Lecture 12 - Mendelian Genetics

Genes

Discrete heritable units passed on from parent

Alleles

the different “versions” of genes that are inherited, reflecting different physical traits and characteristics

Dominant Allele

If the two alleles at a gene locus differe, the ____ determines the organism’s appearance

Recessive Allele

If the two alleles at a gene locus differ, the ____ has no noticeable effect on organism’s appearance

Homozygous

Organism has two identical alleles for a character

Heterozygous

Organism with two different alleles for a character

Phenotype

Observable traits - refers to physiological traits and appearance traits.

Genotype

Organism’s genetic makeup

PP vs Pp both expressed as purple flowers, but have different genotype

Two organisms can have the same phenotype by different genotypes if one is homozygous dominant and the other is heterozygous.

Testcross

Unknown individual is bred with a homozygous recessive individual, if any offspring display recessive phenotype, they unknown must be heterozygous.

Law of Segregation

3:1 rations observed in F2 generation

Two alleles for a heritable character separate during gamete production into different gametes

• the breaking apart of alleles relates to the physical separation of homologous chromosomes to different daughter cells in meiosis I.

• If organism has two identical alleles, then that allele is present as a single copy in ALL gametes

  • if organism has different alleles, then 50% will receive one and 50% receive the other.

Punnett square

Used to predict the probability of results of a genetic cross between individuals with known genotypes. Mendel’s 3:1 ration for F2 generation.

Law of independent assortment

conclusion that the two pairs of alleles segregate independently of each other. Each pair of alleles for different genes segregate independently during gamete formation.

Dihybrid

Individual that are heterozygous for two characters (YyRr) Follows two different traits

When sperm with four classes of alleles and ova with four classes of alleles combine, there are 16 equally probable ways in which the alleles can combine in the F2 generation.

 These combinations produce four distinct phenotypes in a 9:3:3:1 ratio

Monohybrid

Describes an individual that is heterozygous for one character.

Degrees of Dominance

When alleles are not completely dominant or recessive

Multiple Alleles

When a gene has more than two types of alleles

Pleiotropy

When a single gene produces multiple phenotypes

Epistasis and polygenic traits

When two or more genes control a single trait

Incomplete Dominance

Heterozygotes show a distinct intermediate phenotype not seen in homozygotes

EX: 25% white flowers, 25% red flowers, 50%pink flowers

Codominance

Two alleles affect the phenotype in separate distinguishable ways

EX: Both traits are exhibited such as M, N, and MN — MN is not an intermediate but exhibits both M and N phenotype

Prevalence of dominant vs recessive traits

a dominant allele is not necessarily more common in a population than the recessive allele

Many genes exist in populations in more than two allelic forms.

Example: blood type has three different alleles

Epistasis

the phenotypic expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.

Polygenic Inheritance

Additive effects of two or more genes on a single phenotypic character.

Quantitative characteristics

Traits that vary in population along a continuum EX: Heigh or skin color

Genes and environment determine phenotype

What influences phenotype

Pedigree

Mapping of specific traits on a family tree, info about the presence or absence of a particular phenotype trait is collected from as many individuals in a family as possible, across generation — helps understand past and predict the future

Some human disorders are recessively inherited

Genetic disorders are inherited as simple homozygous recessive traits, heterozygotes have a normal phenotype because one normal allele produces enough of the required protein/enzyme

Albinism

Present only in homozgyous recessive individuals, inherit one recessive allele from each parent.

Carriers

Soemone who may transmit a recessive allele to their offspring

Some human disorders are dominantly inhereited EX: achondroplasia (dwarfism)

99.99% of population who are notdwards are homozygous recessive

Lethal dominant vs recessive alleles

 A lethal dominant allele can escape elimination if it causes death at a relatively advanced age, after the individual has already passed on the lethal allele to his or her children

•  Lethal dominant alleles are much less common than lethal recessive alleles.

   If a lethal dominant allele kills an offspring before he or she can mature and reproduce, the allele will not be passed on to future generations.

Some disorders are mutifactorial

Disorders have a genetic component plus significant environmental influence… EX: diabetes, heart disease, certain mental illnesses… etc.

Genetic component of such disorders is typically polygenic.