MENDELIAN GENETICS

INTRODUCTION

HEREDITY - transmission of traits from one generation to the next

GENETICS - study of heredity

GREGOR MENDEL- "Father of Genetics"

             * identified basic principles of inheritance

TERMINOLOGY

A heritable feature is called a CHARACTER

               * Ex: Flower color

A variation of a character is called a TRAIT

               * Ex: Red Flower

Traits are controlled by segments of DNA called GENES

ALLELES

  • Alternative versions of genes; located on a chromosome

  • An organism receives one allele for a trait from each parent

ALLELE BASICS

Alleles are often represented by letters

DOMINANT alleles represented by uppercase letters

             *T=dominant

RECESSIVE alleles represented by lowercase letters

                    * t=recessive

When combined, a dominant allele masks a recessive allele (Tt)

      HOMOZYGOUS

Having two IDENTICAL ALLELES for a gene

Two types

Homozygous dominant = TT

Homozygous recessive = tt

HETEROZYGOUS

Having two DIFFERENT ALLELES for a gene

Also called HYBRIDS

           * Ex: Tt or Rr

GENOTYPE

Refers to the various allele combinations for a particular gene

It can be a homozygous or heterozygous combination

              * Ex: TT, Tt, tt

PHENOTYPE

Refers to an organism's observable, physical, or physiological traits

Results from an expression of genotype, in conjunction with environmental factors

Described in terms of traits themselves, such as flower color, height, etc.

               * Ex. Tall Plant

                .....Possible genotype? TT= Tall, Tt= Tall

MENDEL'S EXPERIMENTS

MONOHYBRID CROSS

  • Examines the inheritance of a SINGLE TRAIT

  • Cross HD vs HR parents (P generation)

  • Results in all heterozygous offspring (F1)

2nd generation (F2) results in 3:3 PHENOTYPE RATIO & 1:2:1 GENOTYPE RATIO

PUNNET SQUARE

  • grid used to predict the genotype & phenotype outcome of a cross

Dihybrid Cross

  • Examines inheritance of two different traits

  • Cross b/w HD vs HR parents (P generation)

  • Results in all heterozygous offspring (F1)

  • 2nd generation (F2) results in 9:3:3:1 phenotype ratio

Mendel’s Law

  • Provide the foundation for understanding genetic inheritance & explain how traits are passed from one generation to the next:

    • Law of Segregation - made of a monohybrid cross, states “two alleles for a trait will separate into different gametes during gamete production”, simplified: a parent contributes only one of two alleles for a trait to each sex cell to produce

    • Law of Independent Assortment - resulted from Mendel’s dihybrid cross, “pairs of alleles assort independently of other pairs located on nonhomologous chromosomes. Simplieifed “inheritance of one trait is not affected by inheritance of another trait, assuming the genes are not linked (located on different chromosomes or far apart on the same chromosome)

Exceptions to the Mendelian Genetics

  • Not all characters are determined simply by one gene, in which there are two alleles

    • Exceptions include

      • 1. Incomplete dominace

      • 2. Codominace

      • 3. Multiple alleles

      • 4. Pleiotropy

Incomplete Dominance

  • neither allele is dominant over the other

  • results heterozygous offspring shows an intermediate blend of the two homozygous phenotypes

Codominance

  • both alleles are fully expressed

  • The resulting heterozygous offspring has a phenotype that shows both traits simultaneously without blending

Multiple alleles

  • the organism only has two alleles for a gene, BUT has more than two alleles exist in a population

  • increases diversity of possible genotypes and phenotypes

  • AB is rare, and receives | O is donating

Pleiotrophy

  • 1 gene affects multiple, unrelated phenotypes

ADDITIONAL EXCEPTIONS

  • This is seen when two or more genes affect a phenotype

    • Ex:

    1. Epistasis - phenotype expression of 1 gene affects another gene expression of another

    2. Polygenic inheritance - 2 or more genes have an additive effect on a trait, and observed phenotype will depend along a continuum (range)

      • Ex: skin color, eye color, height

MULTIFACTORIAL CHARACTERS

  • both genetic & environmental factors influence phenotype

  • genotype either rigidly or loosely determines phenotype

  • environment exert influences

    • height - nutrition

    • build - exercise

    • skin color - sun exposure

HUMAN TRAITS & MENDEL

  • human traits do follow Mendel’s pattern of inheritance

  • patterns can be traced by pedigree

    • determines the genotype of chosen individuals

    • predict the genotype of future offspring

SINGLE GENE DISORDERS

  • happens in the mutation of 1 gene

  • effects range from mild to severe

  • disorders are inherited in different ways

    • 1. Recessice inherited

    • 2. dominantly inherited

RECESSIVE DISORDERS

  • in individuals that inherit two copies of the mutated recessive allele

  • Carrier - heterozygous organism

  • carries one copy of recessive allele, BUT phenotypically normal, does not show symptoms

  • carriers may pass on the recessive allele to future offspring

DOMINANT DISORDERS

  • appear in an individual that inherits one copy of a mutated dominant allele

  • Dominant alleles that cause severe disorders are often subject to strong, negative selection pressures

  • Affected individuals may have reduced survival or reproductive rates, decreasing the frequency of these alleles in the population

  • Therefore, dominant disorders tend to be less common

DOMINANT DISORDER: HUNTINGTON’S DISEASE

  • Degenerative disease of the nervous system

  • appears in ages 35-40, fatal

  • any offspring will have a 50% chance of inheriting the disorder

  • can be identified by genetic tests

GENETIC TESTING

  • Can identify disorders before or after birth

  • diagnostic (yes or no answer)

  • Fetal testing

    1. Chorionic Villus Sampling: placental tissue, early as 10th week

    2. Amniocentesis: amniotic fluid, starting at 15th week