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