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Gregor johann mendal Father of genetics experiments with pea plants.Empirical approach: did not have hypothesis, but analysed data to find relationships
Pea plants:
- It has the ability to self-fertilize
- It was easy to cross-fertilize one plant with another
- It has easily identifiable traits
Hybridization The mating or crossing between two individuals that have different characteristics.
Hybrids: the offspring that results from such a mating.
Mendal two types of crosses Self-fertilization
Cross-fertilization
Characters: the morphological features of an organism, e.g. eye colour
Traits: describes the specific properties of the character, e.g. blue eyes
True breeder: a variety that produces the same trait over several generations.
Differences in plant flower color or plant height are called a variant of a
trait.
Monohybrid cross Crossing two variants of the same characteristic.
Genotype: 1:2:1 (heterozygote parents)
Phenotype: 3:1
Mendel’s law of segregation During gamete formation the paired factors (genes) for a given character segregate
randomly
Dihybrid cross Crossing individuals that differ in two characteristics.
Nonparentals: represent offspring with genetic combinations that were not
found in the parental lines
Genes assort independently: phenotype 9:3:3:1
What is the probability that an offspring will have an ssRR genotype from
a cross of two SsRr individuals? 6,25% -> draw punnet square.
Mendel’s law of independent During gamete formation, the segregation of any pair of hereditary determinant is
assortment independent of the segregation of other pairs (meiosis I); t wo different genes will
randomly assort their alleles during the formation of haploid cells.
Does not apply to linked genes.
Alleles Different versions of genes
Homozygote: two identical alleles in individual
Heterozygote: two different alleles in individual
Genotype Specific allelic composition of an individual
Phenotype Outward appearance of an individual
Dominant (functional)
Recessive (defective)
Punnet square: used with Grid that enables one to predict the outcome of a simple genetic crosses
mono and dihybrid crosses - Write down genotype of both parents
- Write down possible gametes
- Create/fill in punnet square
- Determine relative properties of genotype and phenotype of offspring
Outside of the bocks represents haploid gametes
Forked-line method 3/+ independently assorting genes
Multiplication method Multiplication rule/ product rule: the probability that two or more independent events will
occur is equal to the product of their respective probabilities.
Using Mendel's flower color (purple is dominant, white is recessive), if two
heterozygous plants are crossed, what is the probability that the first two
offspring will have purple flowers? 9/16 want, ¾ X ¾
Inheritance pattern in humans Loss-of-function alleles (e.g. produces inactive protein) = defective copies of a gene
Pedigrees (family tree) are use to study inheritance, cause can’t use parental crosses ->
unethical.
Disease that follow a simple mendelian pattern of inheritance: dominant or recessive:
Recessive:
- 2 normal heterozygote: 25% affected offspring
- 2 affected individuals: 100% affected offspring
Dominant:
- Affected individual inherited gene from at least one infected parent or mutation
, during gamete formation
Chapter 4
Simple mendelian Involves a single gene with two different alleles which display a simple dominant/recessive
inheritance relationship.
WT phenotype heterozygote:
- 50% normal protein is enough
- May produce more that 50% of functional protein; normal gene is up-regulated to
compensate for defective gene.
Wild-type alleles Should produce sufficient amounts of proteins of a gene to bring about WT-phenotype. Common
alleles in a population. Encodes proteins that function normally and is made in proper amounts.
Mutant alleles Alleles altered by mutation. Tend to be rare in a population. Cause reduction in amount or affect
the function of the encoded protein. In not always defective (recessive).
Recessive mutations Null: complete loss of function
Hypomorph: reduced level or protein with weak function
Dominant mutations - Gain-of-function mutation: change the gene or protein encoded by the gene so that it
gains a new or abnormal function.
hypermorph = increased levels or more effective way
neomorphic = new function: gain of function
Ecotopic expression: gene expression in tissue which is normally not expressed.
- dominant-negative mutations: poisonous effect: mutant protein counteracts the effect of
normal protein -> altering phenotype.
- haploinsufficiency
Single gene traits
Traits may skip a Incomplete penetrance = a dominant allele does not always penetrate into/ lead to a phenotype of
generation due to the individual; polydactyly. Always referred to with population. E.g. trait is 70% penetrant means
Only 70% of the individuals who carry the allele(s) for a trait express the trait.
Vary in their expressivity = the degree to which a treat is expressed (environmental influences,
environmentally influenced trait e.g. Phenylketonuria). Variable expressivity means
that the phenotype of a trait can vary between individuals.
Incomplete Heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes..
dominance Whether a trait is dominant or incompletely dominant may depend on how closely you examine
the trait.
- Haploinsufficiency: heterozygote individual has one loss-off-function allele and the
functional allele does not yield enough gene product to bring about a WT condition.
- Four-o-‘ clock plant
Overdominance Heterozygote is more vigorous/higher fitness that both the corresponding homozygotes;
heterozygote advantage. Sickle-cell anemia-materia sensitivity
- Disease resistance
- Homodimer formation
- Variation in functional activity (temperature sensitivity -> fur e.g.)
Codominance and ABO blood group. The phenomenon that two alleles are both expressed in a heterozygote =
multiple alleles codominance. Blood groups A, B AB, O. Safe blood transfusion e.g. type O cannot receive type A, B,
AB; antibodies in blood will react with antigens in donated blood. Different groups can differ, in
antigen presentation, sugar, antibodies.
Human blood groups are determined by antigens on the surface of red blood
cells.
X-LINKED genes Gene is only located on X-chromosome. Pedigree for X-linked disease: shows mostly affected
males with their mothers as carriers.
Males have single copy of X linked gene = hemizygous
Also have
Y-linked or holandric genes e.g. SRY
Let op X-dominant traits.
Sex influenced traits Traits were an allele is dominant in one sex but recessive in the opposite sex, does not mean sex-
linked. Most are due to autosomal genes. Phenomenon of heterozygotes.
- E.g. scurs in cattle
Sex limited inheritance Traits that only occur in one of the two sexes. Genes are controlled by sex hormones or the sexual
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