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Genetics Reference Sheet Gene- unit of heredity (a segment of DNA) Allele- different forms of a gene Punnett Square- tool for predicting probability of offspring with different allele combinations Dominant trait- An allele that expresses its phenotypic effect even when heterozygous with a recessive allele (the big guy always wins) Recessive trait- allele that does not express a characteristic effect when present with a dominant allele. Expresses only when 2 recessive alleles present. (little guy is rarer) Homologous chromosomes-A pair of matching chromosomes in an organism, with one being inherited from each parent. Non-Homologous Chromosomes Homologous Chromosomes In order to be homologous, the chromosomes must code for the same genes and in the same spots. Karotype- picture of a person's chromosomes. Can be used to determine genetic disorders or diseases. 75% of the offspring will express the dominant trait & 25% of the offspring will express the recessive trait when there are two heterozygous parents. Phenotype- the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. (Physical appearance) clipboard(2).galleryitem Genotypegenetic makeup of a cell, an organism, or an individual usually with reference to a specific characteristic under consideration. Must include 2 alleles. Homozygous- a genotype with two alleles that are the same (TT or tt) Heterozygous- a genotype with two different alleles (Tt) rd There are 23 pairs of homologous chromosomes with the 23 pair showing the sex chromosomes (XX= female; XY=male) This karotype shows 3 chromosomes at the 21 pair spot (trisomy 21). This karotype shows the chromosome arrangement of a male with Down syndrome. Incomplete dominance- one allele does not completely dominate another allele, The heterozygous genotype will result in a “blending” of the two alleles. (mixing paint) X-linked traits- gene causing the trait or the disorder is located on the X chromosome. (ie. Color blindness or hemophilia) Both males & females can express X-linked traits. In order for a male to express the trait, his only X chromosome must have the allele. The Red and White alleles show incomplete dominance. The heterozygous combination expresses a “blending” of the two alleles producing a pink phenotype. Co-dominance- A heterozygous combination in which both alleles are fully expressed in the phenotype. (“Co” = Together) The Red and Blue alleles are co-dominant. The heterozygous combination expresses both the red and blue alleles in the phenotype. However, it is less common for females to be affected because both of her X chromosomes must have the allele. Pedigree- a tool used to show how a characteristic might be inherited from generation to generation o Determining between Autosomal or X-linked pedigrees Autosomal- male & females are usually affected equally X-linked- male are affected more frequently than females o Determining between Dominant or Recessive pedigrees Dominant- trait is observed in most generations; cannot be inherited from unaffected parents. Recessive- trait seems to “skip” generations; can be inherited from unaffected parents. A and B are co-dominant. If a person receives an A allele and a B allele, their blood type is type AB. Autosomal Recessive Autosomal Dominant X-Linked Recessive