meiosis I t T T t Meiosis II Meiosis II T T t t dominant allele Separation ofAlleles by Meiosis recessive allele centromere Each sex cell will carry only one allele for each gene

  4. Autosomal Inheritance for One Gene Donald and Darla are members of a population at high risk for carrying the recessive allele, t, for Tay-Sachs disease. Individuals with at least one copy of the dominant allele, T, do not have the disease. A genetic counselor orders tests to determine the alleles that Donald and Darla have for this gene. The results show that they each have one copy of allele T and one copy of allele t. What are the chances that Donald and Darla could have a child with Tay-Sachs disease, inherited with the alleles tt?

  5. phenotype = physical characteristic genotype = set of alleles T t T t Autosomal Inheritance of One Gene TT or Tt = no disease tt = Tay-Sachs disease Donald, no disease Darla, no disease Tt Tt Tt Chances with each pregnancy: 75% no disease25% Tay-Sachs disease TT Tt tt homozygous recessive homozygous dominant heterozygous(carrier)

  6. fW fw FW Fw fW fw FW Fw Autosomal Inheritance for Two Genes WW or Ww = widow’s peakww = no widow’s peak FF or Ff = Freckles ff = no freckles Suppose George and Gina are both heterozygous for these traits, what are the chances that they will have a child with freckles but no widow’s peak? George F f W w x F f W w Gina Each sex cell will carry one allele for each of the two genes, in all possible combinations.

  7. Child with freckles but no widow’s peak Child with freckles and a widow’s peak Child with no freckles but with a widow’s peak Child with no freckles, no widow’s peak

  8. Variations in Genetic Patterns: Incomplete Dominance Neither allele masks the other and both are observed as a blending in the heterozygote Example: Hair Curling HH = curly hairHH’= wavy hairH’H’ = straight hair

  9. Incomplete Dominance Tina and Thomas both have wavy hair. What are the chances that they will have a child with straight hair? Tina, wavy hair Thomas, wavy hair Chances with each pregnancy:

  10. Type A Type B Type O Type AB Neither allele masks the other so that effects of both alleles are observed in heterozygotes without blending Variations in Genetic Patterns: Codominance Example: ABO Blood Type Alleles A and B are codominant. Alleles Aand B are completely dominant over o. Effects of both alleles observed in phenotype

  11. Three or more alleles exist for one trait[Note: A person can only carry any two of these alleles at once.] Variations in Genetic Patterns: Multiple Alleles Examples: ABO Blood Typeand also Rh Factor *Although there are multiple R alleles, R1, R2, R3, etc. all are completely dominant over all of the r alleles, r1, r2, r3, etc. ABO Blood Type and Rh Factor are controlled by separate genes. They are inherited independently.

  12. Example of Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative (mother is Type O, Rh-) (father is Type O, Rh-) Child with Type AB, Rh negative blood

  13. Many genes affect one trait Example: Skin color Variations in Genetic Patterns: Polygenic Inheritance *Based on a study conducted in Jamaica.

  14. Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white)

  15. BB or Bb bb BB or Bb bb Variations in Genetic Patterns: Sex Influenced Genes Autosomal genes that have different dominance patterns in males and females Example: Pattern BaldnessCaused by a dominant allele in males,but a recessive allele in females

  16. Sex-Influenced Genes Neither Frank nor Felicia have pattern baldness. Felicia’s father also did not have pattern baldness. If Frank and Felicia’s first child is a son, what are the chances that he inherited pattern baldness? Frank, no baldness Felicia, no baldness For a boy: For a girl:

  17. Males carry only one copy of genes on the X chromosomeFemales can be homozygous or heterozygous these genes Inheritance for Sex-linked Genes Hemophilic Male Non-hemophilic Female (father is hemophilic) H = no hemophiliah = hemophilia Chance for each pregnancy:

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