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The alignment of one pair of homologs is independent of any other.

The alignment of one pair of homologs is independent of any other. The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment.

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The alignment of one pair of homologs is independent of any other.

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  1. The alignment of one pair of homologs is independent of any other. The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

  2. The Punnett Square for a Dihybrid Cross Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.

  3. 9:3:3:1 ratio that is dependent on: • Two loci, two alleles per locus • Independent assortment between loci (genotypic independence) • Dominance-recessive relationships between the alleles found at each locus • One locus does not affect the phenotype of the other locus (phenotypic independence) Dihybrid Cross (2 loci, 2 alleles) 3:1 ratios are all over this

  4. What Works for Peas Also Works for Humans Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

  5. Some Alleles Are Related Through Incomplete Dominance Dominance relationships may differ, but the Principle of Segregation remains the same.

  6. Pleiotropy – When One Allele Influences Many Traits

  7. Anemia, infections, weakness, impaired growth, liver and spleen failure, death. Traits (phenotypes) associated with the sickle cell allele. Pleiotropy in Action

  8. Height is a polygenic trait Polygenic Inheritance – When a Single Trait is Influenced by Many Genes

  9. Multiple Alleles Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene.

  10. Codominance The human ABO blood group illustrates another genetic phenomenon – codominance. The AB phenotype (genotype IA IB) is an example of codominance Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote.

  11. Pedigree Analysis

  12. Human Traits Table is from http://207.233.44.253/wms/reynolmj/lifesciences/lecturenote/bio3/Chap09.ppt • Most genetic diseases are recessive traits • In other words, there is an absence of a protein function

  13. Autosomal dominant Autosomal recessive Males and females affected? Yes Yes Males and females transmit the trait? Yes Yes Trait skips generations? No Yes At least one parent of affected child must be affected? Yes No Dominant vs. Recessive Note that lethal dominant traits tend to be very rare because affected individuals tend to die before mating

  14. Autosomal Dominant Inheritance No silent carriers Generations are not skipped Typically about half the offspring are affected, but don’t count on this!!!

  15. Autosomal Dominant Inheritance Generations are not skipped

  16. Autosomal Dominant Inheritance Generations are not skipped Huntington’s disease

  17. Pedigree Analysis (dominant)

  18. Autosomal Recessive Inheritance • Heterozygotes carry the recessive allele but exhibit the wildtype phenotype • Males and females are equally affected and may transmit the trait • May skip generations • Note that with rare recessive traits we usually assume that people from outside of a family do not possess the affecting allele

  19. Autosomal Recessive Inheritance Generation skipped

  20. Autosomal Recessive Inheritance Sickle-cell disease Often both parents are silent carriers Cystic Fibrosis Typical is 1/4th affected Generations skipped

  21. Inbreeding unmasks otherwise rare recessive traits because genotypes of parents are not independent Consanguineous Mating Consanguineous mating (=) “With blood”

  22. Autosomal Recessive Inheritance Generations skipped More likely early onset lethal than if dominant

  23. Pedigree Analysis (recessive) Generation skipped

  24. Genetics Problem-Solving Secrets! • Known Genotype can be used to infer unknown Phenotype (but not always, due to complications, e.g., penetrance) • Known Phenotype can be used to infer unknown Genotype (but not always due to lack of 1:1 correspondence: more than one genotype can give rise to a given phenotype) • Genotype (diploid) gives rise to Gametes (haploid) via Meiosis • Gametes (haploid) give rise to “Progeny” (diploid) via Fertilization • Fertilization (syngamy) always results in Diploidy (I.e., >ploidy than haploid) • Meiosis always results in Haploidy (I.e., anaphase I reduction division from diploidy to haploidy)

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