Chapter 20 Genetic Inheritance Genotype genetic make up for a particular trait written with symbols Alleles are alternate forms of a specific gene at the same position locus on a gene e g allele for unattached earlobes and attached lobes alleles occur in pairs A dominant allele eg T will be expressed and will mask a recessive allele Tt or TT A recessive allele eg t is expressed only when a gene has 2 of this type of allele tt A homozygous dominant genotype consists of 2 dominant alleles TT or AA A homozygous recessive genotype consists of 2 recessive alleles tt or aa A heterozygous genotype consists of 1 dominant allele and 1 recessive allele Tt or Aa Phenotype the physical or outward expression of the genotype Phenotype unattached earlobe unattached earlobe Genotype EE Ee ee attached earlobe Crosses One trait monohybrid cross involves inheritance of alleles for 1 gene character e g WW x Ww So in F2 generation for a monohybrid cross originating from parents who are homozygous for the 2 alleles of 1 gene Phenotypic ratio 3 1 Genotypic ratio 1 2 1 Two trait dihybrid cross considers inheritance of 2 genes characters An experimental cross usually involving parents who are homozygous for different alleles of 2 genes e g WWTT x WwTT Results in a 9 3 3 1 phenotypic ratio for offsprings in F2 What would the Punnett square look like for a dihybrid cross between a male who is WWSS and a female who is wwss A Gamete carries only 1 allele so if an individual has the genotype Ww what are the possible gametes that this individual can pass on Answer either a W or a w but not both Punnett Squares The use of a grid to diagram crosses between individuals by using the possible parental gametes These allow one to determine the probability that an offspring will have a particular genotype and phenotype Practicing Punnett squares What would a Punnett square involving a man M with a genotype Ff and a woman F with a genotype Ff look like F freckles f no freckles Practicing ratios Genotypic ratio is the number of offsprings with the same genotype Phenotypic ratio is the number of offsprings with the same outward appearance What is the genotypic ratio 1 2 1 1 FF 2 Ff 1 ff What is the phenotypic ratio 3 1 3 with freckles and 1 with no freckles Pre implantation genetic disorders offspring will be free of the disorder o If prospective parents carry an allele for a genetic disorder they may seek assurance that their o Following in vitro fertilization IVF the zygote divides o When the embryo has 8 cells 1 cell may be removed for genetic testing o Only embryos that will not have the genetic disorders of interest are placed in the uterus to continue developing Autosomal recessive disorder Individuals must be homozygous recessive to have the disorder Homozygous recessive only recessive no dominant alleles Small a small a aa means autosomal recessive disorder Autosomal dominant disorder Individuals that are homozygous dominant OR heterozygous will have the disorder Capital A Capital A or Capital A small A An Autosomal recessive disorder of interest o Sickle cell anemia due to 1 amino acid change hemoglobin is mutated red blood cells are sickle shaped rather than biconcave resulting in clogged blood vessels not enough oxygen supply results in anemia An Autosomal dominant disorder of interest progressive degeneration of brain cells o Huntington disease huntington protein mutated due to too many glutamine amino acids leading to Likewise there are Sex Linked traits or disorders either X chromosome linked or Y chromosome linked Eg Color Blindness X chromosome linked recessive Its expressed X small b Y recessive trait Males are more likely to have this o Polygenic inheritance Polygenic traits 2 or more sets of alleles govern 1 trait Each dominant allele codes for a product so these effects are additive This results in a continuous variation of phenotypes Environmental effects cause intervening phenotypes e g skin color ranges from very dark to very light e g height varies among individual humans Multifactorial trait a polygenic trait that is particularly influenced by the environment e g skin color is influenced by sun exposure e g height can be affected by nutrition o Incomplete dominance Occurs when the heterozygote phenotype is intermediate between phenotypes of the 2 homozygotes Example curly hair CC x ss straight hair Cs wavy hair o Codominance Occurs when the alleles are equally expressed in a heterozygote Example Type A blood AA x BB Type B blood AB Type AB blood that has o Multiple allele inheritance characteristics of both blood types The gene exists in several allelic forms A person only has 2 of the possible alleles A good example is the ABO blood system A and B are codominant alleles The O allele is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles O is always recessive universal blood donors Ex What type of blood would each of the following individuals have in a cross between Ao and Bo Possible genotypes AB Bo Ao oo Phenotypes Type AB blood Type B blood Type A blood Type O blood o Sex linked inheritance Traits are controlled by genes on the sex chromosomes so are inherited X linked inheritance the allele is carried on the X chromosome why because X is always present in both sexes y is small and limited Cross B b B X X x X Possible offspring Y B b X X B B B X X X X normal vision female normal vision female Y normal vision male b Y normal vision male Most X linked disorders are recessive These more often affect males than females because recessive alleles are always expressed in males as they have only 1 X chromosome and female cells undergo Barr body formation by random inactivation of 1 of the 2 X chromosomes Y linked inheritance the allele is carried on the Y chromosome Most sex linked traits are X linked Eg Color blindness Hemophilia
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