Gentics Week 2

why did Mendel’s model organism use peas

• easy to grow

• True-breeding strains

• controlled mating: self-fertilization or cross-fertilization

• grow to maturity in one season

• observable characteristics with two distinct forms

Phenotype

physical expression of a trait

Gene

Unit of inheitance

Allele

Alternative form of single gene

Mendel’s three postulates

• Unit factors exists in pairs

• Dominance/recessiveness

• segregation

What is segregation in Medel’s three postulates

paired unit factors segregate (separately) independently during gamete formation

What do organism inherit one of from each parent

alleles

Homozygous/Homozygote

both alleles are (DD, dd)

Heterozygous/heterozygote

Alleles are different (Dd)

Monohybrid crosses contain

one trait

Arrange P, F, and F2 in generations

P1: Original parents → F1: Offspring first filial generation → F2: offspring gen F1 (Self fertilizing)

Who created the punnet square?

Reginald C. Punnett

Dihybrid cross

two pairs of contrasting traits

Mendel suggested heredity resulted in

discontinuous variation

Darwin and Wallace subscribed to the theory of

continuous variation

Independent assortment leads to

genetic variation

Basis for Mendel’s principles of segregation and independent assortment

Separation of Chromosomes during meiosis

Genetic material in living organisms contained in what?

Chromosomes

During formation gametes meet via

meiosis

Homolongs segregate during

meiosis

What are the four criteria for classifying two chromosomes as homologous pairs

• both are same size and exhibit identical centromere locations

• form pairs or synapse during stages of meiosis

• Contain identical linear order of gene loci

• One member of each pair is derived from the maternal parent and one from the paternal parent

Genetic variation due to

nonidentical homologous chromosomes

Chromosome combination produces

genetic variation

When was mendel’s work rediscovered?

Early twentieth century

Mendel suggested heredity resulted in discontinuous

A dominance-recessive relationship

Darwin and Wallace subscribed to theory of continuous variation

A dominance-recessive relationship

Darwin and Wallace subscribed to theory of continuous variation

offspring were a blend of parental phenotypes

Independent events are not

linked

Sum law

calculates probability of outcomes independent of each other

What are the two factors in analyzing or predicating genetic outcomes

independent assortment and sample size

Independent assortment

subject to random fluctuations due to chance deviations

Sample size

average deviation decreases as sample size increases

Chance deviation

Chance events subject to random fluctuations and expected outcome is diminished by larger sample size

Null hypothesis

expect what happens to happen - assumes data will fit given ration

In the null hypothesis there is no real difference between

measured values and predicted values

Pvalve

probability of event happen randomly

Two postulates are basic principles of gene transmission

• Genes are present on homologous chromosomes

• Chromosomes segregate and assort independently

Gene interaction

single phenotype is affected by more than one set of genes

X-Linkage genes are present where?

X chromosome

Mutation

Ultimate source of alleles. New phenotypes result from changes in functional activity of gene product

Mutations cat do what three things?

• Eliminating enzyme function

• Changing relative enzyme efficiency

• Changing overall enzyme function

Wild-type (w t) allele

Occurs most frequently in nature and is usually, but not always, dominant

Loss-of-function mutations

• New phenotype results from change in activity

• Mutation causes loss of wild-type function

Gain-of-function mutations

• Mutation enhances function of wild type

• Quantity of gene product increases

Neutral Mutations

• No change to the phenotype

• No change to the evolutionary fitness of the organism

Genetic symbols for alleles:

(D) or (Wr)

(d) or (Wr)

(e)

(e⁺)

(D) or (Wr) = dominant

(d) or (Wr) = Recessive

(e) = mutant

(e⁺) = Wild Type

Incomplete or partial dominance

• Intermediate phenotype

• Neither allele is dominant

Threshold effect

• Normal phenotypic expression results

• Certain level (usually 50% or less) of gene product is attained

• In Tay-Sachs disease, < 50% threshold

Codominance has what three things?

1. No dominance or recessiveness

2. No incomplete or blending

3. joint expression of both alleles in a heterozygote

Essential genes have what two things?

• Absolutely required for survival

• Mutations can be tolerated if heterozygous

Will one wild-type of allele sufficient for survival

yes

Mutation behaves as

recessive lethal allele

In essential genes homozygous traits will _________

not survive

Lethal alleles

–Has potential to cause death of organism

–Alleles are result of mutations in essential genes

inherited in recessive manner

Dominant lethal alleles

Presence of one copy of allele results in death

Is Huntington disease dominant or recessive?

Dominant autosomal allele H

Is Huntington disease homo or hetero? When is it delayed till?

Onset of disease in heterozygous delayed until adulthood

How do you characterize Huntingtons Disease?

Characterized by progressive degeneration of nervous system, dementia, and early death

Phenotypic characters are influenced by _________ and their products

many different genes

Several gene’s can influence a particular

characteristic

Cellar function of gene interaction

numerous gene products contributes to development of common phenotype

Epistasis

• Expression of one gene masks/modifies effect of another gene pair

• Gene masks phenotypic effects of another gene.

• Each step of development increases complexity of organ

• Under control and influence of many genes

Pleiotropy

Expression of single gene has multiple phenotypic effects

Porphyria variegata

A type pf pleiotropy that is an:

• Autosomal disorder

• Toxic buildup of porphyrins in body

Phenotypic effects of Porphyria

• Abdominal pain

• Muscular weakness

• Fever

• Racing pulse

• Insomnia

• Vision issues

Hemizygous:

males X Y

Homozygous:

females X X

X-linkage are what and are different from what?

• Genes present on X chromosome exhibit patterns of inheritance

• Different from autosomal genes

Y Chromosome are what and specific to what?

• Relatively inert genetically

• Male-specific genes on human Y chromosome

• Lacks copies of genes found on X chromosome

IS colorblindness X or Y chromosome linked?

X linked

Types of X-link traits in humans

• color blindness

• g-6PD deficiency

• hemophilia A

• hemophilia b

• lesch-Nyhan syndrome

• Duchnne muscular dystrophy

Sex-limited inheritance

Expression of specific phenotype is absolutely limited to one sex

Sex-influenced inheritance

• Sex of individual influences expression of phenotype

• Not limited to one sex or the other

Phenotypic expression of trait

• influenced by environment

• Influenced by genotype

Penetrance of phenotypic expression

Percentage of expression of the mutant genotype in a population

Expressivity of phenotypic expression

• Range of expression of mutant phenotype

• Result of genetic background differences and/or environmental effects

Genetic background: Position effect

1. Physical location of gene influences expression

2. Translocation or inversion events modify expression

3. Gene relocated to condensed or genetically inert chromosome (heterochromatin)

What are conditional mutations

mutations such as temperature effects

Nutritional mutations

• Prevent synthesis of nutrient molecules in microbes

• Auxotrophs (microbe)

• Phenotype expressed or not depending upon diet

Phenylketonuria

• Loss of enzyme to metabolize amino acid phenylalanine

• Severe problems unless low-Phe diet

Galactosemia

a nutritional effect that cannot metabolize galactose

Lactose intolerance

a nutritional effect that cannot metabolize lactose