Patterns of InheritanceChapter 14, 15
Mendelian Genetics and its Extensions
Mendel,Garden Peas, and Heredity
Pre-Mendelian theory of hereditary: Blending theory 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 F1 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
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 Mendelian genes have specific loci on
chromosomes, and it is the chromosomes that undergo segregation and independent assortment
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
Theory of independent assortment– by the end of meiosis, genes on different pairs of
homologous chromosomes are independently distributed to gametes
Extending Mendelian Genetics
Incomplete dominance Codominance Multiple alleles for one gene Polygenic inheritance: multiple gene effect Pleiotropy: multiple effects of a single gene Environmental impact Linked genes
– genetic recombination between linked genes– sex determination and sex-linked genes
Incomplete Dominance
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
It is not support for the blending theory of inheritance, because…
Multiple Alleles– more than two alternative
forms of a gene (IA, IB, i) Codominance
– Inheritance characterized by full expression of both alleles in the heterozygote (IAIB)
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
Two or more genes determine a single phenotypic character
commonly seen in quantitative characters, e.g. skin color
Environmental effects could also affect the phenotype
Flower Color depends on acidity;
Extending Mendelian Genetics
Incomplete dominance Codominance Multiple alleles for one gene Polygenic inheritance: continuous variation Pleiotropy: multiple effects of a single gene Environmental impact Linked genes
– genetic recombination through crossing over– sex determination and sex-linked genes
Recombination due to crossing over
Recombination Frequency and Genetic Mapping
XRXrXRY
XrY XRXR or XRXr
Carrier
An example of Sex Linked Genes
Human Genetics Follows Mendelian Principles
Recessively Inherited Disorders– cystic fibrosis– sickle-cell disease– Tay-Sachs disease
Dominantly Inherited Disorders– Huntington’s disease
Multifactorial Disorders– cancer– diabetes– heart disease