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Patterns of Inheritance and Punnett Squares

Patterns of Inheritance and Punnett Squares. Homologous Chromosomes. One came from the mother, one from the father. Draw the following picture. The loci on the homologous chromosomes represent a pair of alleles for a particular trait. Dominant and Recessive Traits.

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Patterns of Inheritance and Punnett Squares

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  1. Patterns of Inheritance and Punnett Squares

  2. Homologous Chromosomes • One came from the mother, one from the father. • Draw the following picture. • The loci on the homologous chromosomes represent a pair of alleles for a particular trait.

  3. Dominant and Recessive Traits • When an organism has two of the same alleles, it exhibits that characteristic. • Whether the alleles are dominant or recessive determines the trait expressed. • Dominant: represented by an uppercase letter and it masks the recessive allele. • Recessive: represented by a lowercase letter; it is only expressed when no dominant allele is present.

  4. Example • Dark hair color is dominant; light hair color is recessive. • The hair could be described as HH (two dominant alleles), Hh (one dominant, one recessive allele), or hh (two recessive alleles). • So, HH and Hh would have dark hair and hh would have light hair.

  5. Vocabulary • Homozygous (purebred): HH or hh; homo means same. • Heterozygous (hybrid): Hh; hetero means different • Genotype: the possible alleles inherited from the parents • Ex: HH, Hh, or hh • Phenotype: the physical/outward expression of the alleles. • Ex: dark hair or light hair

  6. Punnett Squares • A diagram used to identify possible combinations of dominant and recessive alleles in offspring. • Divide square in 4 sections • Write the alleles of 1 parent across the top (1 per square) • Write the alleles of the other parent down the side (1 per square) • In each box, combine the alleles of one parent with the other parent.

  7. Punnett Square Example • Black guinea pig (dominant  BB) crossed with brown guinea pig (recessive bb). • Provides probability, not definite genotype. • 100% Bb • 100% black B B b b

  8. Example 2 • Two heterozygous purple plants, where purple is dominant to white. • HETEROZYGOUS is the key word that lets you know one large, one small letter  Pp • Genotype: • PP = 25% • Pp = 50 % • pp = 25% • Phenotype: • Purple = 75% • White = 25% P p P p

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