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Mendel's Laws of Heredity and Dihybrid Crosses: Understanding Genetic Inheritance

This article explores Mendel's laws of heredity, including the Law of Segregation and the Law of Independent Assortment. It also discusses dihybrid crosses and how different traits are inherited. Learn about the importance of meiosis and how gametes combine to form offspring. Examples and explanations are given to help understand these genetic concepts.

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Mendel's Laws of Heredity and Dihybrid Crosses: Understanding Genetic Inheritance

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  1. Genetics Mendel’s Laws of Heredity and Dihybrid Crosses

  2. Meiosis (Review) • Reproductive cells produce haploid gametes (egg or sperm) • During meiosis, the homologous chromosomes separate into different cells • Therefore, each gamete receives only one allele for each type of trait • When the gametes (egg and sperm) combine to form an offspring, 2 alleles are inherited (1 from the egg and 1 from the sperm) for each trait. • ie: The Law of Segregation

  3. Mendel’s 1st Law • The Law of Segregation • Organisms inherit two copies of each gene (one from each parent) • Organisms donate only one copy of each gene in their gametes. During meiosis the two copies of each gene separate (“segregate”) during gamete formation.

  4. Mendel’s 1st Law – Explained • The Law of Segregation means that if your mom was heterozygous for a particular trait, she could have given you the dominant allele or she could have given you the recessive allele. You only get one from your mom. • (Same thing regarding alleles from your dad.)

  5. Mendel’s 2nd Law • The Law of Independent Assortment • Different traits are determined by different genes. • Genes for different traits can be inherited separately from other traits.

  6. Mendel’s 2nd Law – Explained • The Law of Independent Assortment means that just because you are tall does not mean you have to have brown eyes… Your alleles for a certain trait are not necessarily linked to your alleles for a different trait. They can get shuffled around during meiosis (gamete formation).

  7. Mendel’s 2nd Law – Example In Fruit flies: • Red eyes are dominant to white eyes • Having wings is dominant to no wings • R = red eyes r = white eyes • N = wings n = missing wings

  8. Mendel’s 2nd Law – Example • R = red r = white • N = wings n = missing wings • What is the genotype of a heterozygous red eyed fly? • What is the genotype of a heterozygous normal winged fly?

  9. Mendel’s 2nd Law – Example • R = red r = white • N = wings n = missing wings • What gametes will a heterozygous red eyed fly produce? • What gametes will a heterozygous normal winged fly produce?

  10. Mendel’s 2nd Law – Example • R = red r = white • N = wings n = missing wings • What is the genotype of a fly that is heterozygous for eye color and for wing type?

  11. Mendel’s 2nd Law – Example • R = red r = white • N = wings n = missing wings • What gametes will a fly heterozygous for eye color and wing type produce? (Hint: Use FOIL method.)

  12. Dihybrid Cross • A dihybrid cross examines the inheritance of two traits at the same time. • We will now examine a dihybrid cross between two flies heterozygous for both eye color and wing type.

  13. Dihybrid Cross – Step #1 1) Assign allele letters to represent the traits R = red r = white N = wings n = missing wings

  14. Dihybrid Cross – Step #2 & 3 2) Determine genotypes of parents. 3) Determine the gametes they will produce.

  15. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  16. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  17. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  18. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  19. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  20. Dihybrid Cross – Step #4 4) Fill in your Dihybrid Punnett Square. Place gametes on outside and F1s on inside just like before. rN RN Rn rn RN Rn rN rn

  21. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn RN Rn rN rn

  22. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  23. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  24. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  25. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  26. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  27. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = 9 Red w/o wings = White w/wings = White w/o wings = RN Rn rN rn

  28. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = 9 Red w/o wings = 3 White w/wings = White w/o wings = RN Rn rN rn

  29. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = RN Rn rN rn

  30. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. rN RN Rn rn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = 1 RN Rn rN rn

  31. Dihybrid Cross – Step #5 5)Determine the gametes that give rise to the same phenotypes and write the phenotype ratio. 9 : 3 : 3 : 1 rN RN Rn rn Red w/wings = 9 Red w/o wings = 3 White w/wings = 3 White w/o wings = 1 RN Rn rN rn

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