Patterns of Inheritance Chapter 14, 15 Mendelian Genetics and its Extensions

Mendel,Garden Peas, and Heredity n Pre-Mendelian theory of hereditary: Blending theory n Mendelian theory of hereditary: Particulate theory –Law of Segregation/Law of Dominance –Law of Independent Assortment

Monohybrid Cross on Flower Color

Genotype versus phenotype

The Results of Mendel’s F 1 Crosses for Seven Characters in Pea Plants

Alleles, alternative versions of a gene

Mendel’s Law of Segregation/Law of Dominance Alternative forms of genes (alleles) are responsible for variations in phenotypes For each character, an organism inherits two alleles, one from each parent (maternal and paternal) If the two alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele) The two alleles for each character segregate during gamete production (meiosis)

Testcross n To determine whether an organism with a dominant phenotype is homozygous or heterozygous, you use a testcross

Testing Two Hypotheses for Segregation in a Dihybrid Cross

Mendel’s Lucky Choices of Characters in Garden Peas 1) Each character is determined by one gene 2) Each gene has only two alleles 3) One allele is completely dominant over the other 4) In dihybrid crosses, the two genes (seed color and seed shape) are located on different pairs of chromosomes

Blending Theory Particulate Theory (discrete heritable factor) Chromosomal Theory Heritable factors are located on chromosomes It is the chromosomes that segregate and independently assort Theory of Inheritance

1860s : Mendel proposed that discrete inherited factors segregate and assort independently during gamete formation 1875: cytologists worked out process of mitosis 1890: cytologists worked out process of meiosis 1902: Cytology and genetics converged as Sutton, Boveri and others noticed parallels between the behavior of Mendel’s factors and the behavior of chromosomes: chromosomes and genes are both paired in diploid cells homologous chromosomes separate and allele pairs segregate during meiosis fertilization restores the paired condition for both chromosomes and genes Genes are located on Chromosomes

Comparing chromosome segregation and allele pairs segregation

Chromosomal Theory of Inheritance n Mendelian genes have specific loci on chromosomes, and it is the chromosomes that undergo segregation and independent assortment n Theory of segregation –diploid cells have pairs of genes, on pairs of homologous chromosomes. During meiosis, the two genes of each pair segregates from each other, and end up in different gamete n Theory of independent assortment –by the end of meiosis, genes on different pairs of homologous chromosomes are independently distributed to gametes

Extending Mendelian Genetics n Incomplete dominance n Codominance n Multiple alleles for one gene n Polygenic inheritance: multiple gene effect n Pleiotropy: multiple effects of a single gene n Environmental impact n Linked genes –genetic recombination between linked genes –sex determination and sex-linked genes

Incomplete Dominance n One allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotypes of the two homozygotes n It is not support for the blending theory of inheritance, because…

n Multiple Alleles –more than two alternative forms of a gene (I A, I B, i) n Codominance –Inheritance characterized by full expression of both alleles in the heterozygote (I A I B ) Human ABO Blood Types

Complete dominance Codominance Incomplete Dominance Full expression of both alleles in heterozygote Intermediate phenotype in heterozygote Expression of dominant allele masks the expression of recessive allele

Pleiotropic effects of the sickle-cell allele in a homozygote

Polygenic Inheritance n Two or more genes determine a single phenotypic character n commonly seen in quantitative characters, e.g. skin color n Environmental effects could also affect the phenotype

Flower Color depends on acidity;

Extending Mendelian Genetics n Incomplete dominance n Codominance n Multiple alleles for one gene n Polygenic inheritance: continuous variation n Pleiotropy: multiple effects of a single gene n Environmental impact n Linked genes –genetic recombination through crossing over –sex determination and sex-linked genes

Recombination due to crossing over

Recombination Frequency and Genetic Mapping

XRXrXRXr XRYXRY XrYXrYX R X R or X R X r Carrier An example of Sex Linked Genes

Human Genetics Follows Mendelian Principles n Recessively Inherited Disorders –cystic fibrosis –sickle-cell disease –Tay-Sachs disease n Dominantly Inherited Disorders –Huntington’s disease n Multifactorial Disorders –cancer –diabetes –heart disease