Applied Genetics and Pedigrees

Copyright Pearson Prentice Hall Incomplete Dominance When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles RR A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Codominance In codominance, both alleles contribute to the phenotype. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Multiple Alleles Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Cell biologists analyze chromosomes by looking at karyotypes. Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs. A picture of chromosomes arranged in this way is known as a karyotype. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Human Karyotype These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex. Females have two copies of an X chromosome. Males have one X chromosome and one Y chromosome. The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes How is sex determined? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis. In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes are called sex-linked genes. More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall X Chromosome Duchenne muscular dystrophy The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Why are sex-linked disorders more common in males than in females? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Colorblindness Three human genes associated with color vision are located on the X chromosome. In males, a defective version of any one of these genes produces colorblindness. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Hemophilia The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia. In hemophilia, a protein necessary for normal blood clotting is missing. Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Pedigree Charts A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall A circle represents a female. A square represents a male. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A circle or square that is not shaded indicates that a person does not express the trait. A shaded circle or square indicates that a person expresses the trait. This drawing shows what the symbols in a pedigree represent. Copyright Pearson Prentice Hall

What is a carrier? A carrier is represented by a half-shaded circle or square. They are heterozygous – they have the recessive gene but it does not show.