Genetics Review Study, study, study!! Test tomorrow… List Mendel’s three proposed principles that are recognized as the basis of inheritance. • The Principle of Dominance • The Principle of Segregation • The Principle of Independent Assortment Define: Homozygous •Having identical alleles or pairs of genes •Ex: BB, bb, DD, dd Define: Heterozygous •One gene in the pair is dominant, and the other gene is recessive. •Ex: Bb, Dd Define: Zygote •A fertilized ovum or egg Define: Dominant •Refers to genes that are expressed whether an allele is identical or similar. •Ex: D, B Define: Recessive • Refers to genes that are only expressed when they are paired with an identical allele • Ex: b, d In swine, the gene for drooping ears (D) is dominant, while the gene for erect ears (d) is recessive. If two swine are mated, one is heterozygous with drooping ears and the other is homozygous with erect ears (Dd x dd), what would be the expected offspring outcome? d d D Dd Dd d dd dd Homozygous drooped ears= 0 Heterozygous drooped ears= 2 Homozygous erect ears=2 When two or more genes influence a trait, an allele of one may have an overriding effect on the phenotype, such as for comb shapes in chickens. This is known as: •Epistasis An exception to the principle of dominance occurs when a heterozygote exhibits traits found in both associated homozygotes, such as a roan coat color in horses. This is known as: •Codominance When a pure, brown-eyed sheep is crossed with a pure, green-eyed sheep, blue-eyed off-spring are produced. This is known as: •Partial dominance Sex-linked genes are carried on what? •The X or Y chromosome Genetic improvement can take up to how long to see an improvement in a trait? • It can take several generations. Is phenotype predetermined? •NO!! Phenotype is not predetermined by the parents. Can genotypic characteristics be measured or observed? •NO! Only phenotype can be measured or observed.