Mendelian Genetics

Gregor Mendel studied pea plants

Alleles: two different versions of the same gene

  • Alleles are created by mutations to a gene sequence

  • Many alleles are due to single-nucleotide polymorphisms (SNPs)

  • Mendel created true-breeding plants - a line of plants that only produced plants with the specific trait when allowed to self-pollinate

  • Mendel’s P Generation consisted of 2 true-breeding plants of opposite traits

    • (ex. a true-breeding purple flower plant and a true-breeding white flower plant)

F1 Generation:

  • Mendel performed the first cross with the P generation where he cross pollinated the true-breeding plants

  • The resulting offspring were called the F1 Generation

  • He called these the hybrids (monohybrids)

  • 100% of the hybrids exhibited only 1 of the two versions of each trait

    • (ex: purple flowers crossed with white flowers produced 100% purple flowers)

F2 Generation:

  • Mendel allowed the hybrids (F1 generation) to self-pollinate in order to produce the F2 generation

  • The offspring in the F2 generation exhibited both versions of the traits in predictable ratios

    • (Ex: 3/4 purple flowers and 1/4 white flowers - 3:1 ratio)

1st Concept of Mendelian Genetics:

2 Alleles

  • Traits that exhibit the patterns of Mendelian Genetics have only 2 alleles

  • Within each organism, there are 2 copies of each gene, one on each chromosome in a homologous pair (at the same loci)

  • Those copies could both be the same, or they could be different

2nd Concept of Mendelian Genetics:

Biparental Inheritance

  • For each trait, an organism inherits 2 alleles, 1 from each parent/gamete

  • Remember - these alleles could both be the same or they could both be different

Autosomal Traits:

  • Traits that exhibit Mendelian genetics have genes that are found on the autosomes (chromosomes 1-22 for humans)

  • This allows them to be inherited in a biparental pattern

  • Traits determined by genes that are found on the sex chromosomes (X or Y) are not considered to be “Mendelian” - why?

  • More on sex linked traits during non-mendelian genetics

3rd Concept of Mendelian Genetics:

Dominance v. Recessiveness

  • Of the 2 different alleles for any trait, one is dominant and one is recessive

  • When an organism has 2 different alleles, the dominant allele will be the one that is expressed

  • The recessive allele is only expressed in the absence of the dominant allele - aka when an organism has two copies of the recessive allele

4th Concept of Mendelian Genetics:

Law of Segregation

  • In the parent, the alleles of each trait segregate (or separate) into each gamete so each gamete only receives one copy of each gene

  • This is because the homologous chromosomes separate during Meiosis I

  • These 2 concepts are tied directly to the process of sexual reproduction: Gamete formation and fertilization

During gamete formation, the homologous chromosomes separate during Meiosis I (segregation of alleles) - because gametes are haploid, each gamete to only contribute 1 copy of each gene to their offspring (biparental inheritance)

  • During fertilization, the fusion of sperm and egg creates a diploid zygote

  • The fertilization process provides the offspring with 1 copy of each gene from each parent/gamete (biparental inheritance)

  • Tied directly to the chromosome numbers:

    • Before Meiosis: Diploid cell

    • After Meiosis: Haploid gametes

    • After fertilization: Diploid zygote

Writing Alleles - Autosomal Traits:

  • The alleles for an autosomal trait are given letters

    • Capital letter for the dominant allele

    • Lower case letter for the recessive allele

  • Example:

    • F = purple flowers, f = white flowers

Genotype - Autosomal Trait:

  • The combination of alleles is the genotype of an individual

  • Genotypes for 1 trait are written with 2 letters to represent the 2 alleles on the homologous chromosomes

  • Possible combinations:

    • FF, Ff, and ff

  • Organisms are homozygous if they have 2 copies of the same allele:

    • FF = homozygous dominant

    • ff = homozygous recessive

  • Organisms are heterozygous if they have 1 copy of each allele:

    • Ff = heterozygous

Genotype & Phenotype:

  • The genotype determines the phenotype (with a few exceptions)

  • The phenotype of an individual is the expression of the genes (ex physical appearance or metabolic characteristics, etc)

  • Homozygous dominant and heterozygous genotypes will result in the dominant phenotype

  • Homozygous recessive genotypes will result in the recessive phenotype

Punnet Square:

  • Developed by Sir Reginald Punnett

  • A statistical tool used to predict the phenotypic and genotypic outcome (offspring) of a cross

  • The outside represents the gametes of the parent - 1 parent across the top, 1 parent down the side

  • The inside boxes represent the possible genotypes of the offspring

Recessive Disorders:

  • Many genetic disorders are inherited in an autosomal recessive pattern - the gene’s locus is on an autosome, and the allele is recessive, so an individual must be homozygous recessive to express the recessive disorder

  • Example: Phenylketonuria or PKU

  • If someone is heterozygous for a recessive disorder, they are said to be a “carrier”

  • Carriers have the normal (dominant) phenotype, but have 1 copy of the recessive allele that they can pass on to their offspring

  • Many partners will get genetically tested to see if they are carriers of the same trait

Phenylketonuria/ PKU:

  • PKU is caused by a mutation in a gene on chromosome 12 (autosomal)

  • This gene codes for an enzyme called phenylalanine hydroxylase (PAH)

  • PAH converts the amino acid phenylalanine (Phe) into tyrosine (Tyr)

  • The mutation creates a malfunctioning enzyme

  • Individuals that are homozygous recessive are unable to break down Phenylalanine (Phe)

  • Phenylalanine builds up to toxic levels and can cause: a musty odour from the skin and urine, fair skin, eczema, seizures, tremors and hyperactivity

  • If the condition is left untreated, brain damage can occur

  • PKU can be managed with a tightly controlled, low-protein diet

5th Concept of Mendelian Genetics:

Law of Independent Assortment

  • Each pair of alleles segregates independently of each other pair of alleles during gamete formation

  • AKA The way each tetrad lines up during metaphase I has zero impact on the other tetrads

  • Assumption: the gene loci are on different chromosomes!

A dihybrid cross: performed if both parents are heterozygous for both traits

  • Imagine that in cats, black fur is dominant to white fur, and long fur is dominant to short fur.

  • Two heterozygous cats are crossed.

  • Predict the phenotypic ratio of the offspring.