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MENDEL AND THE GENE IDEA (Mendel used the scientific approach to identify…

- - - - DOMINANT ALLELES
        
        determines the organism's appearance
      - RECESSIVE ALLELES
        
        has no noticeable effect on the organism's appearance
      - EX: In the flower-color example,the F1 plant had purple, flowers because the allele for that trait is dominant
    - - For example, P is the purple-flower allele and p is the white-flower allele
  - - - F1 progency produced in his crosses of true breeding patents
      - Meaning that they were heterozygous for the one particular character being followed in the cross
      - Monohybrid cross- a cross between such heterozygotes.
      - This law explains the 3:1 ratio of F2 phenotype observed
    - - Individuals heterozygous for the two characters being followed in the cross
      - Dihybrid cross: a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a pachage or independently
      - In one such dihybrid cross, Mendel studied the inheritance of seed color and seed shape.
        
        The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).
        
        The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).
      - Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr).
      - One possibility is that the two characters are transmitted from parents to offspring as a package
        
        The Y and R alleles and y and r alleles stay together.
      - If this were the case, the F1 offspring would produce yellow, round seeds
      - The F2 offspring would produce two phenotypes (yellow + round; green + wrinkled) in a 3:1 ratio, just like a monohybrid cross.
      - Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ratio in the F2 generation.
- - - - Unlike homozygotes, heterozygotes are not true breeding
- - - - Two dominant alleles affect the phenotype in separate, distinguishable ways
        
        The human MN blood group is determined by codominant alleles fro two specific molecules located on the surface of red blood cells, the M and N molecules.
    - - The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
        
        Ex: Mendel's classic pea crosses, The flowers on Mendel's pea plant are an example of complete dominance, or when the dominant allele completely covers up the recessive allele
    - - occurs when phenotypes of the heterozygote and dominant homozygote are identical
        
        Ex: Mendel's classic pea crosses, The flowers on Mendel's pea plant are an example of complete dominance, or when the dominant allele completely covers up the recessive allele
  - - - Ex: pleitropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle cell disease
    - - Ex: in Labrador retrievers and many other mammals, coat color depends on two genes
- - - - Can be performed starting at the 15th week of pregnancy
      - The liquid that bathes thevfetus is removed and tested.
    - - Is that it can be performed as early as the 10th week of pregnancy.
      - A sample of the placenta is removed and tested.
  - - - Show up only in individuals homozygous for the allele
      - Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
      - Most individuals with recessive disorders are born to carrier parents
      - Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair